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LIBRARY 

LIBPAAV -~-~ 

U.S. BUREAU OF MINES We:.~-~"'Tl Fie~ ()peraUon c.,. 

Aerial Photographs in E~t 360 3rd Ava. 

_..._ was~~~~.,..._ 

Geologic Interpretation 

and Mapping 

GEOLOGICAL SURVEY PROFESSIONAL PAPER /"373 

8 U R fA ll 0 F M 1-N E S 

LIBRARy 

SPOKANE WASH • 

JUN- 91961 , 

PLEASE RETURNTO li~RARy 
- ) 

;· 

Aerial Photographs in 

Geologic Interpretation 

and Mapping 

By RICHARD G. RAY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 373 

I'he use of aerial photographs to obtain 

f!Ualitative and quantitative geologic 

inforntation, and instrument procedures 

enzployed in conzpiling geologic data from 

aerial photographs 

BUREAU OF' MINES 

.L 18 R·A·R.Y 

SPOKANE. WASH. 

JUN- 91961 

··p{.E AS E R·E T u R·N 

.TO li£RA~Y 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960 
( 

.... .. 

\ 

UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 

'J'he U.S. Geological Survey Library has cataloged this publication as follows: 

Ray, Richard Godfrey, 1920-

Aerial photographs in geologic interpretation and Jl1apping. Washington, U.S. Govt. Print. Off., 1960. 

vi, 230 p. illus., maps, diagrs. 30 em. (U.S. Geological Survey. 

Professional paper ( 373) 

Bibliography : p. 224-227. 

1. Aerial photography. 2. Photogratlhic surveying. 3. GeologyMaps. (Series·) 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D.C. - Price '$2. (paper cover) 
CONTENTS 

Abstrnct __________________________________________ _ 

Introduction ______________________________________ _ 

Tho aerial photograph __________________________ _ 

l"nctors that affect tho photogmphic image _______ _ 

l"ocal length and flying height _______________ _ 

Film and filter combinations ________________ _ 

Lens anglo _________ - ______________________ _ 

Viewing of photographs _________________________ _ 

Interpretation ______________ - ______________________ _ 

Interpretation factors __________________________ _ 

Recognition elements _______________________ _ 

Photogmphic tone _____________________ _ Color ________________________________ _ 

Tex-ture ______________________________ _ 

Pattern _______________________________ _ 

llelation to associated features __________ _ Shape ________________________________ _ 

Size __________________________________ _ 

Combinations of recognition elements ____ _ 

Vertical exaggeration _______________________ _ 

Scale of photographs _______________________ _ 

Photogeology in geologic nutpping _______________ _ 

Kinds nnd 1.trnounts of information _______________ _ 

.Lithologic character of rocks ________________ _ 

Sedimentary rocks _____________________ _ 

Igneous rocks _________________________ _ 

Metttmorphic rocks ____________________ _ 

Structure _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . 

Flat-lying beds ________________________ ·_ 

Dipping beds __________________________ _ 

Folds _________________________________ _ 

Faults ________________________________ _ 

.Joints ________________ - __ - - ----- - - - -- --

Cleavage nnd foliation __________________ _ 

Unconformities ________________________ _ 

lntcrprettttion of aerial photographs in petroleum 

geologY----------------------~--------------- Folds _____________________________________ _ 

Bedding ______________________________ _ 

Drainage _____________________________ _ 

To!)ography ___________________________ _ Soils _________________________________ _ 

Fnults ____________________________________ _ 

.Facies ____________________________________ .:.. 

.Regiorml studies ___________________________ _ 

lnterprettttion of aerial photographs in search for ore deposits _________________________________ _ 

Structural guides __________________________ _ 

Lithologic guides __________________________ _ 

Physiographic guides _______________________ _ 

Botanical guides ___________________________ _ 

Page 

Interpretation-Continued 

1 Interpretation of aerial photographs in engineering 2 geology _____________________________________ _ 

2 Surficial materials _______________________ -- __ 

2 Ground conditions _____________________ _ 

4 Elements of soil pattern ____________ _ 

4 Landform _____________________ _ 

4 Drainage characteristics ________ _ 

5 Erosional characteristics ________ _ 

6 Photographic tone _______ - - _ - - __ 

6 Color _________________________ _ 

6 Vegetation ____________________ _ 

8 Land use _____________________ _ 

9 Permafrost ________________________ _ 

9 Distribution and type of vegeta- l! tion ________________________ _ 

12 Polygonal relief p~tterns ___ - _----

13 Pingos ___________ ----- _ -- __ -- _-

1:3 Features resulting from thawing __ 

13 Absence of permafrost ___ --------

14 Erosion, transportation, and deposition ___ _ 14 Landslides ________________________ _ 

15 Beach erosion _____________________ _ 

16 Bedrock __________________________________ _ 

1 6 Geologic structure and type of rock_-_----

17 Interpretation of aerial photographs in hydrologio 

18 studies---------------------------~----------

19 Locating potential ground-water sources_-_----

19 Water-loss studies _________________________ _ 

19 Other applications ______________________ - - _ -

19 Instrumentation ___________________________________ _ 

21 Kinds of instruments----------------------------

2~~ Measuring devices for use with paper prints ___ -

24 Plotting devices for use with paper prints __ ----

24 Measuring and plotting devices for use with 

paper prints ____________________ -_--------

24 Measuring and plotting devices for use with glass26 plate diapositives ________________________ _ 

26 Other instruments _______________________ -_-

26 Exaggerated-profile plotter ____ -- ... --------

27 Interval-measuring device _____ -------"---

29 Universal tracing table _________________ _ 

29 Measurement_ ______________________ -- __ -------

30 Principles of vertical measurement_-----------

3!) Determination of altitude differences by the 

parallax method ________ _, __ - - - - - - - - - - -

:30 Use of stereometer-type instruments_-

30 Use of double-projection instruments __ :n Tilt- ___________________________ - - -

31 Determination of altitude differences from a 

32 single photograph ______ ---------------

III 

Page 

33 

:33 

33 

34 

34 

:34 

35 

35 

36 

:36 

37 

37 

37 

:n 

38 

38 

38 

39 

39 

39 

39 

39 

40 

40 

40 

40 

41 

41 

41 

42 

45 

45• 

48 

48 

48 

. 48 

48 

48 

50 

51 

54 

55 

56 
,f 

IV CONTENTS 

Page Page 

Instrumentation-Continued 

Measurement-Continued 

Instrumentation-Continued 

Plotting ______________________________________ _ 66 

67 

67 

Geologic uses of parallax measurements _______ _ 

Determining strikes and dips ____________ _ 

Determining stratigraphic thickness ______ _ 

Isopach mapping __________________ _ 

Facies change _____________________ _ 

Structure contouring ______ -- _____ - _____ _ 

N otom-15 quadrangle _______________ _ 

Discovery anticline ________________ _ 

Determining displacements on faults _____ _ 

Determining stream gradients ___________ _ 

Direct determination of slope _______________ _ 

Dipping platen ________________________ _ 

Stereo slope comparator __________ - _____ _ 

Dip estimation ________________________ _ 

Other methods of determining angles of 

slopes ______________________________ _ 

Other methods for determining quantitative data_ 

56 

56 

57 

58 

59 

60 

60 

61 

61 

63 

63 

63 

63 

64 

64 

64 

Construction of control layout _______________ _ 

Orthographic positioning ____________________ _ 

Plotting on ~rthographic base maps or con- trol layouts _________________________ _ 

Plotting on orthophotographs ___________ _ 

Plotting on gridded base maps __________ _ 

Vertical positioning in cross sections __________ _ 

Interpretation, measuring, and plotting systems ___ _ 

Instrument capability ______________________ _ 

Relation between instrument capability, scale, 

and vertical measurements ________________ ~ 

Horizontal positioning ______________________ _ 

Source and identifying data of aerial photographs ______ _ 

References cited ___________________________________ _ 

Index ____________________________________________ _ 

ILLUSTRATIONS 

67 

69 

69 

69 

69 

72 

74 

74 

222 

224 

229 

Page 

FIGURE 1. Geometry of the vertical aerial photograph___________________________________________________________ 3 

2. Spectral reflectance curves of weathered samples of Bernal formation and San Andres limestone_____________ 8 

3. Drainage map of large area in vicinity of ring dike shown in figure 54 ________ ~___________________________ 10 

4. Generalized soil map of area shown in figme 48-------------------------------------------------------- 12 

5. Sketch showing long tributary streams on updip side of strike valley in area of low dips ___________________ .. 20 

6. Sketch showing long tributary streams on downdip side of strike .valley in area of steep dips________________ 20 

7. Drainage map of area shown in figure 74 and vicinity__________________________________________________ 22 

8. Drainage map of area shown on left half of figure 73 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 25 

9. Drainage map of area shown in figure 94 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 28 

10. Stereo meters ________________________________________________________________________ .______________ 43 

11. Stereo slope meter ________________________________________________________________________________ . 43 

12. Parallax ladder____________________________________________________________________________________ 43 

13. Sketchmaster_____________________________________________________________________________________ 44 

14. Radial planimetric plotter _____________________________________________________________ -_____________ 44 

15. Stereotope _________ ~------------------------------------------------------------------------------ 44 16. Multiplex _____________________________________________________________________________________ -____ 46 

17. ER-55 plotter ___________________________ .:. ______________ ·___________________________________________ 46 

18. Kelsh plotter______________________________________________________________________________________ 46 

19. Exaggerated-profile plotter (lever and fulcrum type) ________________________________________________ --.. 49 

20. Exaggerated-profile plotter (pantograph type) __________________________ .. ______________________________ 49 

21. Interval measuring instrument _____________________________________________________________________ - 49 

22. Universal tracing table ___________ .__________________________________________________________________ 49 

23. Diagram showing relation between absolute stereoscopic parallaxes and horizontal distances actually measured 

with stereometer-type instruments in determining differences in altitude from paper prints__________________ 50 

24. Diagram showing relation between absolute stereoscopic parallaxes and vertical distance measured with doubleprojection type instruments in determining differences in altitude from glass plates ___________ -_----------- 51 

25. Sketch showing correct orientation of photographs for stereoscopic viewing _________________________ -_----- 52 

26. Diagram of gently dipping beds showing relation of stratigraphic thickness to differential parallax determined 

at any two points along dip direction and at the formation contacts_------------------------------------ . 57 

27. Diagram of gently dipping beds showing relation of stratigraphic thickness to differential parallax determined 

by floating-line method __________________________________________________________________ -_------.. 58 

2R. Diagram of steeply dipping beds showing relation of stratigraphic thickness to dip angle and horizontal distance 

between top and bottom of bed ___________________________________________________ ~ _____ ----------- 59 

29. Diagram showing graphic reconstruction of marker bed along line of section ___________________ ------------- 61 

30. Diagram showing graphic construction in positioning structure-contour lines on top of marker bed and the 

projecting of these lines to their relative horizontal positions along the line of section ___________ ----------- 62 

31. Diagram showing relation of structure-contour positions along two lines of section at different altitudes_---- .. -- 62 

32. Dipping platen____________________________________________________________________________________ 63 

33. Stereo slope comparator _________________________________________________________ .. _____________ ----- 63 
CONTENTS v 

Page 

FtGUHl~ :34. Slopt·) conversion chart showing relation of exaggeration factor, exaggerated slope, and true slope_ .. ____________ 65 

:35. Diagram, based on tangent functions, showing relation of true dips to exaggerated dips seen in stereoscopic 

mode~----------------------------------------------------------------------------------------- 66 

:3(1. Photogmmmctric systems for geologists_------------------------------------------- _________________ .. 70 

37. Photogrammctric measuring and plotting systems for geologists showing relation of different instruments to 

vertical-measurmcn t and horizontal-po&itioning errors- ---------- - _- _______________ .. _________________ . 71 

:38. Diu,gmm showing datum-curvature relation to maximum expected error in measuring vertical intervals from 

leveled double-projection stereoscopic modeL-----------------------------------_-__________________ 73 

:39. Poorly resistant pyroclastic rocks overlain by resh:;tant capping formation (Colorado) (stereoscopic pair)______ 77 

40. Area underlain predominantly by gently folded sandstone, shale, conglomerate, and graywacke (northern 

Alaska) (stereoscopic pair)------------------------------------------------------------_- _________ .. 78 

4.1. Extrusive volcanic rocks and associated cinder cone (Utah) (stereoscopic pair)______________________________ 79 

42. Flat.-lying sandstone (southern Utah) (stereoscopic pair) _______________ --------------------------------- 80 

4:3. Sandstone and conglomerate beds intruded by diorite porphyry laccolith (Utah) (stereoscopic pair)__________ 81 

44. Gently dippii1g beds of sandstone, siltstone, and conglomerate that locally contain channel-till deposits (Arizona) 

(stereoscopic pair) ______ -------- --------------------------------------------------------- _- _ _ _ _ _ _ 82 

·l5. Gneiss, schist, and granite flanked by dipping sedimentary rocks truncated by pediment gravel (Colorado) 

(stereoscopic pair) ____ -------------------------------------------------------------------________ 8:3 

46. Landslide area (Colorado) (stereoscopic pair)_---------------------------------_-_-____________________ 84 

47. Orthoj)hotograph and perspective photograph of same area_--------_____________________________________ 85 

48. Area showing relation of residual soils to underlying rock types and structure (South Carolina) (stereoscopic 

pui~------------------------------------------------------------------------------------------- 86 

4!l. Shale und thin sandstone interbeds capped by poorly consolidated gravel and sand (Texas) (stereoscopic pair)__ 88 

50. Recently faulted strand lines of former lake (Nevada) (stereoscopic pair)__________________________________ 90 

51. Steeply dipping sedimentary rocks intruded by quartz diorite (California) (stereoscopic pair)________________ 92 

52. Area showing drainage characteristics in surficial materials (Wyoming) (stereoscopic pair)___________________ 94 

5:3. Area of basalt flows (Idaho) (stereoscopic pair)________________________________________________________ 96 

54. Ring-dike area (North Carolina) (stereoscopic pair)____________________________________________________ 98 

55. Area in Maine showing esker landform on photographs of two different scales (stereoscopic pairs)____________ 100 

5(). Stabilized sand dunes (central Alaska) (stereoscopic pair)_______________________________________________ 102 

57. Morainal deposits of two glaciations (central Alaska) (stereoscopic pair)__________________________________ 104 

58. Morainal deposits of continental glaciation (South Dakota) (stereoscopic pair)____________________________ 106 

59. Gently dipping sedimentary rocks lying unconformably on gneiss-granite-schist complex (Utah) (stereoscopic 

pair)------------------------------------------------------------------------------------------- 108 

60. Gently dipping sedimentary rocks cut by numerous ncar-vertical faults (Utah) (stereoscopic pair)____________ 110 

61. Flat-lying beds of sandstone, limestone, and shale (Texas) (stereoscopic pair)______________________________ 112 

62. Area of faulted gently dipping sedimentary rocks (Utah) (stereoscopic pair)_______________________________ 114 

6:~. Shale in semiarid climatic area (Utah) (stereoscopic pair)-_- __ --_- ___ -_-_-_-_- _______________________ --- 116 

(:i4. Shale and limestone in hamid climatic area (Virginia) (stereoscopic pair) _____ ~___________________________ 118 

()5. Area of gently dipping limestone and shale (Pennsylvania) (stereoscopic pair)_____________________________ 120 

(:i6. Area mantled by glacial deposits (South Dakota) (stereoscopic pair) ____ - _______________________________ - 122 

()7. Glaciated area in southeastern Alaska (stereoscopic pair)----------------------------------------------- 124 

68. Approximately flat-lying limestone and interbedded sandstone and marlstone beds covered locally by alluvium 

and windblown deposits (Texas) (stereoscopic pair)__________________________________________________ 126 

69. Area of basalt flows (Idaho) (stereoscopic pair)________________________________________________________ 128 

70. Area of granite (South Dakota) (stereoscopic pair) ____________________________________________________ - 1:30 

71. Area of granitic intrusive rocks (Wyoming) (stereoscopic pair)___________________________________________ 132 

72. Area showing vegetation differences in igneous and metamorphic terrain (southeastern Alaska) (stereoscopic 

pai~------------------------------------------------------------------------------------------- 1:34 

n. Area underlain by silicic volcanic tuffs and flows and bedded argillite or slate (North Carolina) (stereoscopic 

pai~------------------------------------------------------------------------------------------- 136 

74. Area of folded, faulted, and highly metamorphosed rocks (North Carolina) (stereoscopic pair)______________ 138-

75. Arca underlain by phyllite and slightly metamorphosed slaty rocks (Alabama) (stereoscopic pair) ______ -_--- 140 

7(). Heavily vegetated terrain underlain by metamorphic 'and extrusive volcanic rocks (southeastern Alaska) 

(stereoscopic pair)_______________________________________________________________________________ 142 

77. Flat-lying beds of limestone and shale (Kansas) (stereoscopic pair) _____________________________________ - 144 

78. Flat-lying beds of sandstone, limestone, and shale (Texas) (stereoscopic pair)_____________________________ 146 

79. Area of sparse outcrops underla.in largely by gently folded sandstone, conglomerate, and shale (northern Alaska) 

80. 

(stereoscopic pair)_______________________________________________________________________________ 148 

Gently to steeply clipping sedimentary rocks overlying schist-gneiss-granite complex (Wyoming) (stereoscopic 

triplet) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 150 

8L. Gently folded sedimentary rocks (central Utah) (stereoscopic pair)_______________________________________ 152 

82. Ring dike in New Hampshire (stereoscopic pair) ___ --------------"--------------------------------~---- 154 

8:3. Gently clipping sedimentary rocks offset by near-vertical fault (Utah) (stereoscopic pair) ______________ ----- 156 
VI CONTENTS 

Page 

FwuuE 84. Faulted, gently dipping sedimentary rocks (Utah) (stereoscopic pair)_____________________________________ 158 

85. Vegetated terrain underlain by igneous and metamorphic rocks locally covered by basalt flows (southeastern 

Alaska) (stereoscopic pair) _______________ -________________________________________________________ 160 

86. Area of crystalline rocks (Wyoming) (stereoscopic pair)_________________________________________________ 162 

87. Lava flow lying unconformably on gently dipping sedimentary rocks (New Mexico) (stereoscopic pair)____ 164 

88. Gently dipping sedimentary rocks overlain in part by river gravels (Utah) (stereoscopic pair)_______________ 166 

89. Gently folded sedimentary rocks in heavily vegetated area (northern Guatemala) (stereoscopic pair)_________ 168 

90. Plunging anticline in gently folded rocks (southern Utah) (stereoscopic pair)______________________________ 170 

91. Stratigraphic section across gently folded anticline (Utah) (stereoscopic pair)______________________________ 172 

92. Gently dipping sedimentary rocks, on east side of San Rafael Swell (Utah) (stereoscopic pair)______________ 174 

93. Uncontrolled mosaic of Umiat anticline area, northern Alaska, underlain by shale, sandstone, conglomerate, 

and graywacke-------------------------~-------------------------------------------------------- 176 

94. Subsurface structure reflected in surface drainage characteristics (Texas) (stereoscopic pair)________________ 178 

95. Same photographs as figure 94, but reversed to show pseudoscopic effect as an aid to interpretation (stereoscopic pai~------------------------------------------------------------------------------------- 180 

96. Uncontrolled mo.saic of Wainwright area, northwestern Alaska__________________________________________ 182 

97. Area underlain predominantly by gently dipping sandstone and shale (northern Alaska) (stereoscopic pair)___ 184 

98. Gently dipping sedimentary rocks cut by high-angle faults (Nevada) (stereoscopi·c pair)____________________ 186 

99. Complexly folded and faulted area of greenstone, schist, limestone and marble (southeastern Alaska) (stereoscopic pai~------------------------------------------------------------------------------------- 188 

100. Igneous and metamorphic tern-tin (southeastern Alaska) (stereoscopic pair)_______________________________ 190 

101. Gently tilted and faulted lava flows· (Oregon) (stereoscopic pair)________________________________________ 192 

102. Area of pegmatite dikes (South Dakota) '(stereoscopic pair)_____________________________________________ 194 

103. Metamorphosed sedimentary rocks intruded by quartz monzonite (California) (stereoscopic pair)~----------- 196 

104. Distinctive landform of· gravel terraces along a major stream (northeastern Utah) (stereoscopic pair)________ 198 

105. Areas showing drainage characteristics in surficial material that is predominantly loess (stereoscopic pairs)__ 200 

I 06. Arett of surficial deposits, primarily windblown (central Nebraska) (stereoscopic pair)______________________ 202 

107. Mottled soils of drift plain (Iowa) (stereoscopic pair)__________________________________________________ 204 

108. Coastal plain underlain by clay, sand, and gravel (New .Jersey) (stereoscopic pair)_________________________ 206 

109. Polygonally patterned ground in permafrost area along major stream (northern Alaska) (stereoscopic pair)___ 208 

110. Well-developed pingo in area of permafrost (northern Alaska) (stereoscopic pair)__________________________ 210 

Ill. Area of permafrost (northern Alaska) (stereoscopic pair)_______________________________________________ 212 

1 12. Conspicuous beaded drainage in area of permafrost (northern Alaska) (stereoscopic pair)___________________ 214 

u:3. Landslide area in volcanic and sedimentary terrain (New Mexico) (stereoscopic pair)_______________________ 216 

114. Damsite area (southeastern Alaska) (stereoscopic pair)_________________________________________________ 218 

115. Gridded photograph_______________________________________________________________________________ 220 

11~ Gridded base map_________________________________________________________________________________ 221 
AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

By RICHARD G. RAY 

ABSTRACT 

Aerial photographs today are widely used to obtain both 

qunlitntive nncl quantitntive geologic information; vertical 

aerial photographs nre ·used almost to the exclusion of other 

types. Techniques and procedures 'described herein relate primarily to vertical photogrnphy. 

Geologic interpretntion of aerial photographs is based on the 

fundnmental recognition elements of photographic tone, color, 

texture, pnttem, relntion of associated features, shnpe, and 

size. The scale of photogrnphs, as well as the vertical exaggeration that is present in most stereoscopic models, also are 

significant in photointerpretation. 

The amount of geologic information that may be obtained 

from aerial photogrnphs is primarily dependent on the type 

of terrain (igneous, metamorphic, or sedimentary), climatic 

environment, and stage of the geomorphic cycle. Because 

fentures are more readily recognized where strong differences 

exist in the erosional resistance of adjacent rocks, sediment:nr~· terrain may be expected to yield the greatest amount 

o.f information from aerial photographs. Metamorphic tert•nin may yield the least information because metamorphic 

processes tend to destroy differences that may have existed 

in the unmetnmorphosed rocks. Combinations of criteria such 

ns photographic tone, texture, pattern, and vertical exaggeration penult inferences as to rock type and geologic structure, 

which are important in petroleum exploration, ore-deposits 

search, and engineering geology. 

ln pett·oleum explorntion aerial photographs provide a wealth 

of information primarily with regard to potential structural 

traps: I~olcls commonly may be interpreted from a study of 

stt·ike and dip of bedding and from stream patterns; anomalous stream characteristics, such as stream deflections, may 

suggest: subsurfnce structures. The variety of photorecognit:lon cl'iterin that suggests faults permits aerial photographs 

to he o.f particular use in many ore-deposits studies. Analysis of soil patterns yields information regarding permeability 

of the surficial materials that are a concern of the engineer-

! ng geologist. 

~t;he instruments used for viewing photographs, measuring 

geologic features, and compiling geologic maps range from 

si tuple stet·eoscopes and stereometers, or measuring bars, to 

eomplex tlouble-pro:Jection instruments such as the multiplex 

or Kelsh plotter. Some instruments require the use of papet· prints; others require the use of glass-plate diapositives. 

Measuring devices primarily provide spot heights or differences In altitudes; these quantitative data may be geologically significant in measurement of stratigraphic thickness 

and dip of beds and in structure contouring. Accuracy of 

vet·tical measurements is related fundamentally to the scale 

of photogt·nphy and the instruments used for making measurements. In general, accuracy of measurement is greater 

when large-scale photographs are used and when doubleprojection instruments rather than simple parallax bars are 

employed. 

INTRODUCTION 

Use of aerial photographs to obtain geologic information-popularly called photogeology-has contributed to mineral and fuel discoveries, and to engineering geology studies, as well as to the general geologic 

mapping of many areas; it also has increased the 

efficiency of many geologic mapping groups by adding 

speed, economy, and accuracy to areal mapping as well 

as adding certain geologic information that is impossible, difficult, or economically impractical to obtain by 

routine field-mapping methods. The past financial 

success of consultants in geologic interpretation of 

aerial photographs attests to successful geologic application of photogeologic procedures and techniques, 

particularly in petroleum geolobry, and to lesser extent 

in mining and engineering geology. 

~1a.ny technical papers on the geologic interpretatioH 

of aerial photographs have been published, primarily 

since the end of "T orld 'Var II; on several occasions 

symposia on photointerpretation, and specifically photogeology, have been presented.· ~1uch of the information currently available has been published as individual articles and they are scattered throughout the 

literature; comprehensive treatments on the geologic 

interpretation of aerial photographs are generally llllavailable now (1959) (see Eardley, A .• J., 1942; Smith, 

H. T. lL 194:3b; and Petrusevich, ~1. N., 1954). In 

addition to the published informatio11 much unpublished information also has accumulated in recent· 

years. The collection and synthesis of much of this 

information, together with discussio11 of photointerpretive and related photogrammetric procedures, are 

the main objectives of this paper. The paper is particularly intended as a guide to geologie interpretation 

of aerial photographs as applied as a tool in recmlnaissance geologic mapping, and 1'\S a refere11ce to published papers representative of the photogeologic 

literature. 

1 
( 

2 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

The primary objectives of photogeology are (a) to 

contribute to geologic mapping, which in turn is basic 

to mineral and fuel exploration, engineering geology 

sttidies, some water-resources investigations, and related studies, and (b) to contribute to geologic knowledge through r~search. The objectives of photogeology 

may be economic or acaden1ic, and procedures used 

can be expected to differ widely, ranging, for example, 

from use solely of simple lens stereoscopes for qualitative interpretation to employment of precision stereoplotters for making geologic measurements. Regardless of procedures used, however, photogeology to date 

has contributed principally to the broad aspects of 

geologic study, that is, in ma.pping distribution of rock 

types and structures. The interpreter can only infer 

the composition of rock types from photographs; he 

cannot identify mineral types or absolute ages of 

rocks. Thus, maximum use of photogeologic procedures is attained by combining photogeologic studies 

with various laboratory studies and with field investigations. 

The uses of aerial photographs and the various photogeologic procedures are discussed herein under the 

general headings of interpretation and instrumentation. The part dealing with interpretation is widely 

illustrated with stereoscopic pairs and triplets, single 

photographs, and 1nosaics, together with explanatory 

notes, to demonstrate the kinds and amounts of geologic information that can be obtained from aerial 

photographs of differing geologic terrains and to illustrate some uses of photographs in specific studies, such 

as the search for petroleum. These photogn~phs demonstrate primarily the qualitative uses of the fundamental recognition elements in geologic interpretation; 

they represent only a sampling of the tremendous 

number of illustrations available. Because reproduction processes tend to obscure details only subtly expressed on aerial photographs, selection of photographs 

used in this paper was confined principally to those 

that illustrate clearly defined recognition elements and 

geologic features. All aerial photographs (figs. 39-

114) are at the end of the paper. Agencies holding 

the negatives of these photographs are given in a list 

that follows the photographs. 

l\1ethods and significance of determining quantitative geologic information from aerial photographs are 

described under the heading of instrumentation. 

THE AERIAL PHOTOGRAPH 

The aerial photograph is an instantaneous record 

filters used. It may also be defined as a composite of 

photographic images, which make up the recognition 

elements used for interpretation. The aerial photograph is a perspective photognt-ph that is geometrically related to the type of camera in which it is 

taken; it may be a vertical photograph, taken with 

the camera axis pointing essentially vertically down, 

or it may be an oblique photograph, taken with the 

camera axis purposely tilted from vertical, generally 

20° or more. Vertical photographs currently are used 

almost to the exclusion of oblique photographs for 

geologic interpretation, and most photogrammetric 

instruments used in photogeology in the United States 

are designed to accommodate vertical photographs; 

hence the following discussion is limited to techniques 

applicable to the study of vertical photography. Twin 

low-oblique photographs-those in which the apparent 

horizon is not shown-generally may be transformed 

and the resulting paper prints used as vertical photographs; however, low-oblique photographs cannot be 

used in many stereoplotting instruments unless the 

instruments are specially designed. 

A general knowledge of the geometry and terminology used with vertical aerial photographs is necessary 

if one is to understand and make maximum use of 

photographs for interpretation and mapping purposes. 

The terminology and geometry of the vertical photograph are presented in figure 1. 

FACTORS THAT AFFECT THE PHOTOGRAPIDC IMAGE 

Factors that affect the photographic image, and 

hence interpretation, may be divided into two groups: 

(a) The relatively constant man-controlled factors, 

such as focal length of lens, flying height, film and 

filter combinations, and lens n.ngle; and (b) the variable natural factors including color of objects photographed, position of an object with respect to the 

angle of sun, amount of haze in the atmosphere, and 

others.· The constant factors are discussed briefly below; effects of natural factors on the photographic 

image are described where appropriate throughout the 

text. 

:FOCAL LENGTH AND FLYING HEIGHT 

Focal length and flying height may be considered 

together because of the relation of photograph scalr 

to these two factors. In terms of focal length and flying 

height, the average scale of a photograph is expressed 

as 

of the ground details as determined chiefly by the or 

focal length of the camera lens, the flying height of the 

airplane at the time of exposure, and the film and 

Scale- focal length (feet) 

flying height (feet) .. 
Approximate photobase 

~Plane of the negative 

Focal length of camera lens 

Ground nadir, photograph 1 Ground nadir, photograph 2 

FIGURE 1.-Geometry of the vertical aerial photograph. 

z 

~ 

~ 

0 

t:l 

0 

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~ 1-4 

0 

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(J,j 

... 
./ 

/ 

4 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

Thus, for example, the scale of photographs taken at 

10,000 feet above mean terrain with a 6-inch (0.5 foot) 

focal~length lens is 

10°0~0 = 1: 20,000. , 

It is evident that as focal length increases, the scale of 

photographs becomes larger, but as flying height increases, scale of photographs becomes smaller~ Therefore, at any given flying height, long-focal-length lenses 

result in larger ·scale photographs than short-focallength lenses. It follows also that at a given flying 

height a greater number of photographs are necessary 

to cover a .given ground area when long-focal-length 

lenses are used. Doubling the focal length of the lens, 

for example, will quadruple the number of photographs 

required to .cover a given area at a given flying height. 

It should be noted that if a photogra:ph is enlarged 

or reduced, the "effective" focal length for that photograph is also changed in direct proportion to the 

amount of enlargement or. reduction. Thus, if photographs taken with a 6-inch-focal-length lens are enlarged two times, the effective focal length is changed 

from 6 inches to 12 inches. Parallax measurements 

(see p. 53), used in determining differences in altitude, are increased, but the absolute value for each 

unit of parallax measurement is decreased. Reducing 

or enlarging photographs thus has a significant effect 

with respect to choosing appropriate measuring and 

plotting instruments, and in measuring differences in 

altitudes. 

For determining scale from tl~e formula S = f I II 

the effective focal length must be used where photographs have been reduced or enlarged. Particular 

note should be made in the use of transformed lowobliqtle photographs, in which the effective focal length 

is almost always less titan the focal length of the 

camera. 

FILM AND FILTER COMBINATIONS. 

The appearance of the photographic image may be 

controlled to marked extent by the sensitivity of the 

film and by the light transmission of the filter employed. The sensitivity of a film emulsion may be 

controlled when the film is manufactured so that all 

or only selected parts of the visible spectrum are recorded, or so that part of the invisible spectrum, such 

as infrared light,. is recorded. Also, the wave length 

of light reflected from an object and actually recorded 

may be controlled in part by filter combinations. The 

recording of selected wave lengths of light, controlled 

by filters or type of film, or by combinations of film 

and filters, affects the photographic image, primarily 

by affecting the photographic tone. Conventional panchromatic aerial film is generally exposed through a 

minus-blue filter and permits th~ recording of bluegreen, yellow, orange, and red light. When color film 

is used the entire visible spectrum is commonly recorded and a greater number of color differences can 

be distinguished than on conventional black-and-white 

film. Special color film or color film used with filters 

may distort the color of the objects photographed, but 

if differentiation of geologic features is permitted, 

then lack of color fidelity may not be detrimental to 

interpretation. Idet'tlly, film and filter combinations 

may be used to accentuate speeific features for a given 

type of interpretation (see p. 7-8) . 

LENS ANGLE 

The angle of the camera lens-that is, the apex 

angle of the cone of rays passing through the lensis important indirectly in relation to radial displacement and parallax measurements, which are in turn· 

significant with regard to photogrammetric applications in geologic interpretation. Characteristically, 

long-focal-length lenses (greater than 6 inches) have 

a narrower lens angle than short-focal-length l~nses 

( 6 inches or less). Thus, to maintain a given scale 

and format size, photographs taken with a narrowangle (long-focal-length) lens requires flying at a 

higher altitude than with a wide-angle (short-focallength) lens. Under these conditions, radial d1splacement (also termed "relief displacement") of similar 

image points is less when narrow-angle lenses are used, 

but only because of the greater flying height required 

by the narrow-angle lens to maintain the given scale 

and format size. The ·parallax difference for ·an object 

of specific height is in turn affected as a result of the 

difference in flying height, the paralla.x difference decreasing directly with increased flying height for a 

given focal-length lens (see p. 53). Thus it can be 

said that lens angle indirectly affects parallax meas .. 

urement. 'Vhen a constant flying height is maintained, however, lens angle does not affect parallax 

measurements of similar image points. Image distortion at the margins of photographs taken with a wideangle lens may be greater than on photographs· taken 

with a narrow-angle lens and this is considered by 

many to be detrimental to photointerpretation, especially in high-relief terrain. 

VIEWING OF PHOTOGRAPHS 

Photographs may be viewed singly, as mosaics, or 

as stereoscopic pairs. l\1ost commonly stereoscopic 

pairs of paper prints are viewed in reflected light with 

simple lens, prism, or mirror stereoscopes, notes are 

made directly on the prints or on transpa.rent overlays, 

and information is later transferred to a base map or 
INTERPRETATION 5 

mosa.ic. Many paper prints can also be viewed with 

transmitted light, which may be helpful in revealing 

photogm,phic detail. If vertical measurements are 

made from paper prints a separate measuring device 

must be commonly employed with the stereoscope. In 

recent years .double-projection instruments, employing 

the anaglyph principle in forming the stereoscopic 

model from glass-plate diapositives, have been used 

increasingly in geologic interpretation. 

"'hen photogra,phs are viewed singly or in a mosaic, a two-dimension:tl view is obtained. The single 

photograph or mosaic is normally viewed along a line 

genernJly perpendicular to the photograph surface, 

but for some subtle linear features, an oblique view of 

the surface with a reflected light source moved to different positions ma,y reveal photographic details otherwise difficult or impossible to see (see La.ttman, 1958, 

p. 575). :.Many valuable relations of geologic and associated features can be observed, particularly as a, result 

o:f the overall view permitted by mosaics or single 

prints of small scale, but a two-dimensional view reveals only p:trt o:f the data available to the interpreter; 

the value of three-dimensional, or stereoscopic, examination of aerial photographs in contrast to studying 

the two-dimensional view cannot be overemphasized. 

Conspicuous geologic features are commonly visible 

on single aerial photographs or mosaics of aerial photographs, but the wealth of information seen in stereoscopic view is ·many times greater. Details, such as 

fine lines or textural differences not readily seen on 

single photographs-or even on the ground-commonly. 

are shown clear.ly .in the stereoscopic modeL Such 

clarity is in many places a direct result of the common association of fine lines and textures with relief 

changes, which are exaggerated in most stereoscopic 

models. To take advantage of the exaggerated vertical 

dimension, which is so helpful in photointerpretation, 

aeria,l photographs should of course be viewed stereoscopica;lly. It may be desirable also to view the photographs pseudoscopically by reversing the print positions so that hills appear as v.alleys and vice versa. In 

pseudoscopic view, stream patterns or anomalies may 

stand out (see fig. 95), or because of the unnatural 

view ot·her features may be accentuated. Aschenbrenner ( 1954, p. 401) demonstrated by the use of a seemingly random distribution of clots, which may be 

likened to silver grains of a photographic emulsion, 

that information not visible on a single picture can 

be clearly seen in stereoscopic view. 

INTERPRETATION 

Interpretat-ion was defined broadly by Summerson 

(1!>54, p. ~U7) as the prediction of what cannot actun.lly be seen; and t-hus geologic interpretation of aerial 

photographs is not unlike certain geologic interpretations of field-observed data. Colwell (1952, p. 535; 

1954, p. 43:3) defined interpretation as "* * * the act 

of examining photographic images of objects for the 

purpose of identifying the objects and deducing their 

significance." However, geologic interpretation, more 

often than not, is a result of combined deductive and 

inductive reasoning, based on the principle of cause 

and effect. For example, certain features easily identified on aerial photographs, such as termimiJ moraines 

and kettle holes, are inductively concluded to have resulted from glaciation, and this in turn may lead by 

deductive reasoning to the specific identification of less 

readily recognizable features, such as certain kame 

terraces. Further deduction may lead to conclusions 

concerning the type of material present in these terraces. On the other hand, interpretation may be 

directed mainly toward understanding the geologic 

significance or history of a broad area, and specific 

features such as positions of beds and measurements 

of stratigraphic thicknesses combine by inductive reasoning to reveal the probable geologic conditions that 

resulted in those features. 

Geologic interpretation of aerial photographs can 

be considered generally a two-step process. The first 

step includes observation, fact-gathering, measurement", 

and identification of features on the photographs. The 

second step .involves deductive or inductive mental 

processing of these data in terms of geologic significance. According to Stone (1951, p. 755) a procedural 

or methodical approach to interpretation should be 

used because "The establishment of procedure prepares 

the way for orderly and-complete analyses of complex 

subjects." A procedural or methodical approach _appears to be especially applicable to the observational 

phase of a photogeologic study, and it will give a· firm 

basis on which the interpretive phase is dependent. 

Observational data, also termed "first-order" information (see Hopkins, !Carlstrom, and others, 1955, 

p. 142), may be subjected to either the empirical or 

rational method of processing (Smith, H. T. U., 195:3, 

p. 9-10).. According to Smith (195:3, p. 9) the empirical process is largely mechanical rather than perceptive, and involves matching images of a photograph 

with simiht.r reference images; reasoning is largely by 

analogy and this "Reasoning by analogy alone is notoriously deceptive, unless extreme care is taken to 

make certain that the analogy is truly complete." 

Smith ( 195:3, p. 9) also made the particularly significant statement that "Nature is not always so obliging 

as to r>rovide the desired simplicity and uniformity of 

conditions" that would permit the ready <ipplication 

of empirical met-hods. Indeed, much in nature is com-
6 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

plex and nonuniform, many terrain features are polygenetic, and thus interpreting the geologic signific,ance 

of observational data is dependent largely upon a 

thorough understanding of the principles of the sciences that explain the observational data. "The physical conditions of a particular terrain change, and the 

adjustment to that change is a gradual one, allowing 

all manner of intermediate forms and combinations 

between the norm for the old situation and that for 

the new" (Summerson, 1954, p. 396). 

Surface expressions of geologic features-the observation.al data-range from the very obvious to the very 

subtle. Sorting those data that are significant to a 

particular problem and properly relating these features one to another provide a measure of the ability 

of the individual interpreter. "'\Vhere more than one 

plausib~e explanation of observational data is possible, 

the experience as well as the fundamental geologic 

ability of· the interpreter become especially significant. 

Thus a rational processing of observational data will 

yield the most meaningful interpretive results. This 

does not imply that an· empirical method of processing 

observational data,' exemplified by the numerous photointerpretation "keys," may not yield satisfactory 

results in certain geologic problems and terrains, but 

the logical, scientific approach demands a rational 

processing of observational data, particularly where 

inferences based on subjective interpretations are required. 

If observational data cannot be rationally interpreted, they may still be of some empirical value. For 

example, an interpreter ·may recognize two zones of 

vegetation on aerial photographs. Without knowledge 

of plant ecology, he may infer that a geologic contact 

exists between the two vegetation types. But a kno,vledge of plant ecology, permitting a rational interpretation; might well have allowed a further inference 

that one type of rock or soil was likely to be highly 

permeable in contrast with the other, thus revealing 

information with regard to certa1n physical characteristics of the rock or soil, which in turn may suggest 

something of the lithologic character. Thus "* * * the 

understanding of any particular terraih requires more 

than the knowledge· of the geology * * *'' (Summerson, 1954, p. 397). The greatest understanding will be 

by individuals grounded in the natural sciences and 

who understand, as a result, the various factors that 

contribute to a particular region. Because any geologic terrain is normally a complex that also includes 

features of fundamental interest to the botanist, forester, soil scientist, and others, photogeologic interpretation must take cognizance of these related sciences. 

The combined use of vegetation, soils, and geologic 

features to interpret geology is an example of what 

has been termed the "convergence of evidence" principle (Colwell, 1952, p. 566). 

Interpretation, then, begins .with the observation, 

identification, and measurement of features on photographs. One of the most obvious features is that of 

landform, and it has been stated by Johnstone ( 1953·, 

p. 265) that analysis of landforms constitutes the basis 

of photogeology. Smit Sibinga (1948) also pointed 

out the importance of landform, or geomorphic expression, in interpreting the lithologic character of 

rocks; but analysis of landform, however important, 

cannot be the complete basis for photogeologic interpretation, for there is additional "first-order" information such as vegetation patterns, soil patterns, and 

stream patterns; photogramnietric measurements also 

provide first-order data. Features· other than landform may actually be more important for certain structural interpretations. For example, vegetation may 

grow along and mark the location of a fault. Thus, 

interpretation is based not only on landform but also 

on photographic tone differences, color differences, 

drainage patterns, erosion patterns, soil patterns, vegetation patterns, and any other surface expression of 

underlying geology. It is fundamental that photointerpretation is only applicable to those geologic features 

that do develop such surface expressions. 

The many features of surface expression used in 

photogeologic interpretation are identified on the basis of recognition elements-characteristics of the photographs that result from the scale selected; the color 

of the rocks, vegetation, and soils of the terrain photographed; the kind of film and filters used; the 

processing of the film; and related factors. The most 

significant recognition elements are relative photographic tone, color,· texture, pattern, and association 

of features. Shape is important in identifying many 

constructional landforms. Finite size as a recognition 

element for geologic interpretation has been used very 

little, although relative size is commonly considered 

by the interpreter. The increasing use of photogrammetry in geologic interpretation, however, portends an 

increasing importance for size, not so much as a recognition element in the usual sense of the word, but as 

an interpretation element (see p. 13). Shadows may 

help in distinguishing certain tree types, which in turn 

may have geologic significance. 

INTERPRETATION FACTORS 

RECOGNITION ELEMENTS 

:PHOTOGRAPHIC TONE 

Photographic tone is a measure of the relative 

amount of light reflected by an object and actually 
INTERPRETATION FACTORS 7 

recorded on a black-and-white photograph; it is fundamental to all other recognition elements, except color. 

Tones on conventional photographs are usually shades 

of gray, but may be. black or white. Because of the 

ability o:f the human eye to differentiate subtle tone 

cha.nges, rela.ti ve photographic tone is tt significant 

asset in geologic interpretation of aerial photographs. 

:However, photographic tone is subject to wide variation and there are limits to its usefulness as a recognition element because of the many factors that can 

.influence it. 

Photographic tone depends on light reflectivity, 

which .in turn depends on location of an object with 

respect to the sun; thus the time of day and month 

of yea.r are influencing factors. l-Iaze strongly scatters 

light, pn.rticulu.rly at the short or blue end of the 

spectrum and hence affects photographic tone unless 

corrective filters are used. Geographic latitude, angle 

of reflected light, sensitivity of film, light transmission 

of filters, a.nd processing also may exert considerable 

effect on photographic tone. l-Ienee a standardized 

tone scale (Daehn, 1949, p. 287) would appear to have 

little usefulness unless relative photographic tones on 

different photographs could be equated in terms of the 

tone scale. It is probable that the use of a densitorneter to measure relative photographic tone in terms 

of the density of the negative would be a satisfactory 

rneasure of tone, even though many factors influence 

the density. Particularly in quantitative studies the 

use o:f. a densit:.o'met:er to measure relative photographic 

tone nmy prove very worthwhile in differentiating 

certain geologic features (Ray and Fischer, 1960). 

Photographic tone, despite the many factors that 

may rdfect it, can be a particularly useful interpretation element; .its usefulness depends on· the problem 

under consideration and on how tone is used in conjunction with other recognition elements. For example, if the fracture pattern of an area were studied 

and it were known that a lush and dark-toned vegetation grew along the fractures because of concentration 

o:f. ntoisture, the relative photographic tone probably 

would be a significant recognition element regardless 

of the limiting factors mentioned above. On the other 

hand, as a Jneans of distinguishing between two types 

of trees that reftect similar amounts of light but which 

•night signify dift'erences in the underlying geology, 

photogr:tphic tone might be of little value unless used 

.in conjunction with other recognition element's, such 

as crown texture. 

Raup and Denny ( 1950, p. 120), .in work along the 

Alaska. IIighway, considered photographic tone the 

least satisfactory criterion in .identi.fying and interpret·ing vegetation. But Tn.t'or (1951, p. 717) considered tonal contrasts to be major keys for recognizing 

landscape features of low relief in the coastal region 

of the Gulf States, such tonal differ~nces resulting 

mainly from soil and vegetation differences. Schulte 

( 1951, p. 697) pointed out, with regard to pure stands 

of trees, that at large scales photographic detail is 

more impor.tant, b'l1t at small scales tone becomes relatively more important. 

~ecause the analysis of photographic· tone is highly 

subjective it has been suggested that photogrttphic 

tone might not be as useful as other recognition elements in interpretation. Because of differences that 

may arise from position, angle of reflected light, proc-· 

essing, and other factors, Stone ( 1956, p. 125) stated 

that photographic tone in interpretation probably has 

been overemphasized. This is true if the factors that 

affect tone prevent photographi~ distinction of images 

that might otherwise be made. But if changes in relative photographic tone are present on a photograph, 

the interpreter should expect that these changes might. 

be geologically significant. Indeed, photographic tone 

is probably the most used recognition and interpretation element, although it is generally used in conjunction with one or more other recognition elements. The 

absence of photographic-tone changes, as a restl.lt of 

processing and other factors,· where such tone changes 

might normally be expected, does not detract from the 

possible usefulness of tone changes on other parts of 

a photograph. It merely suggests that the potential 

of photographic tone differences has not been accomplished for the photographs in question. · 

It is worthy of note that in interpretation strmi.g 

tonal GOntrasts are generally desirable. Yet modern 

automatic dodging devices commonly used in printing 

photographs generally reduce overall tonal c~:mtrasts, 

although .locally, as in. darkly shaded areas or brightly 

lighted areas, automatic dodging may strengthen tonal 

contrasts. Because most aerial negatives are slightly 

lighter in the corners than in the center, hand dodging 

is commonly necessary if automatic dodging is not employed in making prints. However, undodged or 

hand-dodged prints are probably the best for general 

photo interpretation. 

For special interpretation probleml' it may be possible to enhance tonal contrasts within a limited range 

of the tone scale by film intensifiers or other photographic processes, such as use of selected filters. Ray 

and Fischer (1960) used selected filters in photographing weathered samples of the Bernal and San Andres 

formations of New ~1exico. The Bernal formation is 

a red thin-bedded shale and shaly siltstone; the San 

Andres limestone is primarily a grayish limestone. 

Spectral reflectance curves, based on spectrophotome-
8 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

ter studies, show significant differences for these two 

formations (fig. 2). Particularly at the short or blue 

end of the spectrum a strong difference in intensity of 

reflected light is present; the San Andres limestone 

reflects almost twice as much light as the Bernal formation. A noticeable difference is also present at the 

long or red end of the spectrum. But the spectral 

reflectance curves of the Bernal and San Andres formations within 500 to 675 millimicrons-the wave 

lengths generally recorded on conventional black-andwhite aerial photographs-show that the overall light 

reflectance from these two formations would average 

out to be generally similar and little photographic 

distinction should be expected within this range. It 

is significant that strong reflectance differences ·which 

exist in one part of the spectrum may be masked by 

reflectance characteristics in another part of the spec1-

::c 

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20 

---- Bernal formation 

• • • • • • San Andres limestone 

.... ······ 

400 450 500 550 600 650 700 

WAVE LENGTH, IN MILLIMICRONS 

l!~IGURE 2.-Spectral reflectance curves of weathered samples 

of Bernal formation (red thin-bedded shale and shaly siltstone) and San ·Andres limestone (primarily a grayish limestone), New·Mexico. 

trum, and therefore to achieve photographic distinction of two features, it would probably be necessary 

to record a restricted part of the spectrum. Photographs of the Bernal and San Andres formations 

taken through filters that permit recording only of reflected light at the short or long end of the spectrum 

show a marked photographic distinction of the two 

formations. 

COLOR 

Color as a recognition element could be one of the 

most useful, if not the most useful, criterion for interpretive purposes (see Kent, 1957; Ray, 1958; Fischer, 

1958; and ~iinard, 1960). The significance of differences within a photograph is interpretation and the 

greater the number of differences, such as color contr·asts, the greater is the potential for detailed interpretations. It is a well-established fact that the human 

eye can differentiate about 1,000 times as many tints 

and shades of color as it can tints and shades of gray, 

characteristic of black-and-white photographs, and 

hence it may be concluded that color aerial photographs will permit a greater amount of detail to be 

recognized and interpreted. However, it is possible 

that two rock types of similar color might be differentiated more easily on certain black-and-white photogrttphs than on color photographs because of mineralogic, chemical, or other differences that may permit 

significant contrasts in reflected light to be recorded 

when selected films and filters are used. 

Petrusevich and J(azik ( 1955, p. · 5, 8) reported that 

where color contrasts of rocks are not strong, boundaries between these rocks are more clearly seen on 

black-and-white photographs than on color photographs. Where color contrasts are strong, interpretation is facilitated in areas where individual beds are 

rather thick and the color of formations is retained 

for considerable distances along the strike. Petrusevich and Kazik (1955, p. 8) also reported that "A comparison of aerial color photographs of scales 1:15,000; 

1 :10,000; 1 :8,000; and 1 :5,000 for the same area, taken 

under. the same conditions, shows that * * * the practical use of aerial color photography for geological 

purposes should· be limited to a scale of 1:10,000 and 

larger." This conclusion was based on a study of photographs taken with an 8.25-inch lens. Recently developed short-focal-length lenses ( 3.5 inches and 6 

inches) no'v permit useful color aerial photography to 

be achieved at a scale smaller than 1:10,000. 

Aerial color negative film may prove particularly 

useful in photointerpretive studies. Unlike positive 

transparencies (reversible-type color film), which require use of filters during exposure of the film, negative-typ~ color film is processed with filters inserted 

during the printing process. It is thus possible to 

vary the color contrasts through the simple medium 

of printing, with the possible advantages that certain 

features of geologic significance, such as alteration 

zones, may be emphasized. Petrusevich and J(azik 

( 1955, p. 5) noted that a greater contrast often can be 

obtained in a black-and-white print from a color negative than in a print from the corres.ponding black-andwhite negative. Fischer (1958, p. 546) in turn stated 

that black-and-white positive prints from Ektachrome 

aero positive transparencies also show stronger tone 
INTERPRETATION FACTORS 9 

contrasts between various rock units than the conventional black-and-white photographs. 

TEXTURE 

Texture was defined by Colwell (1952, p. 5=38) as 

"* * * the frequency of tone change within the image 

* * * [and] * * * is produced by an aggregate of unit 

features too small to be clearly discerned individually 

on the photograph." The scale of the photographs 

thus luts an important bearing on this definition of 

texture. For example, a network of fine lines described 

as a texture on one scale of photographs may well be 

recognizable ns a network of joints on ~t larger scale 

of photogr:tphs; this bears out Schulte's statement 

(1951, p. 697), made with regard to plant distribution, 

that nt large scales photographic detail is more important, whereas at small scales tone becomes relatively 

rnore important. Such a texture, due to tone change 

within the image, may be called a "photographic texture" (figs. 43, ·56, and 85). Photographic texture is 

therefore a comparn.ti ve feature within any one general sc:de of photography. Although photographic 

tone is a fundamental element of photographic tex-. 

ture, the photographic conditions that affect tone may 

vary and yet permit such a. texture to be a diagnostic 

recognition element. This is due to the fact that texture in the photographic sense is really a composite 

of several fundmnentaJ characteristics, namely photogrnphi'c tone, shape, size, and pattern, and, of these 

elements, slight variations of tone would least significant'ly affect the recognition of a given texture. 

Photographic textures have been described by various workers (Spurr, 1948; Raup and Denny, 1950) as 

coarse, .fine, rough, and fluffy, but such descriptive 

terminology would seem to have very limited use unless accompanied by photographic illustrations. Furthermore, unless these descriptive terms are themselves 

clearly defined, comparisons of photographic textures 

with specific commonly known objects might be more 

usefuL It may also be suggested that because photographic textures are normally considered as two dimensional, the use of a densitometer in conjunction with 

a coordin:ttograph-type instrument to determine tone 

changes per millimeter within the ima(J'e mi(J'ht permit 0 0 

mathematical classification of such textures. However, 

S\lCh a classification might fail where a stron(J' textural 

f b. . 0 

.a nc pre~ra1led, for two textures ditrering primarily 

because of shape or preferred orientations within the 

image might well be assigned to the same mathematical 

classification. 

In addition to the use of the term "texture:' as described above, texture has other meanings, as in reference .to soils. r~he ~1.pplicntion of aerial photographs 

to smls and engmeermg geology studies has resulted in 

t~1e introduction of "soil texture" in some photogeologic 

hterat.ure; the obvious distinction between photographic texture, which can be directly observed, and 

soil texture, which must be interpreted from other 

recognition elements, must of course be made. Textur~ also has been used in relation to the density of a 

drauutge network; wide spacing of streams results in 

a "coarse" texture of drainage and close spacing of 

streams results in a "fine" texture of drainage (figs. 

3, 42, 49, 51, 62, 63, 79, and 106). Unless quantitative meaning is given to drainage texture, the term 

must be used on a comparative basis within any one 

general scale of photography. Horton (1945, p. 283-

284) defined drainage density in quantitative terms 

but little use has been made of this classification by 

geologists. 

Topographic texture has been used to describe the 

degree of dissection of the land surface (see Smith 

K. G., 1950, p. 655-668). Like texture of drainage: 

topographic texture may be described as fine or coarse 

(figs. 39, 51, 62, 75, and 106). Quantitative study of 

topographic texture, based on map study, has shown 

that the texture ratio, a mathematical expression of 

the dissection of the ground surface, bears a logarithmic function relation to drainage density (Smith, 

K. G., 1950, p. 667-668). Therefore, because draina(J'e 

d . 0 

ens1ty can be determined readily, aerial photographs 

may be useful in a comparative study of erosional 

topographies. 

"Vhen texture is used to designate drainage density 

or erosional characteristics of the terrain, rather than 

to designate a photographic texture, the use should be 

so qualified as to make the definition clear. Although 

textures are commonly considered in terms of the twodimensional plan view, it is important to note that 

erosional texture may well be significant or at least 

more readily recognized when viewed from the third 

dimension of the stereoscopic model (see fig. 39). 

PATTERN 

Pattern, as used herein, refers to the orderly spatial 

arrangement of geologic, topographic, or vegetation 

features. The spatial arrangement of pattern is normally considered by interpreters to be a two-dimensional or plan-view arrangement of features, but it 

may also be a three-dimensional arrangement. If features that make up a pattern become too small to 

identify, as on small-scale photographs, they may then 

form a photographic texture. 

Patterns resulting from particular distributions of 

gently curved or straight lines are common and are 

frequently of structural significance; they may represent faults, joints, dikes, or bedding. But a single 

line, or lineation, is also an illustration of ·pattern; it 
10 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

FIGURE 3.-Drainage map of Large area in vicinity of ring dike shown in figure 54. 

A separate tracing of drainage over a large area in the 

vicinity of the ring dike of figure 54 shows a marked decrease in drainage density within the ring-dike area. The 

contrast in drainage densities inside and outside of the 

ring dike is more readily seen on a separate tracing of 

drainage than in the stereoscopic model. (See also fig. 9.) 

Inset shows area of figure 54. 
INTERPRETATION FACTORS 11 

may result from an orderly arrangement of stream 

segments, trees, depressions, or other features (figs. 

74 and 84). This arrangement may be a continuous 

alinement of geologic, topographic," or vegetation feahn·es, but more commonly it is a discontinuous alinement. Because lineations are especially important as 

expressions of fractures they may indicate areas of 

considerable economic significance. 

Drainage patterns are an important element in geologic interpretation of aerial photographs (figs. 48, 

94, and 96). In bedrock areas these patterns "* * * 

depend for the most part on the lithologic character of 

the underlying rocks, the attitude of these rock bodies 

* * * and the arrangement and spacing of the planes 

* * * of lithologic and structural weakness encountered 

by the runoff" (Tator, 1954, p. 412). Stream patterns 

thus reflect. the control exerted by underlying structure 

a.nd rock type. But stream characteristics may be influenced by thickness and kind of surficial material 

where structural control is at a minimum. Under 

these conditions the drainage may reflect differences 

in surficial materials that are significant to the e:ilgineet·ing geologist. (See Belcher, 1945, 1948; Jenkins, 

Belcher, Greeg, and '~roods, 1946; Frost, 1946; Frost 

and "roods, 1948; !-little, 1949; and Parvis, 1947, 

1950.) In areas where resistance to erosion may be 

more or less uniform, as in ma.ny surficial deposits or 

in bedrock without pronounced structure, the drainage 

pattern is commonly dendritic or modified dendritic 

(see Jigs. 70, 77, and 106). 'Vhere structures are well 

developed, as in folded mountains, characteristic trellis, annular, or other drainage patterns may develop 

(see .fig. 64) . · 

Because the sensitivity of drainage to strike and dip 

directions is pronounced, changes in a drainage pattern, or deviation from an established norni, may be 

as .important or more important than interpretation 

of :tny one stream pattern. "It is ·therefore important 

that relatively large areas be examined so that local 

drainage patterns [or anomalies] may be readily differentiated from regional drainage patterns" (Alliger, 

1955, p. 180). A change in drainage may thus have 

significant structural implications (see figs. 9, 94, and 

99). But a difference in stream pattern may also suggest variations in lithologic character of the underlying 

rocks. 

P:'ttterns of vegetation are also commonly of geologic 

significance and may reflect structural conditions or 

lithologic character of rock types (see figs. 40, 43, 54', 

72, a.nd 85). "r oolnough ( 1934a., p. 213) suggested 

that a distinction be made between two types of vegetation distribution which he called "blocks" and "alineInents ;" the former occurs on extensive outcrops of 

rock of uniform character, the latter along narrow rock 

bands, faults, or similar limited features. Alinements 

further may be subdivided as linear, parallel, and 

curved (see W oolnough, 1934a, p. 213). It is suggested 

that narrow linear or palallel alinements of vegetation 

may represent fractures (figs. 50, 60, 71, and 101), 

whereas wide, curved alinements may signify the distribution of outcrop of low, moderately or even steeply 

dipping beds (figs. 40 and 72); where curved alinements form closed loops they generally represent horizontal or nearly flat lying beds (figs. 61 and 77). 

The term "pattern" has also been used in the literature in quite a different sense than described above. In 

the study of surficial mater_ials the term "soil p"h.ttern" 

-a poorly chosen term-refers to the combination of 

surface expressions, such as landform, drainage characteristics, and vegetation, that are used in the interpretation of ground conditions (see p. 34-37). Thus 

. pattern as defined above may be included as an element 

of the broad term "soil pattern," commonly used in 

engineering geology descriptions. Soil patterns may 

reveal information of direGt use in engineering geology studies, as in the location of granular materials 

(figs. 52, 88, 104, and 108; and p. 34-37) ; in addition 

soil patterns may be important in suggesting the distribution of certain rock types; this distribution in 

turn may reflect geologic structures (see figs. 4, 48, 

and 81). 

RELATION TO ASSOCIATED FEATURES 

The relation of one feature to its surroundings is 

commonly important because a single feature by itself 

may not be distinctive enough to permit its identification. The significance of this relation may be spatial 

or genetic. For example, depressions may be identified 

as kettle holes because of their location near readily 

identified terminal moraines, and because of their 

genetic association with glaciated terrain. vVith regard to spatial association this recognition element 

has also been called the "site factor" (Col well, 1952, 

p. 540) ; it may be particularly useful in determining 

the significance of vegetation with respect to the underlying geology. For example, certain tree groups growing on topographically high areas in interior Alaska 

suggest well-drained underlying materials. The landform, especially, in combination with the vegetation 

characteristics· suggests stabilized sand dunes (fig. 56). 

The relation of associated features may thus permit 

specific identifications and inferences concerning surficial materials or rock types. 

Photograph scale may be important in the interpretation of associated features; that is, a geologic feature on small-scale photographs may be interpreted 

because of its position with regard to other features, 
12 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

Iredell loam 

Mecklenburg loam 

FIGURE 4.-Generalized soil map of area shown in figure 48 (modified from soil survey map of Abbeville County, 

South Carolina) . 

The circular area underlain by Mecklenburg loam and 

Iredell loam is conspicuous. These soils delimit an area of 

mafic intrusive rocks from a surrounding area of silicic rock 

types. Particularly where outcrops are sparse or lacking, 

soil maps may provide significant geologic information, espeall of which can be seen at the same time (fig. 55), 

whereas on large-scale photographs direct identification might be made because photographic details are 

visible. The usefulness of association- as a recognition 

element also will vary not only with photograph scale 

but with the objective of the study. The engineering 

geologist may be confronted with the immediate identification and location of kames as a source of gravel 

near a proposed highway route, whereas the geologist 

mapping the entire area would be interested i1i the 

identification, orientation, and distribution of all kames, 

with respect to geologic history of the area. 

SHAPE 

Shape as a recognition element in geologic interpretation is significant primarily only in its broadest deficially in areas of residual soils. The soil maps may provide 

structural or lithologic data with respect to underlying 

rocks, or they may provide pedologic data concerning the 

surficial materials that is useful to the engineering geologist. 

nition, which involves relief or topographic expression. 

In this regard shape is very important in recognizing 

constructional landforms, such as volcanic cones, sand 

dunes, river terraces, and certain glacial features (figs. 

41, 55-58, 60, and 104) ; it is also useful in differentiating rock units where one formation is expressed in 

bold cliffs and overlies a second formation that shows 

a lesser angle of slope (figs. 62, 84, and 91), or where 

adjacent rocks show significant differences in topographic relief (fig. 65). Rectilinear depressions may 

be expressions of faults (figs. 60, 85, 86, 99, and 103). 

The general surface configuration, as in the "beehive" 

weathering of some massive sandstones, also ma.y be 

regarded as an expression of shape that is useful in 

recognition. According to Belcher (1945, p. 140-141) 
~TERPRETATION FACTORS 13 

shapes of gully cross sections are important to the interpretation of surficial materials for engineering purposes. I-Iowever, in its strictest definition, as "a spatiaJ form with respect to a relatively constant contour 

or periphery" (vVebster's New International Dictionary, 1950) shape has been of little importance as a 

recognition element because nature may reveal the 

same geologic feature in an infinite number of different 

shapes, and because common scales of photography 

(1 :20,000) restrict an interpreter's ability to see details 

of shape. 'Vhere local field criteria have been established, shape may be useful, for example, in the interpretation of patterned areas (fig. 53) or in the study 

of gu11y cross sections, particularly when large-scale 

photographs are available (fig. 88). 

Shadow as n. recognition element is primarily a use 

of shape and photographic tone. Because vertical 

aerial phot.ogrn.phs are commonly used for interpretation, shadows may permit an effective side view of some 

features. For example, shadows may be useful in interpreting tree types, such as conifers, which generally 

have pointed tops; the type of tree in turn may have 

u. relation to underlying geologic materials and particularly to soil-moisture conditions (Stoeckeler, 1952, 

p. 4). For interpreting most natural features, however, the shape of shadows probably co~1tributes little 

in direct identification. Insofar as shadows contribute 

to textures and patterns, they are of course of great 

importance. Even slight topographic breaks may be 

accentuated by shadows (figs. 88 and 97). 

SIZE 

The qu'a.ntitati ve element of size, as it relates to 

surface or voJume dimensions of an object, has been 

little used in recognition of geologic features from 

n.erin.l photographs, although relative size has been 

usefu 1 as an interpretation aid. Size as an interpretation element is being used increasingly with the 

adoption of photograinmetry as a fundamental part 

of photogeology; it is most appropriately considered 

in relation to interpretations based on thickness of 

strata., amount of offset along faults, or other finite 

measuremerits (see p. 56-63). Geologically significant measurements may be directly related to topographic expression; that is, measurements between 

topographic positions may yield direct information 

such as stratigraphic thickness. But generally other 

influencing factors, such as dip of beds or other structural elements, must be considered just as in field 

measurement (see p. 56-58) . If the thickness of a 

formation is known, it may aid in location of formation contacts on aerial photographs, because measurements may be made from known points that would 

permit the positioning of the obscured contacts (fig. 

39) ; or it may aid in correlation of beds across fault 

zones. . Determining the range of thicknesses over a 

broad area may be essential to understanding the regional geology of that area (see p. 59-60) . 

Studies in quantitative geomorphology suggest that 

size, in terms of finite measurements, may be. an extremely important parameter of geologic interpretation of aerial photographs. Measurement of geologic 

data can be expected to become an even more important part of photogeology in the future as techniques 

of quantifying geologic information become more 

widely used. 

COMBINATIONS OF RECOGNITION ELEMENTS 

The usefulness of recognition elements, of course, is 

enhanced where they may be used in combination. 

Ind~ed, photographic tone, useful alone as a recognition factor under some conditions, is a fundamental 

photographic characteristic without which there could 

be no recognition elements at all on black-and-white 

photographs. As mutually supporting recognition 

elements an association of photographic tone, topographic expression, and texture may permit the interpretation of bedding, even in areas of sparse or no 

outcrops (fig. 40). Faults are commonly expressed as 

straight or gently curved lines; they may be further 

identified by an abrupt photographic tone change on 

opposite sides of these lines as well as by a pronounced 

offset of recognizable rock types (fig. 83). Surficial 

materials are commonly identified on the combined 

basis of stream patterns, shapes of gully cross sections, 

texture, and photographic tone as well as by their relation to associated features (see p. 34-37) . 

VERTICAL EXAGGERATION 

In addition to those recognition elements that are 

based on factors, such as film and filter used and 

processing, that directly affect the photographic image, 

certain other factors are important in interpretation. 

Especially significant is vertical exaggeration-the 

exaggeration of vertical distances with respect to horizontal distances-which is characteristic of almost all 

stereoscopic models; this exaggeration makes slopes 

appear steeper than they are and objects seem taller 

than they are. Vertical exaggeration is generally 

present not only in viewing stereoscopic models formed 

from paper prints but also in viewing projected stereoscopic models formed from glass-plate diapositives in 

double-projection instruments. 

An understanding of vertical exaggeration is fundamental in the study of stereoscopic models, not only 

because of the exaggerated heights and the somewhat 

unnatural view that results, but because in many geologic problems the interpreter relies on visual dip 
14 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

est1mations rather than on any measuring devices (see 

p. 64). · The. amount of exaggeration of a dip angle 

in a stereoscopic model is fundamentally related to the 

tangent of the dip angle; that is, in a stereoscopic 

model exaggerated 3 times, for example, any true dip 

angle would be exaggerated to an angle whose tangent 

function . was equal to 3 times the tangent of true dip. 

Exaggeration of low dips is generally an aid in structural interpretation and in detecting tilt from aerial 

photographs. 

Minor topographic differences, which may reflect 

underlying geologic structure, are also exaggerated and 

in turn easily recognized (figs. 40, 88, and 97). The 

exaggeration of relief in a stereoscopic model of 

1 :20,000-scale photographs taken with a 6-inch-focallength lens commonly permits an interpreter to distinguish differences in altitude as small as 1 foot. As 

a consequence vertical exaggeration then is most important where small but significant differences in altitude may be present in the stereoscopic model. 

Vertical ~xaggeration ·results from the wide spacing 

of the camera positions at the time of exposure, as 

contrasted to the spacing of the human eyes with 

respect to normal viewing arrangement in examining 

stereoscopic models. Vertical exaggeration in normal 

viewing arrangement is fundamentally related to baseheight ~atio, which is the ratio of the air base distance 

t<? the flying height (see fig. 1). As this ratio increases, vertical exaggeration increases. Thus the 

greater the distance between· camera stations ( equivalent to air base distance), flyi~g ,height remaining constant, ·the ·greater is the vertical exaggeration: The 

resulting photographs will have an increasingly smaller 

overlap area and consequently a lesser area within the 

stereoscopic model. It should be noted, however, that 

in norJ?lal photographing procedure when flying height 

is increased for any given focal-length lens, the air 

base distance is also increased so that a relatively constant base-height ratio is maintained. Under these 

conditions vertical exaggeration is not affected by flying height. "Then the base-height ratio is increased 

through the use of short-focal-length lenses, vertical 

exaggeration will also increase, the photographs taken 

with the shorter focal-length lens showing the greater 

exaggeration. Effective vertical exaggeration may also 

be increased by moving the two photographs farther 

apart at the time of viewing, or by increasing the 

v1ewmg distance. 

SCALE OF PHOTOGRAPHS 

Small-scale photographs may be advantageous for 

interpreting some geologic terrains, particularly where 

reconnaissan~e information is desired. Small-scale photographs are normally taken at relatively high altitudes 

and have scales of 1 :60,000 or smaller; their usefulness results chiefly from showing a large area in a 

single view (approximately 80 square miles on a single 

photograph having a scale of 1 :60,000), which may 

reveal overall relations, as in geologic structures, drainage patterns, or other features, that could not be readily seen otherwise (figs. 48 and 55) . Photomosaics 

commonly serve this same purpose (fig. 96). The 

association of features over a large area provided by 

the small scale is. the significant factor to be considered. However, the reduction in image size due to 

high flying heights or short-focal-length lenses may 

be detrimental to the identification of some features, 

and hence make such photographs of little use for some 

interpretation problems. For example, details of jointing may be entirely lost on small-scale photographs. 

Also, use of small-scale photographs obtained by using 

extremely short-focal-length lenses, that is, 2 inches 

or shorter, may be undesirable, especially in areas of 

moderate to high relief, because many topographically 

low areas ·will be masked from view. Nevertheless, 

small-scale photographs have definite advantages in 

qualitative interpretation (Hemphill, 1958b) and may 

be especially useful if employed in conjunction with 

large-scale photographs or with instruments that enlarge the photography scale (see p. 47) .. 'Vhere 

quantitative study is involved, small-scale photographs 

are advantageous in that less control is needed within 

a given area than with large-scale photographs; but 

this advantage may be offset by the decrease in the 

reliability of measurements (see p. 72-74). 

PHOTOGEOLOGY IN GEOLOGIC MAPPING 

In geologic mapping maximum use of aerial photographs is attained by closely integrating field and photogeologic studies. It generally is desirable to precede 

the field phase of geologic study by a study of aerial 

photographs, which should include the compilation of 

a preliminary map on which all interpretations, however reliable or questionable, have been noted. Such a 

preliminary photogeologic study affords several advantages: it may point out areas that must be mapped 

primarily by field methods; it may eliminate or reduce 

extensive field surveys in certain areas; it may direct 

attention to anomalous areas where detailed field study 

is particularly warranted; and it commonly provides 

a basis for organizing the geologic plan of field study. 

In addition, attention is directed to those areas where 

field study will most likely result in establishing criteria that will permit a refinement of further photogeologic mapping. Also, a preliminary study of photographs gives a geographic familiarity of the area 
KINDS AND AMOUNTS OF INFORMATION 15 

that would be useful in choosing camp sites, routes of 

traverse, and optimum locations of instrument survey 

stations. 

Continuing use of aerial photographs during the 

field phase of geologic study permits a firsthand correlation of geologic features and photographic images, 

which corroborates or refutes preliminary interpretations and provides information for subsequent detailed 

photointerpretive study. Refinements in interpretation 

may be made in the field or in a followup photogeolog.ic study. 

The delineation of rock types and structures from 

aerial photographs may involve mere observation; or 

more commonly interpretation is involved. In some 

areas it is possible only to map similar photographic 

ui1its, as no diagnostic or suggestive criteria as 

to rock type may be present. Such areas are usually 

poorly exposed because of heavy vegetation or surficial 

debris; relie~ may be very low. But in other areas, 

where wide expanses of rock crop out, specific formations or rock types may be easily delimited on the 

photographs; thus much of the routine work of mapping geologic contacts, as well as determining structurn.l men,surements, can be accomplished readily by 

photogeologic methods. vVhere geologic structures 

are small and complex, aerial photographs may provide only limited information as contrasted to areas 

where geologic structures n,re large and simple. Depending chiefly on factors related to geologic environment, climate, and erosional cycle, the extent to which 

photogeologic methods can be applied thus may vary 

widely. 

KINDS AND AMOUNTS OF INFORMATION 

Geologic information that can be interpreted from 

ner.ia.l photographs may be grouped broadly into two 

categories: lithologic and structural. In general or 

reconnaiss.ance mapping the geologist is interested 

largely ·in determining the distribution of rock units, 

including surficial deposits, and in delineating geologic 

structures. Special studies may require a more rigorous application of photogeologic procedures, which 

includes quantitative as well as qualitative study of 

.the stereoscopic models. But regardless of the objective of a particular study, the kinds and amounts of 

geologic information available from aerial photographs 

will depend primary on the type of terrain-whether 

igneous, metamorphic, or sedimentary-, the climatic 

environment, and stage of the geomorphic cycle. 

It is generally believed that sedimentary rock areas 

will yield more information from aerial photographs 

than igneous rock arens, and that regions underlain by 

metamorphic rocks will reveal the least information. 

Putnam stated ( 194 7, p. 560, 562) that "* * * geologic 

features * * * tend to be more recognizable where 

strong differences exist in the erosional resistance of 

adjacent rocks. Thus, sedimentary rocks, as sandstop.e, 

shale, and limestone, are likely to be more apparent 

than relatively homogeneous rocks like granite, especially if the sedimentary rocks have a moderate dip." 

Bentor (1952, p. 162), based on his work in Israel, was 

of the opinion that "As a whole, air-photographs 

are much less useful in regions of crystalline rocks 

than in those composed ·of sedimentary formations." 

"Tith regard to igneous and metamorphic terrains, 

which are generally believed to be more difficult to 

interpret than sedimentary terrains, Melton ( 1956, 

p. 57) concluded that "Discovery of hitherto unknown 

or abnormal structures and [mineral] deposits~ rather 

than detailed mapping, will probably be the most rewarding use of aerial photographs * * *." 

Except for high mountainous areas where vegetative 

growth is restricted by altitude, arid and semiarid 

regions generally will have the largest areas of rock 

outcrop, and tropical regions the least; hence the arid 

and semiarid regions may be expected to yield the 

greatest amount of geologic information from aerial 

photographs. In addition to relatively wide areas of 

rock exposure, the arid and semiarid regions may be 

expected to show a greater number of significant plantrock associations than other climatic areas, 'because 

weathered material in the arid and semiarid regions 

is not excessively leached and a close relation of soil 

to parent formation therefore persists. 'Vhere this 

close association of parent rock and residual soils exists the distribl.1tion of different vegetation, reflecting 

the effect of bedrock on the composition of the soils, 

may facilitate the mapping of different rock types (see 

Levings, 1944, p. 27-30). In areas of much precipitation~ under temperate or tropical climatic conditions, 

soils tends to be leached of salts, and there is a further 

tendency for soils from different parent formations 

to become more similar at maturity (Murray, 1955, 

p. 104). l\1urray stated (1955, p. 104) that under 

tropical conditions where rainfall is abundant, the 

ratios of potassium, sodium, and calcium to aluminum, 

which are widely different in silicic and mafic rocks, 

become smaller on weathering and are nearly equal 

in mature soils. Thus differe.nces in vegetation as related to specific formations might be less well developed in tropical areas, except for those plant associations that may depend primarily on physical characteristics of the soil rather than chemical. Grantham 

( 1953, p. 329) noted that in semiarid areas of Africa 

the soils reflect geology through vegetation differences 

more strongly than in wet temperate areas. 
16 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

Sedimentary terrains yield a greater amount of lithologic and structural information than igneous and 

metamorphic areas because of the generally nonhomogeneous nature of sedimentary terrain, which results 

in marked differential erosion characteristics that 

stand out on aerial photographs. Sedimentary rocks 

of strongly differing physical characteristics commonly 

crop oui:- within short distances whereas plutonic rocks, 

particularly, are likely to be relatively homogeneous 

over wide areas. Locally, diagnostic landforms, such 

as volcanic· cones, may be important in study of igneous terrains, especially where extrusive rocks prevail. l\1etamorphic terrains, on the other hand, may 

reveal the least amount of information from aerial 

photographs because of the very nature of the metamorphic processes, which tend to destroy the erosional 

and landform characteristics of sedimentary and igneous rocks from which the metamorphic rocks are 

derived. 

In any area where vegetative cover is dense and 

where surficial debris is widespread, aerial photographs generally can be expected to yield more structural information than lithologic information. The 

amount of structural information in turn generally will 

be greater, for any one type of terrain and climatic 

environment, during the mature stage of the geomorphic cycle, at which time streams show their greatest 

adjustment to and reflection of structure, and at which 

time a greater third dimension of the terrain is visible for study in the stereoscopic model. Where the 

lithologic character of rocks is interpreted on geo-. 

morphic or landform expression, maturely dissected 

areas may reveal the greatest amount of information. 

Although the extent to which photogeologic procedures contribute to the geologic mapping of an area 

depends on the climatic and geologic environment: as 

well as the stage of the erosional cycle, some generalizations may be stated with regard to differentiating 

and interpreting sedimentary, igneous, and metamorphic terrains. These generalizations are grouped into 

two categories: lithologic character of rocks and geologic structure. 

LITHOLOGIC CHARACTER OF ROO~S 

SEDIMENTARY ROCKS 

0 onsolidated.-The presence of bedding in most 

sedimentary rocks is fundamental to their interpretation from aerial photographs. Because of differential 

resistance to erosion of sedimentary beds the typical 

banded patterns of these rocks are seen on aerial photographs. This is a result of what Rich (1951, p. 189) 

termed the "etching concept" wherein more resistant 

beds are brought into relief and less resistant beds 

lowered as a result of weathering and removal of materials largely by sheet wash and creep (figs. 87, 91, 

and 97). Back slopes of steeply dipping beds, especially, may contrast strongly because they commonly 

expose to erosion within a given area a wider variety 

of lithologic types than dip slopes of gently dipping 

beds. Although topographic expression is thus important in recognition of bedding, banding due to 

vegetation or soil differences, expressed by photographic tone may likewise delineate beds, in absence 

of topographic expression or in combination with it 

(figs. 40 and 81). Bedding is especially prominent in 

the mature stage of the geomorphic cycle, particularly 

in terrain underlain by interbedded hard and soft 

sedimentary rocks that are tilted. However, bedding 

may be masked by the massive character of some 

sedimentary rocks, such as certain sandstones, in which 

case the sedimentary rocks 1nay appear to be homogeneous and similar to some metamorphic rocks or 

intrusive igneous rocks as seen on aerial photographs 

(contrast figs. 71 and 92). A notable exception is 

massive limestone in which sinkholes have developed 

(fig. 68). 

As seen on aerial photographs shales and similar 

fine-grained sedimentary rocks tend to have relatively 

dark photographic tones, a fine-textured drainage, and 

relatively closely and regularly spaced joints (figs. 49, 

62, 63, and 91). Coarse-grained clastic rocks, in contrast, tend to have relatively light photographic tones, 

a coarse-textured drainage, and relatively widely and 

regularly spaced joints (figs. 42, 91, and 92). These 

generalizations are probably most applicable to marine 

sedimentary rocks. HO\vever, some fine-grained clastic 

rocks may be light toned and some coarse-grained 

sedimentary rocks, such as the continental red beds, 

may pe dark colored rather than light colored and be 

expressed as relatively dark photographic tones on 

aerial photographs. Drainage density in a given climatic area is related primarily to resistance of a rock 

to erosion; drainage density increases with decrease 

of resistance to erosion. Resistance to erosion in turn 

is fundamentally related to permeability; generally 

resistance to erosion is less in rocks of low permeability. Hence, fine-grained clastic rocks commonly 

have a fine-textured drainage (fig. 79). 

Physico-chemical precipitates are in many places 

closely associated with fine-grained clastic sedimentary 

rocks; where these rocks are interbedded the drainage texture and joint pattern both may be fine, as with 

fine-grained clastic rocks alone, but photographic tone 

of physico-chemical precipitates may commonly be 

relatively light, as with some fresh-water limestone, 

rather than dark, despite the fine grain size. 
\ 

... 

I 

! 

\ 

,J 

\ 

KINDS AND AMOUNTS OF INFORMATION 17 

Alt-hough fine-textured drainage commonly characterizes nonresistant fine-grained sedimentary rocks and 

coarse-textured drainage suggests resistant coarsegrained sedimentary rocks, drainage is probably more 

signi fica.nt as an indicator of structure than it is of 

lithology. Drainage characteristics may suggest broad 

rock groupings, but no one drainage pattern is diagnostic of sedimentary rocks, although annular and 

trellis dru.inage genernJly suggest sedimentary rocks or 

close tnetamorphic equivalents. The association of 

specific drainage characteristi~s with rocks of certain 

lithologic or physical characteristics presents an interesting field of study; quantitative study of stream data 

.measureable from aerial photographs, such as drainage 

density, may indicate certain drainage-lithology relations. 

The type of vegetation may be useful in differentiating specific rock types (see figs. 43 and 61). Hemming ( 1937) noted that striking vegetation differences 

were exhibited by limestone and quartzite in northern 

Rhodesia where the trees Aoaoia and Albizzia grew on 

limestone and B'raol~ystegia and I soberlinia grew on 

quartzite. 

Unoonsolidated.-Most surficial deposits-the unconsolidated sedimentary rocks-are readily distinguished on aerial photographs from consolidated rock 

types, although thin veneers of surficial material may 

be diflicult to identify. ~1a,ny recognition criteria are 

used in identifying and interpreting surficial deposits 

from aerial photographs, but the most significant criterion is probably landform. Landform is significant 

because many surficial deposits, particularly those useful to the engineering geologist, are transported materials that have diagnostic topographic form. The 

irnport:ance of landform is seen in the identification of 

sand dunes, eskers, kames, nJluviaJ fans, river terraces, and similar constructional geologic features 

(figs. 52, 55, 56, 58, and 104). On the other hand extensive deposits of surficiaJ debris, such as the drift 

pln.ins of north-central United States, may show little 

or no diaf,rnostic landform, but their general n1akeup 

rnn.y be .inferred from drainage characteristics, photographic· tone, analysis of slope, and related criteria 

(fig. 107). These criteria are discussed more fully 

1.111der t.tses of ~te.ri~tl 1)l1otogrt~pl1s i11 e11giJ1eeriJ1g 

geology studies (p. 33-37). 

IGNEOUS ROCXS 

Ewt'rttsive ?·oo/cs.-Extrusive igneous rocks commonly have diagnostic landforms (figs. 41, 60, and 76) 

in contrast to the intrusive igneous rocks, which a.re 

revealed on aerial photographs primarily by other recognition features such as drainage, texture, massive 

character of the rock, and crosscutting association with 

country rocks. 

In addition to landform certain other criteria may 

be useful in identifying extrusive rocks, if the terrain 

is relatively undeformed. The structural relation of 

flows to associated rocks, for example, may be 

diagnostic in little-deformed areas, but where flows 

have been strongly tilted, folded, or otherwise disturbed, recognition and interpretation from aerial 

photographs may be extremely difficult or impossible. Only those relatively undeformed flows, 

mainly of Tertiary age and younger, are readily identified from aerial photographs without some knowledge from ground surveys. The ground plan of flows 

showing lobate patterns of vegetation and topography, 

especially at the terminations of suspected flows, is 

commonly diagnostic (figs. 76, 85, and 113), and association with volcanic cones makes misinterpretation 

difficult (figs. 41, 60, and 76). Locally flow channels 

may be visible. The surface of a flow may be rather 

hummocky and irregular, in contrast to surfaces of 

sedimentary strata, and where several flows have piled 

up, present topography may show several terracelike 

forms with irregular surfaces. The unconformable 

contact between flows and underlying rocks may aid 

in recognizing extrusive rocks (fig. 87), and where 

flows c~scade down over older topography their identity is obvious (fig. 60). The basaltic or mafic flows 

may. show dark photographic tones on conventional 

photographs, particularly in well-exposed areas, 

whereas rhyolitic or silicic flows may have light photographic tones. Locally distinctive patterns are present on mafi<; flows (fig. 53), and in some areas erosional characteristics permit differentiation of flows 

and associated rocks (fig. 69). 

vVhere flows have been extruded over deformed 

igneous and metamorphic. terrains, a low-dipping surface associated with lobate patterns of vegetation and 

topography is suggestive of extrusive rocks, and in 

. some areas faults in the underlying rocks are seen to 

terminate abruptly at the edge of a lava flow (fig. 85) . 

lnt1·u8ive roolcs.-Intrusive igneous rocks have a 

wide variety of structur~l relations to surrounding 

rocks that commonly aid in their identification from 

aerial photographs. Dikes may be revealed on aerial 

photographs as rectilinear or curved ridges that stand 

up because of greater resistance to erosion than surrounding rocks (figs. 81, 82, 87 and 90) ; they may 

also contrast in photographic tone with the surrounding rocks either because of the difference in rock types 

(fig. 90), or because of distinctive vegetation that covers dikes in some areas (Mogg, 1930, p. 662 and 668, 

pl. 5). Dikes may follow fr~ctures in other igneous 

rocks and contrast in photographic tone with the host 
18 AERIAL PHOTOGRAPHS. IN GEOLOGIC INTERPRETATION AND MAPPING 

rock. Where dikes are less resistant than the country 

rock, they may be expressed as rectilinear depressions 

that appear similar to fault expressions. 

Because sills occur parallel to bedding in sedimentary rocks, they may be difficult to interpret as igneous 

from aerial photographs, ·even though strong tonal 

contrasts of the sills and sedimentary rocks are present. ~fogg ( 1930, p. 656) pointed out that the diabases 

of Pretoria, South Africa, although relatively hard 

rocks, weather easily to deep soils that support distinctive deep-rooted vegetation, but the vegetation 

type differs from area to area. The tree [{ irkia wilmsii 

·grows conspicuously and exclusively on diabase sheets 

in the sedimentary rocks of the Pyramid Hills area of 

Pretoria (~fogg, 1930, p. 658), but in the ~fagaliesberg 

area Acacia caffra characteristically marks the diabase 

(Mogg, 1930, p. 662). Vegetation thus may be important locally as an indicator of rock types, but commonly it must be interpreted with caution because 

the same rock type may carry a different flora in 

different areas. 

Igneous plugs, laccoliths, stocks, and batholiths, by 

nature of their mode of origin, commonly tilt the sedimentary rocks which they intrude, and in areas where 

plutonic rocks are exposed the pattern of dips in surrounding rocks may support other criteria suggestive 

of igneous rocks. Domal areas clue to intrusion may 

also be reflected in radial drainage pa tter11s (fig. 48) , 

just as are certain folds in sedimentary rocks. But if 

areas of plutonic rocks are large, the characteristic 

homogeneous nature of such rocks may result in a 

dendritic drainage pattern, unless dr:ainage is controlled by a fracture system (figs. 51, 70, 71, and 86). 

Because of the generally homogeneous nature of plutonic rocks as contrasted to sedimentary rocks they 

will have a massive appearance on aerial photographs, 

and banding typical of sedimentary and some metamorphic rocks will generally be lacking (figs. 59, 70, 

and 71). Also because of the homogeneous character 

of many igneous bodies, elements essential to plant 

growth may be distributed more or less uniformly 

throughout the l}lass (Murray, 1955, p. 103) and significant differences in vegetation can be expected to be 

absent from any one plutonic rock mass (fig. 70), 

except for those differences controlled by altitude or 

by structural conditions, such as faults that may trap 

an abundance of moisture (fig. 71). 

In addition, topographic and erosional characteristics may aid in the interpretation of some igneous 

terrains. Granite at low altitudes in some tropical 

areas is expressed in hummocky, rounded topography 

on which a uniform type of vegetation is present; 

dendritic drainage may characterize this terrain. 

Similar rounded topography may also be present at 

high altitudes, but where glaciers have been active all 

rocks may stand out in sharply angular topography. 

Regardless of surface expression, however, a crisscross 

pattern ·of joints, nearly always present in igneous 

rocks, commonly can be seen not only in the wellexposed areas (see figs. 70, 71, 100, and 103) but also 

in many vegetation -covered areas (fig. 72) . Topographic expression of most plutonic rocks, as with 

other rock types, is dependent in large part on climate 

and stage of the erosional eyele, although within an 

area underlain by different igneous rock types difference in resistance to erosion may be significant and specific rocks may stand topographically high (see fig. 

82). The homogeneous character of many intrustive 

rocks and characteristic jointing, on the other hand, 

are related primarily to the rock type and its mode of 

occurrence, and are independent of climate and 

erosional cycle. 

METAMORPHIC ROCKS 

The identification and interpretation of metamorphic 

rocks from aerial photographs is commonly difficult 

because large-scale distinguishing characteristics are 

generally lacking. Bedding, so necessary to structural 

interpretation, may. be difficult or impossible to recognize because of physical changes brought about by 

metamorphi~m, including the superposition of metamorphic structures of small scale. Indeed, many structural trends that can be interpreted from aerial photographs represent foliation rather than bedding (figs. 

73 and 74). Where exposures are good, differences in 

photographic tone offer the best clue to recognition of 

bedding, especially where the metamorphic rocks are 

derived from thin-bedded sedimentary rocks of contrasting lithologic makeup (fig. 103). Thick formations of quartzite anrl marble might readily retain the 

photographic eharacteristic!? of the original sedimentary beds, however, because the metamorphic changes 

of induration of sandstone and recrystallization of 

limestone would likely have little effect on the gross 

physical appearance of these rock types as seen on 

aerial photographs. It is possible, however, that talus 

from quartzite ledges might take the form. of large 

blocks in contrast to smaller chunks of rubble at the 

base of sandstone cliffs. The preference of vegetation 

for on£>; rock type such as limestone, as a result of its 

chemical makeup, might well persist, even if the formation were metamorphosed to marble; but vegetation 

that might show a preference for sandstone because of 

its physical makeup, especially its high porosity, might 

well be lacking on its metamorphic equivalent, 

quartzite. 

Strong parallel alinements of ridges and intervening 

low areas may be due to regional Cleavage, foliation, 

I 

I 

( 

l 

I 

'j 

\ 

l 
KINDS AND AMOUNTS OF INFORMATION 19 

or fold axes, and thus the "topographic grain" of an 

area may suggest underlying metamorphic rocks (figs. 

73, 76, 100. and 102), but in glaciated areas this evidence. must be used with caution (fig. 67). In an area 

where a pronounced regional trend, or "topographic· 

grain," is shown on the aerial photographs, the likelihood that metamorphic rocks are present is supported 

by the occurrence of widely spaced lineations at right 

angles to the regional trend. These lineations commonly represent regional cross joints and may be reflected in abrupt deflections of drainage along conspicuous straight stream segments of major streams 

(fig. 74) or in the development of tributary streams 

nJohg these joints (figs. 73-75). Where bedrock is well 

exposed the profuse fractures in certain metamorphic 

rocks may be visible (figs. 46 and 99). Cady (1945) 

pointed out that in some areas in central Alaska talus 

slopes are developed much more extensively where the 

peaks are of metamorphic rocks rather than of granitic 

rocks; this distinguishes these two rock types locally 

(fig. 103). 

STRUCTURE 

FLAT-LYING BEDS 

Flat-lying or nearly horizontal beds are most easily 

recognized where different sedimentary rock types exhibit contrasting photographic tones expressed as 

bands that extend along the topographic contour (figs. 

77 and 78). 'Vhere both resistant and nonresistant 

rocks· are present slope characteristics commonly are 

suggestive of horizontal strata-the resistant rocks 

have steep slopes or vertical clifl's, and the nonresistant rocks have a lesser angle of slope; the significant changes in slope between two beds extend genernlly along the topographic contour (figs. 77 and 78). 

Photographic tone and slope characteristics are associated in many places as mutually supporting evidence 

of flat-lying beds. 

Because of the uniform resistance of one rock type 

to erosion, particularly in a horizontal direction, the 

drainage on flat-lying beds is generally dendritic unless 

controlled by joints or faults (figs. 42, 61, and 77). 

I-Iowever, dendritic drainage alone, although suggestive, is not necessarily diagnostic of flat-lying sedimentary rocks; other rock types, such as granite, also 

may be uniformly resistant to erosion and may . also 

exhibit dendritic drainage. 

DIPPING BEDS 

Numerous expressions of dip of sedimentary beds 

may be seen on aerial photographs. Dip direction is 

readily apparent where topographic surfaces coincide 

with bedding surfaces (figs. 87, 90, 92). Even in 

heavily vegetated areas the direction of dip commonly 

553966 0-60-2 

stands out ·as a result of the overall view afforded by 

the aerial photograph, which shows the tree crowns 

to fall in a dipping plane (fig. 89). In those areas 

where dips are very·low, the aerial photographs are 

advantageous, if the beds are clearly expressed, because vertical exaggeration, which is present in most 

stereoscopic models, permits low dips to be readily 

interpreted (see fig. 91 and p. 64). The degree of 

dip may be estimated from the stereoscopic model by 

experienced photointerpreters or it may be measured 

photogrammetrically. 

'Vhere bedding is expressed by bands of differing 

photographic tone or by topographic breaks in slope 

due to resistance of beds, the rule of V's may be applied t.o determine the direction of dip; that is, where 

the trace of a bed intersects a stream valley a V in the 

outcrop pattern will point in the direction of dip 

(figs. 80, 81, and 98) unless the direction of dip and 

direc.tion of streamflow are the same and the stream 

has a gradient greater than the amount of dip (fig. 

59). 

In· many areas, particularly those of low relief, 

where beds are obscured by surficial materials or vegetation the direction of dip may be inferred from drainage characteristics. Where dips are gentle the· relatively long tributary streams commonly flow down the 

dip slopes (Lattman, 1954, p. 361-365), whereas short 

tributary streams will characterize back slopes (figs. 

5, 7, and 74). It is interesting to note that the relation 

of short and long tributary streams may be reversed 

where dips are steep, that is, about 45° or more (fig. 

6). ·Other photorecognition criteria, such as distribution of photographic tones, should be sought to indicate whether dips are gentle or steep; or scattered 

field observations may provide sufficient information 

for making an analysis of dip direction based on 

stream characteristics. In such an analysis it may be 

desirable to trace all streams, however small, on a 

separate over lay (fig. 7). This is best done from the 

stereoscopic model. Exact positioning of tributaries 

is not essential to this technique of determining dip 

direction, but lengths of tributaries must be sketched 

carefully as the length relations of updip to downdip 

tributaries is significant. Viewing the stereoscopic 

model pseudoscopically (fig. 95) may be helpful in 

d~lineating the drainage. 

FOLDS. 

Folds are commonly expressed as obvious features 

on aerial photographs especially where a fold is shown 

in its entirety within a single view and an orderly 

structural arrangement of beds is discernible (fig. 90) 

or where erosion has exposed a cross section of the 
20 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

FIGURE 5.-Sketch showing long tributary streams on updip side of strike valley in area of low dips. 

FIGURE 6.-Sketch showing long tributary streams on downdip side of strike valley in area of steep dips. 
KINDS AND AMOUNTS OF INFORMATION . 21 

structure (fig. 91). Drainage characteristics may also 

aid in recognizing and delimiting the fold. "Tengerd 

( 194 7, p. 596) noted that in parts of northern Alaska 

streams flowing in synclinal valleys were sluggish and 

meandering, and had low banks and swampy border 

areas with meandering tributaries; in contrast, streams 

flowing in anticlinal valleys were steep walled and less 

men.ndering. ''Tengerd also noted that dip slopes and 

back slopes in some areas commonly exhibited different drainage densities (fig. 79). 

"There folds are plunging, major streams commonly 

curve around the nose of the fold; the convex sjde of 

the curve indicates the direction of plunge of anticlinal folds (figs. 81, 89, and 90). However, in areas 

where relief is low or bedrock is obscured by vegetation or surficial materials, drainage patterns or drainage anomalies may be the primary expression of folds 

(see figs. 9, 94, and 96; and p. 26-27). Drainage pat-· 

terns or anomalies generally are best seen on smallscale photographs or mos::tics where segments of 

streams combine to form significant patterns when observed in a single view (fig. 96). The significance of 

draina.ge characteristics may not be immediately apparent when large-scale photographs are viewed. 

FAULTS 

One. of the advantages in study of aerial photographs is that of delineating high-angle faults or suspected faults. This advantage is a direct result of the 

aerial view, which allows a large area and the gross 

features within it to be seen at one time. For example, 

many a.linement.s that are inconspicuous on the ground 

are readily seen on aerial photographs (figs. 74 and 

83). Although many a.linements are conspicuous because of their independence of stream valleys or ridges 

and uplands which they ~ross (see Barton, 1933, p. 

11D8), a.linements of stream segments and small drainage courses are probably the most common indicators 

of faults. l\fost high-angle faults are expressed on 

photographs as straight or gently curving lines, and 

this eharacteristic is probably the most important clue 

that a fault may exist. All linear features should be 

examined with care even though other criteria are 

needed to prove the existence of faults. 

Lines indicn.tive of faults may be expressed as alinements. of vegetation, straight segments of stream 

courses, and waterfalls across streams; alinements of 

]a kes, ponds, and springs; conspicuous changes in photographic tone or drainage and erosional texture on 

opposite sides of a linear feature, or tone chan<re alono· b b 

a. linear feature because of vegetation which may appear darker; nJinements of topography, including 

saddles, knobs, or straight scarps; rectilinear depressions; or any combination of these. Faults may also 

be expressed by horizontal or vertical offsets of beds or 

recognizable rock units (figs. 50, 60, 62, 74, 79, 80, 82-

84, 86, 98, 99, 101, and 114). "Vhere bedrock is well 

exposed the actual physical break may be seen. In 

some places ·a lineament, or broad linear depression, 

may be conspicuous but not distinct. "There such 

areas of possible faults occur, beds on one side of the 

general lineament should be examined for change of 

dip and strike as wen as abnormal stratigraphic position with respect to beds on the other sid.e (see fig. 80). 

Commonly in sedimentary terrain, offsets on opposite 

sides of a linear feature are easily detected, owing to 

the vertical . exaggeration which is inherent in most 

stereoscopic models and which generally assists in the 

identification of correlative beds (figs. 83 and 84). 

Suspected faults should be examined carefully because other features may be represented, at least in 

part, on aerial photographs in the same manner as 

faults, particularly if a single photograph is being 

viewed. In some are.as jointing in one direction within 

a formation may be prominent and where an apparent 

offset is present owing to the occurrence of an erosion 

scarp along a joint plane, the scarp line may be misinterpreted as a fault trace. 'Vhere channels have been 

cut in sedimentary formations, and a rock type of 

different lithologic character now occupies the channels, a distinct lineation between the channel filling. 

and the original sedimentary rock, now truncated, may 

suggest a fault trace. A line between two distinct 

types of vegetation, although possibly reflecting the 

presence of a fault, may well be due to difl'erences in 

underlying rock types in normal sequence, or to the 

moisture content of the soil, or even to manmade features. In some areas the simple erosion of dipping 

strata may result in an alinement of topographically 

high points that suggests a fault. Or vegetation and 

drainage alinements may be present along ea~ily eroded 

dikes. The need for caution in interpreting faults 

from aerial photographs is thus dictated by the very 

presence of numerous other geologic features that are 

expressed photographically in a similar manner. 

Low-angle faults are more difficult to interpret from 

aerial photographs than high-angle faults. Discordance of structures within different rock types combined 

with strongly curving or irregular traces of the fault 

contact of the rocks involved offer the best clue to the 

presence of low-angle faults. Angular unconformities 

may show similar photographic expression. 

In a study of azimuth, frequency, and length measurements, Gross ( 1951) demonstrated that in ·some 

covered areas of the Ca.nadian Shield certain lineations 

very likely r~present faults. The study involved the 
22 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 
KINDS AND AMOUNTS OF INFORMATION 23 

construction of histograms in which the length and 

frequency of topographic lineations as seen on aerial 

photographs were plotted against geographic orientation within 10° segments of the compass. Similarly, 

histograms were plotted from a sampling of field data 

·of strike of strata, schistosity, dikes, glacial striae, 

faults, and other features considered to have a bearing 

on the configuration of the topography. One peak on 

the histograms compiled from aerial photographs was 

found to represent a direction of prominent faulting. 

JOINTS 

Joints, like faults, are most commonly expressed o:ri 

aerial photographs as linear features. In flat-lying or 

gently folded sedimentary rocks, most joints are 

steeply dipping or vertical, evenly or regularly spaced, 

and commonly consist of two prominent sets that are 

expressed on aerial photographs as lines intersecting 

approximately at right angles, which gives a blocky 

appearance to the topography (fig. 42); however, 

joints may be well developed in only one direction, 

which gives a strong linearity to the fracture plan 

(fig. 92). In fine-grained clastic rocks the joints are 

generally more closely spaced than in coarse clastic 

rocks. ,Joints may become widened through weathering and erosion processes and appear as particularly 

conspicuous linear :features; the. similarity o:f these 

joints to faults is superficially striking, especially 

where well-developed regional cross joints have formed 

in metamorphic rocks and other criteria o:f faulting 

are lacking. Identification of many joints is enhanced 

by the presence of vegetation along them, due perhaps 

to greater nioisture along these lines. Lattman a:r:td 

Olive (1955, p. 2087) described the use o:f solutionwidened joints in. Texas as a guide to indicate steepening of dip and to locate changes in dip direction. 

Joints more nearly parallel to the dip direction were 

widened by solution o:f meteoric water. Rubble, soil, 

and water trapped in these joints gave rise to dense 

vegetation e~pressed on aerial photographs as conspicuous, even-spaced parallel lines. A good clue to 

t.he identification o:f joints. in relatively undisturbed 

sedimentary rocks is the general abundance of short 

lineations on a photograph, and a more or less uniform 

trend of these lineations within a given ·area of a few 

square miles extent. 

Lattman and Nickelsen (1958) described azimuthfrequency measurements :from aerial photographs in a 

study of fracture traces in western Pennsylvania. The 

fracture traces are expressions of natural linear features that consist of topographic, vegetation, or soil 
: 

24 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

alinements exposed continuously for less than 1 mile. 

The term "fracture trace'~ is used by Lattman ( 1958, 

p. 569) to diffe.rentiate these alinements from natural 

linear features exposed continuously for at least 1 

mile or more, called "lineaments." A histogram of 

frequency and orientation of the fracture traces 

showed maximums closely corresponding to those of 

prevailing directions of joints as seen in the field, with 

the exception of one fracture-trace maximum which 

had no counterpart in exposed rocks of the area (Lattman and Nickelsen, 1958). This one maximum of frequency and orientation of traces, however, was found 

to correspond with joints in underlying coal beds; the 

joints apparently were reflected through an overlying 

thickness of shale and sandstone to be expressed at the 

surface in a manner discernible on aerial photographs 

but not in routine field-mapping procedure. The close 

correspondence .of fracture traces and joint directions 

in the test area suggest that aerial photographs may 

be used to extend mapping of joints into nearby areas 

of no outcrop. 

Joints in igneous rocks are likely to be seen readily 

on aerial photographs (figs. 70, 71, 100, and 103). 

Commonly three and sometimes four prominent joint 

sets are developed. vVhere these joints are primary 

joints~ dips may be low, moderate, or steep, depending 

on the orientation of flow structures within the igneous 

mass; this contrasts with the generally steep dipping 

joints of little-disturbed sedimentary rocks. Because 

of the wide range of dips of joints in igneous rocks, 

and because at least three joint sets are almost always 

developed, the surface traces of these joints, shown as 

lineations on aerial photographs, intersect at widely 

varying angles. The resulting crisscross pattern is 

distinctive of intrusive rocks in many areas. Joints in 

igneous rocks also may be irregularly spaced in contrast to those in sedimentary rocks . 

. ,Joints in metamorphic rocks may not be as conspicuous on photographs as joints in sedimentary or 

igneous rocks. Joints previously formed in igneous or 

sedimentary rocks may be destroyed by metamorphic 

processes and much of the energy of metamo.:-phism 

spent in development of schistosity, fracture cleavage, 

or other small-scale metamorphic structures, rather 

than conspicuous joints. This is particularly true of 

metamorphic rocks formed from fine-grained clastic 

roeks~ which comprise more than 50 percen·t of the 

sedimentary rock group. Where metamorphic rocks 

are recognizable by strong lineations that represent 

fold axes or trend of foliation, widely spaced regional 

cross joints perpendicular to the fold-axes direction 

may be prominent; streams developed along the cross 

joints accentuate· these structu~es (figs. 8, 73, and 74). 

CLEAVAGE AND FOLIATION 

Cleavage or foliation, which is almost universally 

present in metamorphic rocks, especially in those that 

were originally shale or other thin-bedded clastic rocks, 

is difficult to differentiate on most aerial photographs. 

The "grain" of metamorphic terrain as expressed on 

aerial photographs by stream_ and vegetation patterns, 

as well as by parallel ridges and gullies, however, may 

well reflect pronounced cleavage or foliation ( fi[:,rs. 76 

and 100). Turner ( 1952) pointed out that prominent 

topographic alinements in the schists of central Otago, 

New Zealand, are clearly expressed on aerial photographs. These alinements, generally parallel to the 

trend of b lineations, are believed to have resulted 

from differential weathering controlled by cleavage, 

foliation, and other elements of the schist fabric. 

UNCONFORMITIES 

Unconformities are generally difficult to interpret 

from aerial photographs. An angular unconformity 

may be inferred from the discordance of bedding, 

either in strike, dip, or both, at a contact line that has 

an irregular trace across a wide area. But discordance 

alone does not prove the presence of a stratigraphic 

unconformity since a discordance of bedding may be 

due to faulting, or locally to crossbedding. The areal 

distribution of rocks or structures as seen on smallscale photographs or mosaics, or after plotting geologic data over a large area, may also suggest the 

presence of an unconformity. On a local scale, the 

separation of unconsolidated, nearly horizontal, river 

gravel from the truncated, underlying strata is a good 

example of an unconformity (fig. 88). . Pediment 

gravel that overlies truncated bedrock (fig. 45), or 

channel-fill deposits that truncate the underlying bedrock (fig. 44), may be clearly expressed on aerial 

photographs. 

INTERPRETATION OF AERIAL PHOTOGRAPHS IN 

PETROLEUM ,GEOLOGY 

Aerial photographs have been widely used by geologists of the petroleum industry particularly in the 

mapping of many large-scale geologic structures, 

which are readily recognized on aerial photographs 

because of direct surface expressions of dipping beds. 

The nse of photogeologic techniques in the search for 

structures that may contain oil is continuously increasmg. However, as the obvious and well-exposed 

structures in many areas have long been discovered, 

considerable attention is now being given to indirect, 

or geomorphic evidence of subsurface structures. In 

addition, photogrammetric methods for making measurements needed to compile structure contours and to 
INTERPRETATION OF ·AERIAL PHOTOGRAPHS IN PETROLEUM GEOLOGY 25 

FIGUR~<: 8.-Drainage map of area shown on left half of figure 73. 

~'he t·egionnl. cross-joint trend, expt·essed by the main 

streams on figure 73, is strongly brought out on the drainage 1nap. Note tt·ihutary streams flowing at right angles 

to the main streams. Stt·eam trends are obscured commonly by the abundant imaget·y of the photograph and it 

may be desi mhle to cmnpile separate drainage maps when 

stt·earu characteristics are used to interpret lithologic or 

structural features of an area. 
26 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPR~TATION AND MAPPING 

determine strike, dip, stratigraphic thickness, and 

closure are being used increasingly. 

The indirect evidence yielded by photointerpretive 

studies is based on the premise that subsurface structures may be reflected in surface expressions of various kinds, such as stream or vegetation patterns or 

anomalies. Indirect evidence is particularly useful in· 

the interpretation of areas of few or no outcrops, such 

as areas of low relief, or areas of dense vegetation. 

Commonly in such areas, the evidence points not directly to a structure, but to an anomaly, such as a 

variation in drainage characteristics from the norm 

established for a local region. The anomaly in turn 

suggests a possible subsurface structure. Thus even 

though surface evidence alone may not be conclusive 

in the interpretation of subsurface structure, it may 

provide clues and stimulus for exploration (Alliger, 

1955, p. 183). However, vVheeler and Smith (1952, 

p. 112) cautioned that "* * * strong deformation 

periodically rejuvenated, is restricted to the mobil; 

belts * * *," and structures farthest away from maximum deformation are'"* * * therefore more difficult 

to infer from phy~iographic abnormalities at the surface * * * that without the genetic background of those 

events in the geologic history of the tectonic province 

responsible for surface and subsurface indications of 

structure, photogeology may be unfavorably speculative." 

FOLDS 

Fol~s ~ay. be express~d on aerial photographs by 

t?e distributiOn of bedding, by drainage characteristi.cs, ~y topography, by distribution of soils, or by combinatiOns of these criteria. The surface manifestations 

of folded or domed structures thus may range from 

obvio?s, where direct surface expression of dipping 

beds IS present, to subtle, where subsurface structures 

are reflected only indirectly by physiographic criteria. 

BEDDING 

Many folded structures can be delineated readily 

because bedding is clearly expressed at the surface. 

Particularly in areas where rocks are well exposed and 

structural deformation has been mild, beds may be 

clearly defined by photographic tone together with 

topographic relief or hogback landform (figs. 90, 91 

and 92); strike and dip of beds are easily determined 

and the structure delineated from the distribution of 

beds (see also p. 19) . . 

In areas where bedding may be obscured by low 

plant and grass vegetation, differences in vegetation 

together with slight topographic relief chan()'es en- o ' hanced by vertical exaggeration of the stereoscopic 

model, may permit bedding to be delineated and structures to be structure contoured (see fig. 40 and p. 6). 

DRAINAGE 

In areas where bedding is largely obscured, for 

example by vegetation or surficial materials, streain 

patterns, anomalies, or textures may be helpful in determining dip slopes and outlining folds. In some 

areas dip-slope streams have a coarse-textured arrangement in contrast to the fine-textured arrangement of 

back-slope streams (fig. 79). These textures probably 

reflect coarse-grained and fine-grained rocks, respectively. The distributions of such textures in turn may 

suggest the location of structural axes. 

In many areas stream adjustment to structure will 

result in a sweeping curve of the stream particularly 

across a structural axis and around the nose of a 

plunging fold; the. convex side of" the curve indicates 

the direction of anticlinal plunge (figs. 81 and 90). 

But such curves may also develop on dip slopes and 

thus not be indicative of the location of a structural 

ax1s. This situation may prevail where anticlinal 

structures are breached into softer material. Streams 

developing on the more resistant dip slope ·of the 

struc:.ural flank may curve on that flank and along 

soft 1nterbeds, and not across the axis of the structure 

(fig. 93). Such a condition would probably be most 

pronounced during the mature . phase of the O'eomorphic cycle. In geomorphically young areas ~treams may not have had sufficient time to adjust to folded 

structures, and may haye a tendency to follow joints 

or preexisting faults (Howard, 1940, p. 195). In areas 

where stream pattern or anomaly is the primary sur-

~ace reflection of underlying structure, as in some 

JUngle-covered areas, other criteria, such as vegetation 

, pattern, should be sought to corroborate the location of 

a strJ.Ictural axis suggested by stream characteristics. 

Stream patterns or anom~lies are undoubtedly the 

most useful indirect evidence of subsurface structures 

in many areas. This is especially true for coastal 

areas of low relief, mnny interior basins, or areas of 

dense vegetation or other surficial cover. Tator ( 1951, 

1954) and DeBlieux ( 1949) discussed the structural 

significance of drainage anomalies in the coastal region 

of the Gulf States; in these low, swampy areas of 

aggradation it is important to establish the drainage 

norm by studying the drainage characteristics over a 

large area. Deviations from this norm may give the 

first indication of a structural anomaly. DeBlieux 

(1949, p. 1259) described abrupt locally restricted 

changes in drainage considered to be the result of subsurface structure. Over the Lafitte field in Louisiana 

a relic distributary of the Mississippi River forms two 
INTERPRETATION OF AERIAL PHOTOGRAPHS IN PETROLE~ GEOLOGY 

shn,rp meander bends, although for miles upstream and 

downstream from this area, the stream channel is 

simple and straight. Over the. Scully dome"'* * * two 

relic distributaries of Bayou Lafourche followed 

simple courses, but suddenly changed to an anastomosing pattern over the dome, then resumed simple 

courses south of the structure." (DeBlieux, 1949, p. 

1259.) 

StructuraJ implications of stream pattern or 

anomaly may be corroborated by other geomorphic 

evidence. Over the Lafitte. field the presence of distinct remnn,nts of natural levees on either side of the 

channel, the only such ridges over the length of the 

relic course of the stream, was considered significant 

supporting evidence of underlying structure (DeBlieux, 1949, p. 1259). The levee-ridge remnants presumably were held up by subsurface structure as surrounding areas subsided. 

In a study of tonal and textural anomalies in westcentral Texas DeBlieux and Shepherd (1951) described anomn1ous compressed meanders. over structural highs. In the same area annular and radial 

dndnage patterns and stream deflections in the flank 

areas were important criteria that supported an interpretation of subsurface structure (fig. 94). The importance of abrupt changes in direction of streamflow 

in relation to possible subsurface structures has recently been emphasized by Buttorff ( 1958) and Elliott 

(1958), based on work in the Powder River Basin, 

'V'yoming. Such deviations from the norm for an area 

commonly stand out when a separate drainage map of 

an area is studied (see fig. 9) or when stereoscopic 

models are vie.wed pseudoscopically (see p. 5 and fig. 

95). 

In areas of aggradation and active subsidence it is 

to be expected that abnormal drainage characteristics 

ntt:her than any specific drainage pattern might be 

indicative of subsurface structure. The combination 

of emergence, due to recent rise of salt plugs, and 

subsidence of the general coastal-plain area is responsible for many of the anomalous drainage characteristics of the coastal area of the Gulf States; such 

characteristics are anastomosing stream segments, 

abruptly widened levee ridges, or ttbrupt changes in 

direction of streamfiow. 

In areas of active degradation, patterns of drainage 

rather than abnormal drainage characteristics can be 

expected to be indicators of structure. However, 

vVheeler a,nd Smith stated (1952, p. 82) that the an-

~dysis of stream pattern with regard to structurewhich they ca1led "creei{ology"-is somewhat outmoded as a photogeologic procedure; they implied that 

anomalies in drainage characteristics may be far more 

important than specific drainage patterns. However, 

drainage pattern may actually be anomalous with respect to the drainage norm for an area; such examples 

of drainage patterns have been found along the northwest coast of Alaska on the Arctic coastal plain. 

Here, in an area of northward-flowin(J" streams sliCTht b ' 0 

topographic highs have resulted in radial-dendritic 

drainage patterns, which outline areas of subsurface 

structure (fig. 96). Commonly no other surface expression of structure is present. Because of low relief, 

outcrops are few. 

TOPOGRAPHY 

The presence of topographic highs has been significant in locating subsurface folds or domal structures 

even in many areas where relief is very slight. Many 

of the "islands" of the coastal area of the Gulf States 

are surfa~e expressions of salt domes. Topographic 

features such as natural levees, which represent areas 

of higher ground only a few feet or tens of feet above 

the delta or flood plain, are also significant interpretation criteria in the search for subsurface structure 

(Tator, 1951). Tator reported (1951, p. 717) that 

"* * * cheniers and natural levees * * * provide linear 

patterns of higher ground supporting distinctive vegetation growths." These higher areas are also favored 

for cultivation. According to Tator ( 1951, p. 717) 

"Tonal contrasts in the photographs are the major 

keys used for recognition of landscape features of extremely low relief, such tonal variations being largely 

the result of soil and vegetation differences." The 

significance of low relief, is shown by DeBlieux's deset·iption (1949, p. 1254) of Felicity Island, a 2-milelong natural levee ridge separated by several miles of 

marsh from the area where the levees normally could 

be expected to die out. The preservation of the 

"island" in an area of active subsidence has been 

attributed to local subsurface structure. · Abrupt 

widening of levees on one or both sides of a channel . ' 1n areas of subsidence, also has been noted over subsurface doma-1 structures (DeBlieux, 1949, p. 1254, and 

fig. 2, p. 1255) ; widening of levees may be due to other 

causes, such as coalescence of two levees, however, and 

this feature of the flood plain must be interpreted 

carefully. 

In a study in northern ·Alaska, Fischer ( 1953) used 

topography as the· principal interpretation criterion in 

locating the Square Lake anticline. The area is a 

treeless waste of low relief that is underlain by pennafrost; tundra grasses cover the region, and no bedrock 

is exposed. Fischer (1953, p. 208) reported that the 

original photogeologic study showed small hills in the 

n,rea elongate in the same direction as the reCTional t:l 
28 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

FIGURE 9.-Drainage map of area shown in figure 94. 

Radial drainage over structure, A of figure 94, stands 

out on the drainage map which is unobscured by the 

abundant details of the photograph. Note that the radially 

draining streams flow into larger streams that have an annular pattern around the structure. Stream deflections, as 

at D in figure 94, are commonly more readily observed on 

the separate drainage map or in pseudoscopic view of the 

model (see fig. 95) . 
INTERPRETATION OF AERIAL PHOTOGRAPHS IN PETROLEUM GEOLOGY 29 

structural trend. Normal to this trend other elongate 

hilltops sloped in the direction of the inferred dip of 

beds. Also, streams flowing in the dip direction had a 

slightly less well developed dendritic pattern than 

those flowing opposite to the dip direction. There also 

n.ppea red .to be a correlation between slopes of the tops 

o:f cut banks and the direction of dip. Plotting of all 

such data gave a consistent apparent structural pattern 

-a plunging anticline-of unknown reliability. A 

subsequent seismic survey corrobrated the photogeologic interpretation. 

SOILS 

In some areas devoid of outcrops, soil patterns or 

soil distributions may be useful in delineating or suggesting structures. Both W oolnough ( 1934b, p. 221, 

224) and Christensen (1956, p. 858-859) cited examples of the usefulness of aerial photographs in this 

regard. '\Theeier and Smith (1952, p. 106) stated that 

contmsts in soil tone accentuate unusual drainage relations on the northern flank of a structure in the southwest :Maysville area, Oklahoma; and in southwestern 

Alabama differences in cultivation pattern reflect relief 

and soil characteristics by which geologic units of 

Tertiary age beneath younger unconsolidated materials 

can be differentiated. 

DeBlieux and Shepherd (1951, p. 98) described 

tonal and textural anomalies in west-central Texas 

that resulted from differences in soil types, soil-moisture content, and erosional characteristics. They cited 

an anomaly at Salt Fork on the Brazo River where 

""' * * dark and coarsely hummocky surface strongly 

contrasts with the lighter color and finer texture of 

the surrounding surface * "' *" (DeBlieux and Shepherd, 1951, p. 98). The anomaly was strikingly similar to that of a reef producer in Scurry County. DeBlieux and Shephard ( 1951, p. 88) emphasized that 

how a structure forms is unimportant in a study of 

tonal, textural and other anomalies, because the surface 

evidence is identical, whether the structure has resulted 

from upward movement, differential downwarping, 

reef or residual topography, or combination of these 

features; it is important initially that a surface anomaly can be recognized. 

FAULTS 

IFnults are significant in the geologic interpretation 

of petroleum areas because they may 'be deep seated 

and of major importance in localizing oil accumulation. 'Vhere faults are genetically related to structures 

that may be oil bearing the distribution pattern of 

faults may be of considerable use for interpreting the 

locations of those structures. The many surface expressions of faults are described above (p. 21). 

Desjardins (1952, p. 82) stated that surface fault 

and fracture lines, expressed as lineations on aerial 

photographs, form patterns that bear a definite relation to deep-seated salt domes and to subsurface faults 

of the Gulf Coastal Plain. Disturbance centers, believed to be underlain at depth by salt domes, may be 

indicated by the fault patterns, shown on photographs 

as "Sharply curving lines which are strikingly concentric" or as "Prominent transverse lines radiating 

usually from some marginal point rather than from 

the center of the structure" (Desjardins, 1952, p. 82). 

Desjardins believed that such fault patterns were the 

result of continuous application of small stresses caused 

by rising salt masses rather than an accumulation of 

stresses that would produce large amounts of movement; he also felt that faults probably would form 

in interdome areas as well as over the domes. The 

genetic significance of the fault pattern is thus critical 

if distribution of faults is used as a basis for locating 

the dome itself. Desjardins (1952, p. 83) made a 

strong distinction between surface expressions of faults, 

which he associated with deep-seated domes, and other 

criteria, such as topographic expression, widening of 

natural levees, and looping of stream channels, which 

he associated with shallow-seated domes. 

Although most interpretations of faults are based on 

qualitative characteristics of observed data, such as 

the dislocation of correlative beds, or the contrast in 

light and dark photographic tones on opposite sides 

of a straight line, a quantitative approach to such 

studies may provide additional useful data. On the 

basis of azimuth-frequency studies Blanchet (1957, 

p. 17 48) suggested that structural and stratigraphic 

anomalies in some areas may be located by a statistical 

analysis. of fractures, which he defined as the generally abundant, natural lineations discernible on aerial 

photographs. The basis for analysis is a comparison 

of local deviations in the statistical mean direction 

of fracture sets with regional norms for each fracture 

set. Regional norms are first established by plotting 

fracture-azimuth frequency diagrams of the region involved. Then statistical mean directions of fracture 

sets are determined by analyzing samples within circular areas centered at relatively small horizontal intervals of a few miles. These local mean directions are 

in turn studied for local deviations which are entered 

'into empirical equations that yield structure-intensity 

values. These structure-intensity values are then contoured, to form a structure-intensity map, which may 

reflect structural anomaly. The structure-intensity 

map is supported by a fracture-incidence map, prepared in contour form, and by an analysis of drainage 

characteristics of the area in question. Blanchet applied this method to a study of petroleum structures. 
30 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

FACIES 

Although the widest application of aerial photographs in petroleum geology has been with regard to 

structural traps, a study of aerial photographs may 

contribute to the location of stratigraphic traps, particularly if photogrammetric measurements of formation thicknesses can be made at many points within 

the streoscopic model (see p. 59-60). From a qualitative standpoint it can be seen in figure 97 that a 

rapid loss of sandy section can sometimes be detected 

on aerial photographs. Such "shaling" or loss of sandy 

section due to facies change may be important in the 

search for stratigraphic traps and in some areas it 

may be measured photogrammetrically. In other 

areas well-defined formational units may thin or 

thicken without facies change, but this also may be 

significant in regional studies with regard to stratigraphic traps (Christensen, 1956, p. 861, fig. 6). Measurements of stratigraphic sections needed in such studies commonly can be made from aerial photographs. 

REGION.A,L STUDIES 

The significance of aerial photographs in regional 

petroleum studies is widely recognized, but. very little 

infoqnation. is present in the literature with regard 

to this phase of photograph use. Not only may measurements be made from aerial photographs with regard 

to strike, dip, stratigraphic thickness, and closure, but 

the regional view of the mosaic or small-scale photographs permits a continuity of photographic criteria 

t}:lat may be of structural or stratigraphic significance. 

Overall geologic relations are brought together, and 

areas where detailed study· is needed may be indicated. 

However, the limitations of the photogeologic technique must be constantly-borne-in_f!l_ind. As Wheeler 

and Smith (1952, p. 112) pointed out, the validity of 

the technique "* * * depends a good deal upon the 

human element and the utilization of all applicable 

geologic data." They warned that "* * * the student 

of photogeology may * * * become overly impressed 

with geometric design of lines, masses, and tones so 

that, in the absence of understanding of the tectonic 

history he. may find compatible alignments of stream 

courses, structurally inexplicable lineations or shapes 

and soil patterns that are not consistent with the 

known or expectable products of diastrophism" ( 1952, 

p. 112). An emphasis on regional study is indeed justified, however, if maximum use is to be made of aerial 

photographs, and this is true whether photographs 

are used in long-range exploration programs, as in 

reconnaissance mapping of broad basin areas, or in the 

ultimate study of local structures. 

INTERPRETATION OF AERIAL PHOTOGRAPHS IN 

SEARCH FOR ORE DEPOSITS 

The search for ore deposits, similar to that for 

petroleum, requires favorable host rocks or structures, 

or both, and it is therefore necessary to determine the 

general lithologic character and distribution of rocks 

and associated structures as a basis for carrying out 

a mineral survey of an area. Aerial photographs may 

contribute geologic data not only with regard to the 

fundamental guides of lithology and structure in ore 

search, but they also may reveal details of physiography, which may be an important ore guide in the 

search for placer deposits. Botanical guides to ore 

bodies may also be detected on aerial photographs of 

some areas, although little "~ork has been done in this 

field. 

STRUCTURAL GUIDES 

vVhere fractures have been significant in localizing 

ore deposits aerial photographs may reveal significant 

structural data relating to trends and offsets of fractures, even in areas where surficial cover is extensive. 

This is due in large part to the great variety of criteria suggestive of fractures (see p. 21). In many 

areas it is possible to determine spatial relations of sets 

of fractures, and to infer the relative ages of the fractures from the aerial photographs, although results of 

field study are necessary to det.ermine the significance 

of the patterns. 

Aerial photographs are presently inost useful in 

study of those ore deposits associated with fractures. 

In many mining districts where mineralized fractures 

form a vein pattern that is repeated throughout the 

district, an analysis of the fracture pattern and distribution may provide significant data in the search for -' 

new ore bodies. Reed (1940) described the use of 

aerial photographs in a mineral-deposits study of the 

heavily timbered Chichagof mining district of Alaska 

where gold-bearing quartz veins are associated with 

faults and shear zones (see fig. 99). There the faults 

are commonly expressed as depressions owing to greater 

erosion of soft material from fault zones (see also 

Twenhofel and Sainsbury, 1958, p. 1433). Reed (1940, 

p. 44) stated that many faults of moderate surface 

expression would probably have been overlooked on 

the ground, and that many faults of weak surface 

expression were seen only on aerial photographs. 

,Joliffe (1945, p. 604), in a discussion of photogeologic 

prospecting in the Yellowknife area of Canada, noted 

that ·some of the largest gold-ore bodies "* * * lie in 

strong shear zones that are marked by rather distinctive gullies in relatively massive, resistant volcanic 

rocks." Many of these shear zones and other lineaments are conspicuously shown on aerial photographs. 
INTERPRETATION OF AERIAL PHOTOGRAPHS IN SEARCH FOR ORE DEPOSITS 31 

Gross (1951) pointed out that in the glaciated Drysden-Kenont area of northern Ontario and the Goldfields area of northern Saskatchewan, Canada, there 

are many 1 in ear features seen on aerial photographs 

but that only certain ones represent faults. He devised a scheme of statistical analysis to show the likelihood that certain linear trends were faults (see p. 21). 

Concentration of prospecting along these linear features :in the Goldfields area resulted in discovery of 

pitchblende deposits. 

Because faults commonly are interpreted readily 

from 1teriaJ photographs, photogeologic study may aid 

in locating extensions of faulted ore bodies. ·Faults 

that offset an ore body may show an orderly spatial 

arrangement, and information regarding direction of 

displacement may be extrapolated to similar faults 

whose displacement is masked by vegetation or sur-· 

ficial debris. "Tillett ( 1940) found that aerial photographs provide significant information on .the location 

of faults cutting the Invincible Lode, a gold-quartz 

deposit in the Glenorchy district of New Zealand.· The 

photographs strikingly show the offset of a lode east 

of the Invincible Lode by a series of parallel faults, 

which suggest that similar conditions exist at the Invincible Lode where the evidence is not as readily 

observed because of surficial debris. In the Invincible 

Lode arett one major stream flows parallel to the strike 

of the faults, and a second stream is deflected along 

It :fault for about 1,000 feet, then bends back to its old 

course; the direction of deflection also indicates the 

horizontal direction of displacement of the quartz lode. 

The pattern of displacement, based on photogeologic 

evidence together with ground evidence, suggests the 

location of faulted extensions of the lode. 

~1aj?r anticlinal and domal structures, readily interpreted from aerial photographs of many areas on the 

basis of drainage characteristics or strike and dip 

patterns, are with notable exceptions of relatively little 

importance in ore search. 'Vhere folds have been n. 

factor in localizing ore, as in some metamorphic rocks, 

they commonly are too small or the geology too .complex :for existing aerial photographs to provide useful 

guides in prospecting. 

LITHOLOGIC GUIDES 

:Lithologic guides may be considered from the standpoint of the parent and host rocks, or from the standpoint of mineralized or altered zones, which are more 

a.ppropriately termed mineralogic guides. Photogeologic interpretation of rock types (see p. 16-19) may 

outline broad target areas of favorable parent and 

host rocks (fig. 100). Lueder (1953, p. 823) suggested 

that various locations can be rated on the basis of 

favorable photogeologic evidence; the most favorable 

area would become the prime target area for further 

investigation. ''There some local field criteria are available, specific ore-bearing formations may be traced on 

photographs (fig. 98). At Ross Lake, Canada, the 

fact that rai·e-earth-bearing pegmatites of small areal 

extent were recognizable on photographs was a major 

factor in the discovery of certain tantalite deposits 

( J oliffe, 1945, p. 604). 

The use of color aerial photography will increase 

many times the amount of lithologic information that 

can be obtained from a photogeologic studx. This 

additional information will contribute not only to the 

reliable interpretation of host or parent rocks, but may 

permit local guides, such as alteration zones, to be 

readily recognized. In a study of the hydrothermally 

altered volcanic terrain near Tonopah and Goldfield, 

Nevada, information obtained from reversible-type 

color transparencies was contrasted with data obtained 

from thin sections of rock types. Kent ( 1957, p. 868) 

stated that "In both areas a general correspondence 

between coloration and alteration was observed. Zones 

of different coloration within a lithologic unit represent different stages of alteration. Colors of rocks in 

early stages of alteration are useful in recognizing 

lithology, but highly altered rocks have the same color 

within several different lithologic units. The· colors 

associated with highly altered rocks are easily recognized on color aerial photographs; thus their distribution and their relationship to structural fea:tures can 

be studied." In addition Ray (1958, p. 37) wrote that 

"In one area of altered andesites a pale green color 

on color aerial photographs was found to represent an 

intermediate stage ·in the alteration resulting from 

partial silicification, with pa.le green chalcedony, and 

deuteric alteration and la.ter oxidation of iron opaques. 

A light color in another zone wa.s found to result from 

brecciation ·of dacite and attendant alteration . to carbonates. A rather intense orange color was indicative 

of highly altered zones in all localities observed. However, the orange coloration did not indicate the rock 

type involved nor the type of alteration present, but 

rather was the result primarily of weathering of iron 

minerals following their partial breakdown by deuteric alteration." 

PHYSIOGRAPHIC GUIDES 

Some ore deposits are topographically expressed or 

are related to structures, rock types, or surficial deposits that are so expressed (fig. 102). Hence, topography may be considered a. guide in ore search, although 

it does not by itself indicate the presence of ore ... ,Because fractures commonly are topographically ex-
32 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

pressed as rectilinear depressions, a possible physiographic guide to ore as interpreted from aerial photographs may be said to exist in many areas. Where 

fractures have been mineralized, and the vein filling 

stands in relief, topography may be a useful guide in 

ore . search. However, many mineralized veins are 

poorly resistant to weathering and erosion, and occur 

as topographically low areas. 

Where topography results in a photographic texture 

it may be possible to delineate areas of favorable host 

rocks, as the paraschists in Surinam ( Zonneveld and 

Cohen, 1952, p. 155). Grantham ( 1953, p. 336) pointed 

out that in the wet Sierra Leonne area of Africa, the 

"* * * sharp change in topography from linearly arranged hills, controlled by schist and migmatite, contrasts with the irregular features of structureless 

granite." 

An analysis of the physiography of an area may 

provide significant information, particularly in relatiqn 

to placer deposits. As many placers have a complex 

history, it may be necessary to work out the alluvial 

history of a valley or valley system for maximum information in locating placer pay streaks. The arrangement and positions of old stream channels may be interpreted from meander scars, oxbow lakes, and different ter_race levels that are commonly seen on aerial 

photographs. Photographs also may provide gradient 

measurements of present or past streams; this information may· be significant inasmuch as placer concentrates occur especially where stream gradients flatten. 

Buried stream channels, also the site of some ore deposits, may be interpreted from photographs of some 

areas on the basis of topographic expression (fig. 101). 

In many parts of the western Sierra Nevada, Tertiary 

stream valleys have been filled and gold-bearing gravel 

buried by younger lava flows. The greater resistance 

of t!le lava flows compared with the surrounding rocks 

has resulted locally in sinuous cappings of volcanic 

rocks that stand above the present-day stream system. 

Table Mountain, California (Loel, 1941, p. 384, 386-

387), is an outstanding example of basalt capping 

auriferous gravel of a buried channel. 

The location of stream-channel deposits recently has 

received wide attention in the search for uranium deposits on the Colorado Plateau in Western United 

States. There the Triassic Shinarump member of the 

Chinle formation is thicker in channels cut in the 

underlying Moenkopi formation, which is also of Triassic age. Some channels can be observed directly on 

aerial photographs (fig. 44) ; others may be inferred 

from is<;>pach measurements, which in some areas may 

be obtained from aerial photographs. (Seep. 58-59). 

The uranium is associated with carbonaceous materials, 

largely plant remains, sc.attered throughout the channels. 

BOTANICAL GUIDES 

Much field and laboratory work has been accomplished with regard to plant distribution and plant 

concentration of certain elements as indicators of ore. 

Little has been done, however, on plant characteristics 

or associations that may be observed or interpreted 

from aerial photographs, and herein lies a field of research that has much potential. There appear to be 

three aspects of botanical study from aerial photographs that may contribute to the location of ore 

deposits. These involve (a) color differences in vegetation brought about by concentration of certain chemical elements, (b) species differences or absence of vegetation owing to concentration of certain chemical 

elements, and (c) distribution of vegetation suggesting 

favorable structures or rock types. Color differences 

in vegetation, due to mineral and chemical concentrations, that may be reflected on aerial photographs have 

received almost no attention, and offer an interesting 

field of study. Species differences or lack of vegetation 

has likewise received little attention in ore search, a1-

though Walker (1929, p. 50-51) noted that air search 

was made for "dambos" as indicators of copper deposits in central Africa. Dambos are open spots in the 

bush cover, where vegetation does not grow because of 

the concentration of poisonous copper salts in the soils. 

Walker noted that the Roan Antelope copper deposit 

was marked by such a dambo, although the deposit 

extended beyond the limits of open area. Identification 

of species froni color differences due to concentrations 

of certain chemical elements may be particularly amenable to detection on color or other special aerial photography. 

Plant and tree distribution may be useful indicators 

of structures or of rock types favorable to ore deposition; vegetation distribution may reflect physical characteristics of the rocks, such as porosity or presence of 

fractures, although some rocks, such as limestone, may 

be preferentially covered by a certain type of tree because of chemical concentrations in those rocks (see 

fig. 72) rather than because of physical properties. Or 

vegetation may grow preferentially or trees grow 

higher and be alined along a fault or shear zone, which 

might in turn be a favorable site for ore deposition 

(see fig. 71). Such indirect guides in ore search 

contrast highly, however, with color or species differences, which might suggest more strongly the presence 

of ore minerals. 

Levings and Herness ( 1953, p. 456-457) stated that 

heavy timber growth in the Corbin-Wickes area of 

Montana shows a preference for unaltered quartz mon-
INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING GEOLOGY 33 

zonite, which has a darker photographic tone than the 

surrounding andesite and dacite; ore occurs in the 

volcanic rocks. Zonneveld and Cohen ( 1952, p. 153) 

mention that vegetation differences permit photographic difl'erentiation of the clay savannahs and sand 

sa,vannahs of Surinam, South America. Bauxite production at Surinam's Billeton mine (Hemphill, oral 

communication, 1958) is from a clay area, and hence 

it is important to know the distribution of such clay 

areas. 

INTERPRETATION OF AERIAL PHOTOGRAPHS IN 

ENGINEERING GEOLOGY 

Aerial photographs have been used for many years 

to provide topographic and land-use information in 

locating routes for pipelines, highways, or other rightsof-way, but only in recent years have photographs 

attained wide recognition as a source of geologic information for engineering purposes. ~1any kinds of 

geologic information for engineering use may be obtained from aerial photographs. These include (a) 

identification and location of soil materials, particularly granular materials, which would provide suitable 

foundation sites for industrial and airport developments, or highway rights-of-way; (b) identification 

and location of clay and clayey silt soils, which would 

have detrimental properties with regard to building, 

airport, and highway sites; (c) location of aggregate 

materials; (d) delineation of areas that may be underlain by permafrost; (e) interpretation of distressed 

conditions in landslide areas; (f) analysis of structural 

geology in areas of tunnel, dam, and reservoir sites; 

and (g) lo.cation of sample areas for detailed investigation of soil and rock materials. 

SURFICIAL MATERIALS 

Surficial materials generally are interpreted in terms 

of gross physical characteristics, rather than the detn.iled physical characteristics used by the pedologist. 

l(now ledge of the gross· physical properties is sufficient 

for many reconnaissance studies in engineering 

geology. Indeed, it is the grouping of several pedologic soil types into a few classifications that permits 

aerial photographs to be of significant engineering use. 

In the preparation of soil-engineering maps from agricultural reports of an area in Illinois, Thori1burn 

(1951, p. 91) was able to reclassify 99 different pedologic soil types into 13 engineering groups. Lueder 

(1951) described the use of aerial photographs as a 

major tool in preparing soil-engineering maps. 

The inability to interpret fine details with regard to 

physicnJ characteristics of the soil thus does not preclude the use of aerial photographs in many engineering geology studies. From the standpoint of engineer~ 

ing significance a granular material, for example, 

which exhibits certain diagnostic photographic characteristics such as coarse-textured drainage, will generally react favorably to drainage, compactability, and 

other engineering tests, regardless of wh~ther it is a 

residual granular ~aterial that has resulted primarily 

from weathering in place or whether it is a transported gravel derived from different rock types. However, Jenkins, Belcher, Greeg, and Woods (1946) 

pointed out exceptions to the general thought that 

photographic characteristics common to granular materials always signify suitable granular materials for 

engineering purposes. For example, the soil profile of 

well-drained limestone has a granular texture due to 

aggregation of clay particles into lumps. The surface 

drainage may be coarse textured; but the soils break 

down on com paction and react as plastic soil material, 

and not as a granular material (Jenkins, Belcher, 

Greeg, and vVoods, 1946, p. 80). 

"\Vhere pedologic data are already available they 

may be very useful to the engineer when interpreted in 

terms of engineering test data (fig. 4). Indeed, certain soil mechanics tests by the engineer, such as 

plasticity or soil-texture tests, are similar to those·used 

by the pedologist. Greenman ( 1951) suggested that 

the pedologic method can become the nucleus around 

which aerial photo interpretive and field geologic information can be accumulated. But Eardley ( 1943, 

p. 567) pointed out that "In many places conditions 

unfavorable for the development of a normal soil, 

such as rugged mountains, wide sandy plains, or 

swampy basins may cover rather large areas. The 

imporbtnt construction materials in such places may be 

better interpreted as a result of geological processes 

than from the standpoint of a soil classification * Jl' *." 

· GROUND CONDITIONS 

The interpretation of ground conditions probably 

has received more attention in recent years than any 

other engineering geology application of aerial photographs. Much of the literature concerns reconnaissance studies of soils in general and granular materials in particular; granular materials are essential 

fo}: building sites and highway and airport development (Jenkins, Belcher, Greeg, and "\Voods, 1946; and 

Purdue University, 1953). Special attention also has 

been given to photointerpretation of permafrost conditions in northern latitudes. 

vVith regard to highway engineering, Belcher ( 1945, 

p. 144) stated that "* * * the soil problem resolves itself into two parts, one in which the soil in place is 

satisfactory as subgrade, and the second, where the soil 

is unsuitable. In areas of unsuitable soils some form 
r 

34 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

of improvement, by stabilization or insulation, is required. Insulation involves the economic location of 

granular materials in the form of rock to be crushed, 

cinders (volcanic), or sand and gravel** *. Inasmuch 

as sand and gravel or other granular deposits exist to 

some extent in nearly every county in the country the 

sigr.ificance of soil patterns indicating granular matP.cials is of considerable economic importance to engi-

!teers." In fact, a large percentage of construction and 

maintenance expenses are in the costs of sand, gravel, 

crushed rock, and similar materials. As a tool for 

interpreting areas of gravel, sand, silt, and clay, aerial 

photographs offer an inexpensive and rapid method of 

reconnaissance, and according to Schultz and Cleaves 

(1955, p. 370) "* * * planning and exploration for 

large engineering projects should almost never be 

undertaken without a thorough study of aerial photographs of the region * * *." 

Photointerpretation of surficial materials must consider not only the surface expressions of these materials but also the geologic factors reflecting the origin 

of these materials. A ·knowledge of the origin of materials permits inferences as to what kind of materials 

cari. be expected in an area, and in what locations with 

respect to other geologic features. Field -sampling programs also could be based on the analysis of the photogeologic data, and subsequent mineralogic and related 

studies by the engineering geologist could provide the 

soils engineer with information that would eliminate 

the need for many of the soil mechanics tests now 

performed as part of the routine laboratory analysis 

of all soil samples. 

ELEMENTS OF SOIL PATTERN 

The surface expressions of granular and other soils 

materials have been termed collectively the "soil pattern." The soil pattern consists of several elements of 

which the most important ones for nonpermafrost areas 

are considered to be landform, drainage, erosion, relative photographic tone, and color; vegetation cover and 

land use may also be significant (Belcher, 1944, 1945, 

1948; Jenkins, Belcher, Greeg, and Woods, 1946; Frost, 

1946; Mollard, 1947; Hittle, 1949; and Purdue University, 1953). Schultz and Cleaves (1955, p. 371) considered that "interpretation of soils by means of aerial 

photographs rests largely on the interrelationships of 

~oil~;an.d geomorphic features"; consequently landform 

IS considered by some to be the most important element in photointerpretation of soils. Landform, however, is only one element of the soil pattern and it is 

clearly evident that the association of several elements 

of the soil pattern will provide the greatest amount of 

information from aerial photographs. 

Landform 

Identifying the landform commonly identifies the 

natural process that formed it, and commonly limits 

the type of soils or soil characteristics that can be expected. The emphasis on landform is probably valid 

in reconnaissance soils investigations where broad areas 

of several different landforms.are studied and mapping 

is at a small scale. But where detailed engineering information is needed, as in foundation studies, it is 

likely that the area of study will be within one landform, mapping will be on a large scale, and other 

elements of the soil pattern, such as differences in relative photographic tone, which may reflect soilmoisture content, will be of prime importance. 

In small-scale mapping the emphasis on landform is 

probably warranted particularly for constructional 

landforms of transported materials, such as sand 

dunes, river terraces, alluvial fans, and many glacial 

landforms, all of which have distinctive shapes, although not necessarily unique (see figs. 52, 55-57, and 

104); here the transported soil materials make up the 

landform. In contrast residual soils associated with 

destructional landforms may be more difficult to interpret from aerial photographs, for here the parent rock 

makes up the landform, and the significande of soils 

rests in part on the subjective interpretation of rock 

types. However, the interpretation of other elements 

of the soil pattern with regard to physical characteristics of soils are just as valid for residual as for transported materials; the engineering significance of many 

residual soil areas can be interpreted readily from 

aerial photographs on the basis of these other elements, 

such as d·rainage arid erosional characteristics (figs. 

63 and 65). 

Drainage charactertsttcs 

Highly permeable soils will have good drainage regardless of their origin, provided that topographic 

positions will permit draining. In well-drained soils 

surface drainage normally will be coarse textured or 

even absent (figs. 49, 52, 66, 104, 106, and 108) ; in 

soils of low permeability drainage will normally be 

fine textured (fig. 63) or ponds may be numerous (figs. 

57 and 66) .. The presence of fine- or coarse-textured 

drainage in surficial materials, like that in bedrock 

(p. 16), is primarily related to the relative resistance of the different materials to erosion, and hence 

related to permeability and grain size. Coarse materials characteristically are resistant t<;> erosion, are 

permeable, and have a coarse-textured drainage; fine 

materials commonly are less resistant to erosion, are 

impermeable, and have a fine-textured drainage. These 

relations are significant in the in.terpretation of engineering soils from aerial photographs because drain-
INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING GEOLOGY 35 

age texture can be readily observed. However, exceptions to these relations may exist, as with loess, which 

is commonly characterized on aerial photographs by a 

relatively fine-textured drainage (see figs. 105 and 

106) ; yet, loess in its natural environment is generally 

considered well drained internally. 

Flow or channel markings, for example on terraces, 

indicate that the material was deposited by running 

wnter n,nd further suggest that the material may be 

granular. Frost (1946, p. 121) described flow markings in nn aren in the midwest. as consisting of light 

and dark streaks usually parallel to the· direction of 

flow. The light streaks are ridges of sand or fine 

gravel thnt nre probnbly sand and gravel bars; the 

d~rk streaks are fine clay or organic material deposited 

in depressions that once contained standing bodies of 

water. Frost ( 1946, p. 121) stated that "The chief 

significance of such markings lies in the fact that they 

suggest stream deposition and in order to have stream 

deposition at such elevated positions and occurring on 

* * * a large scale, there must have been large volumes 

of swiftly flowing water. Since swiftly flowing water 

allows only coarse material to settle, the terrace must 

contain a large percentage. of gravel." It may be debatable tlutt channel markings always indicate swiftly 

flowing water, but where some doubt exists other elements of the soil pattern may reveal the character o£ 

the terrace material. 

In broad nreas of aggradation, such as the Mississippi embayment, drainage characteristics revealed by 

aerial photographs are significant to the engineering 

geologist, ns different soil characteristics are associated 

with the several features of valley alluviation. For 

exnmple, bnckswamp areas contain clay and silty clay 

of high organic content, whereas natural levees consist 

of silt, silty sand, and some silty clay (Fisk, 1944, p. 

18, 20). The levees commonly "* * * provide linear 

patterns o~ higher ground supporting distinctive vegetation growths" (Tator, 1951, p. 717). Low areas may 

be swn.mpy and also contain distinctive vegetation. 

Grnveliferous deposits fill the .old valleys and in turn 

are covered by various other deposits; sandbars contain 

permeable sand that grades downward into the gravel. 

The distribution of features of the flood-plain drainage may be significnnt in analyzing ground conditions 

and locating favorable engineering materials in highway, rail road, and airport development. 

Erosional chara.ctertsttcs 

Closely related to the drainage characteristics of a 

soil are its erosional characteristics; these are important in photointerpretation studies chiefly from the 

standpoint of gully erosion, which is controlled largely 

by the physicnl properties of the soil and especially by 

cohesion. The usefulness of gully erosion characteristics in photointerpretation primarily involves the shape 

of transverse profiles, although longitudinal profiles 

may provide additional information. According tor~­

search workers at Purdue University (1953, p. 15-16). 

"* * * there are three basic gully characteristics which 

are associated with three major soil textural groups." 

Granular soils commonly develop sharp, V-shaped 

gullies that have short, steep gradients. Nongranular 

cohesive and plastic soils are generally indicated by 

uniform gentle gradient of gullies that extend well 

back into the upland, and by broadly rounded shallow 

V-shaped transverse profiles. The loess soils and 

sandy-clay soil exhibit U-shaped cross sections of 

gullies that have flat bottoms and low gradients. 

Although there may be a general adherence Qf gully 

characteristics to the major soil textural groups, exceptions are not uncommon. For example combinations of the basic gully characteristics "* * * occur in 

'layered soils' or in soils exhibiting a 'strong profile'". 

(Purdue University, 1953, p. 16), and compound 

gradients may develop. In addition to the steep or 

vertical slopes in deep gullies, windblown silt may develop silt pinnacles and catsteps or terracettes (Purdue 

University, 1953, p. 45). It has been pointed out 

(Purdue University, 1953, p. 27) that climatic environment under which gullies form is an important consideration; for example, soft clay shale in an· arid 

region where flash floods may occur commonly exhibit 

abnormally steep slopes in contrast to clay shale in. 

areas of uniform rainfall where such slopes are u~ually 

softly rounded. Under certain conditions, such as 

gravel capping on clay so"ils in the New Jersey coastal 

plain (fig. 108), gullies in the underlying clay soils 

may be steep and deeply incised, in contrast to the 

usual broad shallow transverse gully profile of the clay 

soil group in humid regions. 

In permafrost areas different materials may yield 

the same cross-sectional profile. Gravel deposits cemented by ice may have vertical gully walls, or may 

have gullies. that are asymmetric in cross section if one 

side is preferentially exposed to the sun and thaws at 

a faster rate than the shadow side. The insulating 

materials overlying frozen deposits also affect the rate 

of thawing and hence the erosional characteristics of 

gullies. Interpreted carefully, however, the erosional 

characteristics-gully cross section and gradient-may 

reveal useful information on texture and other engineering characteristics of the soil. 

Photographic tone 

On black-and-white photographs the significance of 

color must be interpreted in terms of relative gray 

photographic tones. Belcher stated ( 1948, p. 486) that 
36 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

"Light color tones (grays) are usually associated with 

well-drained soils, while clays, principally because of 

their water-retention capacity, appear dark." Frost 

(1946, p. 122), in describing the mottled soils in In~iana, stated that mottling nearly always indicates 

gravel. The mottling is due to "dark areas" which are 

"* * * small clay-like pockets that have been formed 

by the normal weathering processes of the gravel-sized 

material" (Frost, 1946, p. 122). These pockets are 

depressions that "* * * act as small infiltration basins 

which result in a higher moisture content and a darker 

color pattern" (Frost, 1946, p. 122). This mottled soil 

pattern contrasts with that of the drift plains in other 

areas where light-toned patches of slightly higher and 

dryer silty soils adjoin lower areas of wet plastic silty 

clays. Somewhat similar mottled patterns have been 

described by Gwynne ( 1942, p. 202-205) from the drift 

plain in Iowa (see fig. 107). But light photographic 

- tones also may result from lack of moisture, owing to 

lack of precipitation as in many arid regions, rather 

than because of good drainage, and thus tone may be 

of little significance in evaluating soil texture in some 

areas. Or light tones may be the result of _topographic 

position, and · it is probable for example that a 

"* * * sand dune on a sand plain in a humid region 

will photograph light ih color against a dark background" (Purdue University, 1953, p. 26) not because 

of differences in soil textures but because of topographic position. Climatic conditions also influence the 

colors of soils, especially the residual soils, and dark 

color rather than light color may be associated with 

well--drained materials; as certain red soils developed 

from limestone (Jenkins, Belcher, Greeg, and Woods, 

1946, p. 84). Yet, provided climatic conditio~s are 

considered and provided relative photographic tone is 

borne in mind, the generalization that light photographic tones suggest good drainage and dark tones 

poor drainage may be useful in the overall interpretation of soils with regard to moisture content. Where 

tone is due to moisture, infrared photography will result in strong tonal contrasts between soils of different 

moisture content. 

Color 

Color has been cited as a principal element of the 

soil pattern that may yield information on the texture 

and drainage character of the soil profile1 but to date 

very little interpretive work has been done with color 

photography. Belcher (1945, p. 138) briefly mentioned 

experimental color photography taken in different 

parts of the United States and stated that "The value 

of color * * * lies in the added detail that it furnishes 

with respect to these two elements of the soil pattern" 

(that is, .soil color and details of vegetative cover). It 

would seem that color photography would reveal pedologic information significant in engineering evaluation 

of soils, especially the residual soils, which may be 

more difficult to interpret in general than transported 

soils, particularly fro.m black-and-white photographs. 

Vegetation 

Vegetation as an element of the soil pattern has been 

considered one of the most difficult to interpret. It is 

known, for example, that certain trees, such as aspen, 

are tolerant of many different soils and soil conditions, 

but some types of vegetation are strongly influenced by 

soil type. Furthermore, vegetation may be influenced 

significantly by climatic factors. Where permafrost is 

close enough to the surface to affect tree and shrub 

roots the kind of vegetation that will grow may be 

restricted (see p. 37). Species identification may 

be difficult in any area unless large-scale photographs 

are available or unless pure stands are present. Nevertheless, vegetation may reveal significant information 

in engineering studies provided valid interpretations 

are made with regard to how vegetation is associated 

with soil textures, soil-moisture content, and topography; thus a knowledge of plant ecology is necessary if 

maximum information on soil conditions is to be obtained from aerial photographs. Willows, for example, 

indicate wet ground, poplar indicates dry ground, and 

jack pine implies sand and gravel beds (Belcher, 1945, 

p. 139). However, where abrupt changes in soil conditions exist, it is likely that vegetation changes will 

also occur (figs. 56 and 72) , and areas needing further 

ground surveys may be easily delineated by inspection 

of aerial photographs without any basic knowledge of 

plant ecology. For example, stabilized sand dunes in 

interior Alaska can be easily delineated because they 

are marked by stands of dark-toned spruce (see fig. 

56), in contrast to light-toned brush and grass of the 

interdune areas. This relation may be found wh~re 

the topographically higher, well-drained sand of the 

dunes contrasts with frozen silty deposits of the interdune areas. 

Where phreatophytes-plants and trees that obtain 

water from the water table-can be delineated (see p. 

40), aerial photographs may p~ovide significant 

en<-rineerinO" data because phreatophytes may choke b b . 1 overflow channels and thus increase the flood potentia 

of some areas. Information pertaining to the downstream de-silting effect of phreatophytes with regard 

to dams and reservoirs may also be obtained from 

aerial photographs of some areas (see Robinson, 1958, 

p. 28-29). However, little interpretative work in this 

field has been done. 
INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING . GEOLOGY 37 

Land use 

Land use may also reveal information on the soil 

conditions of an area. Belcher (1945, p. 142) reported 

that "Orchards thrive in well-drained locations and 

therefore, when observed on level ground, good subdrn,inage is implied." Shallow drainage ditches in 

areas of little relief commonly signify plastic, poorly 

drained soils. Schultz and Cleaves (1955, p. 378) 

stated that in eastern France and western Germany 

""' * "' rugged topography and associated sandy soils 

developed on sandstones are generally left in forest; 

the comparatively level topography and associated 

clayey soils developed on shales and limestones are 

cul t.i vated * * *." On the flood plain of the Mississippi River the silty sand of the natural levees is cultivated primarily beca,use of its topographic position 

above the lower backswamps. In parts of the Valley 

and Ridge province of Eastern United States the limestone areas commonly are farmed whereas shale areas 

may be left in forest. 

PERMAFROST 

Permafrost has a significant effect on engineering 

characteristics of soils of the far northern latitudes 

and has received considerable attention in recent years, 

particularly from the standpoint of photointerpretation (Cabot, 1947; "Voods, !-little, and Frost, 1948; 

Benninghoff, 1950; Frost and Mintzer, 1950; Frost, 

1951; Sager, 1951; Black, 1952; Hopkins, J(arlstrom, 

and others, 1955). Considerable disagreement exists, 

however, on the significance of photointerpreted · ~ata 

.in the study of permafrost areas (see Black,· 1952, 

p. 126-127, 129), although it is generally agreed that 

aerial photographs can be particularly useful in such 

studies. The following discussion of ground conditions 

in permafrost areas is taken from the work of Hopkins 

ltnd J(arlstrom (I-Iopkins, J(arlstrom, and others, 

1955), except as noted by specific references. 

1\1uller (1947, p. 219) defined permafrost as "a thickness of soil or other surficial deposit or even bedrock, 

at It variable depth beneath the surface of the earth in 

which a te.mpentture below freezing has existed continuously for a long time (from two to tens of thousands of years)." 1\1ulle.r (1947, p. 30) also stated that 

"Frorn the standpoint of engineering, the content of 

ice in frozen ground is of paramount importance * * * 

[because] ·* * * thawing of this ice produces excessive 

wetting a:nd undesired plasticity of the ground and 

renders it unstable and susceptible to settling, caving, 

and even flowing." Building foundations, highways, 

rnilbeds, and airport runways may be considerably 

damaged unless the presence of permafrost is taken 

into consideration and measures taken to eliminate 

or minimize the effect of the thawing that results from 

the disturbance of the natural thermal regimen. Where 

bedrock or thick frozen gravel deposits that contain 

local drainage channels are the primary foundation 

materials, permafrost is not normally detrimental to 

construction. 

Although aerial photographs are useful in the interpretation of areas that may be underlain by permafrost, criteria used for interpreting soil conditions of 

nonfrozen areas apply only to a limited extent to 

permafrost zones. Gully characteristics of an impermeable gravel deposit cemented by ice, for example, 

will not necessarily be the same as the characteristics 

of gullies in impermeable nonfrozen soils. Because of 

the altered permeability and porosity of soils owing to 

permafrost, erosional characteristics, as applied to the 

interpretation of nonpermafrost areas, may have little 

significance in the perenially frozen regions. Other 

criteria, such as the common landforms, although suggestive of type of material present, are of limited use 

in determining the presence. or absence of permafrost. 

Few diagnostic indicators of permafrost appear to 

exist. More often than not the criteria are suggestive 

rather than indicative; they include distribution ancl 

type of vegetation, polygonal relief patterns, pingos, 

and features resulting primarily from thawing. 

Distribution and type of vegetation 

Distribution patterns of trees and shrubs commonly 

supplement topographic expression in recognizing 

landforms; these patterns may contribute indirectly to 

the jnterpretation of probable permafrost conditions 

based upon inferences concerning the age of the land 

surface, climate, and character of the underlying material. In addition, the type of vegetation may be 

useful in delineating areas where permafrost is likely 

to be present. Hopkins and Karlstrom ( 1955) reported that recognition of shallow-rooted species on 

aerial photographs aids in delineating the areas where 

permafrost is most likely to occur at shallow depth, 

and that recognition of deep-rooted species helps 

delineate areas least favorable for the formation or 

preservation of. permafrost. Thus, black spruce may 

suggest areas underlain by permafrost at shallow 

depth, whereas tall willow shrubs and isolated pure 

stands of balsam popular on river flood plains generally indicate unfrozen ground. The distribution patterns of trees and shrubs, however, are most significant 

only when evaluated together with other information, 

such as type of landform, concerning permafrost distribution in the region under study. 

Polygonal relief patterns 

Polygonal relief patterns commonly can be identified on aerial photographs, but as they may be present 
38 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

in both permafrost and nonpermafrost areas this criterion is not, by itself, always diagnostic in evaluating 

permafrost conditions; however, in conjunction with 

other features it may be very significant. Polygonal 

patterns may be divided into two broad groups: those 

formed by frost-stirring and those formed by contraction. The formation of frost-stirred polygons is 

favored by the presence of permafrost at shallow 

depth, but these polygons are not necessarily an infallible indicator of shallow permafrost as they are 

also known to occur where permafrost is absent or is 

at a considerable depth. 

Contractional polygons are subdivided into lowcenter, high-center, and frost-crack polygons. Lowcenter polygons, one of the most dependable indicators 

of permafrost, are commonly found in poorly drained 

depressions. The presence of pools of water in the 

centers of some polygons is a diagnostic recognition 

feature, and it is common to find low-center polygons 

expressed on aerial photographs as dark-toned centers 

surrounded by light-toned marginal ridges (figs. 109-

112). High-center polygons occur in slightly better 

drained areas than those in which low-center polygons 

are found; generally, the centers are relatively level, 

although they may be domed where frost is extremely 

active. In contrast to low-center polygons, drainage 

around high-center polygons is commonly concentrated 

in marginal trenches, and pools of water are present at 

trench intersections. Because of the high centers, 

these polygons are commonly expressed on aerial photographs .as light-toned centers surrounded by darktoned margins (figs. 40 and 109-112). Frost-crack 

polygons may be difficult to distinguish on aerial photographs, as they are similar to high -center polygons; 

they may be present in some .areas where permafrost 

is absent. 

Other relief patterns in conjunction with vegetation 

distribution result in striped slopes that are commonly 

present in permafrost areas (fig. 40). 

Pingos 

Pingos are rounded or elliptical steep-sided hills 

formed by the "downward freezing of a body or lens 

of water or of semi-fluid mud" (Muller, 1947, p. 59). 

They may be as much as 300 feet high. Fissures may 

develop along the axis of a pingo or radiate from the 

crest. Pingos usually are found in poorly .drained 

areas and normally are readily distinguished on aerial 

photographs by their distinctive local landform (figs. 

110 and 111); they are almost always a reliable indicator of permafrost, although locally they may be confused with erosional knolls or hilis. 

Features resul tlng from thawing 

Two features associated with thawing are suggestive 

. of the presence of permafrost. These are thaw lakes 

(sometimes called thermokarst lakes) and beaded 

drainage. Thaw lakes occur in frozen fine-grained 

sedimentary deposits that generally contain clear lenses 

and masses of ice whose volume is greatly in excess of 

the ·porosity of the unfrozen material. As this ice 

melts the ground subsides and forms a basin in which 

a thaw lake may accumulate (figs. 110 and 111). When 

caving is progressing, banks are ragged and steep or 

overhanging. Tilted trees known as "drunken forests," 

along the margins of such lakes are indicative of 

active caving .. In treeless areas active caving may be 

indicated by tension cracks parallel to the banks. 

Other indications of thawing, such as serrated margins 

of lakes, also may be present (figs. 109-111). The 

usefulness of thaw lakes as permafrost indicators, 

however, is limited by the difficulty of distinguishing 

on aerial photographs between active thaw lakes, relict 

thaw lakes, and similar-appearing lakes that are in no 

way related to permafrost. 

Beaded drainage occurs particularly in perennially 

frozen peat and silt containing ice wedges; it is characterized by small pools connected by short watercourses that may be sharply incised (figs. 40 and 112). 

The pool banks commonly are steep as a result of 

thawing and caving of frozen materials. 

Absence of permafrost 

In areas of disconti:r:mous permafrost it is not only 

desirable for the engineer to know where permafrost 

is present, but it may be equally important to determine where permafrost is absent. Certain hydrologic 

phenomena and elements of the soil pattern commonly 

indicate or suggest the absence of permafrost. For 

example, subterranean drainage in unfrozen zones 

within permafrost may be detected on aerial photographs of some areas. The recognition criteria are in 

part similar to those applied in evaluating soil conditions in nonpermafrost areas of the world. That is, 

streams :may disappear into the bottoms of closed depressions or bj percolating into a gravel-covered surface. In addition, the presence of dry depressions in 

permafrost areas may indicate locally well-drained 

areu.s: inasmuch as there is generally a source of water 

during the summer months. Such areas contrast with 

other areas of the earth's surface where dry depressions may be due to absence. of precipitation or other 

immediate source of water. Locaily unfrozen zones 

within permafrost areas may also be suggested by 

springs; which are commonly indicated by flood-plain 

icings; shown as flat, white surfaces on some aerial 
INTERPRETATION OF AERIAL PHOTOGRAPHS IN ENGINEERING GEOLOGY 39 

photographs, or indicated by more luxuriant vegetation n.long a pronounced lineation or within a darktoned area on low hill slopes. These unfrozen zones 

may be favorable as a source of ground water. In 

addition, because permafrost generally causes soils to 

be impermeable, drainage chn.racteristics indicative of 

permeable materials (see p. 34) may be interpreted 

to mean lack of permafrost, at least in the immediately underlying materials. Certain tree species may 

nlso suggest absence of permafrost (see p. 37). 

EROSION, TRANSPORTATION, AND DEPOSITION 

J,ANDSLIDES 

Nat ural movement or potential movement of materin.ls is a significant consideration in many construction engineering problems. Foremost in this regard 

is the movement of soils and rock materials by landslides. Ritchie. (1958, p. 67) stated that "All landslide 

investigations must start with recognition of a distressed condition in the natural or artificial slope * * *." 

Distressed conditions are commonly interpretable from 

aerial photographs. "The evidence for distressed conditions that may be present, or that may be induced, 

lies chiefly in evidence of movements, minor or major, 

that have already taken place or of geologic, soil, and 

hydrologic conditions that are likely to cause movement in the future" (Ritchie, 1958, p. 67). Geologic, 

soil, and hydrologic conditions signifying potential 

landslide areas include the presence of unfavorable 

geologic structures and rock type, prevalence of finegrained materials, and an abundance of water or conditions that would permit access of water to finegrnined materials. 

Because r~medial measures for landslides or prevention of ]n.ndslides are generally not only difficult but 

costly, avoidance of landslide areas is important in 

highway route layout. Existing landslides are commonly identified on the basis of. landform (figs. 46 and 

113). Liang and Belcher ( 1958, p. 70) stated that 

some landforms are more susceptible to landsliding 

than others, and thus identifying the landform is 

highly important. "Potential slides of the rockfall 

and soilfnll type cnn commonly be f~reseen simply by 

recognizing geologic conditions that are likely to produce overhanging or oversteepened cliffs" (Ritchie, 

1958, p. 51). Basalt cappings underiain by easily 

eroded shale (figs. 53, 87, and 113) illustrate a condition where rockfalls may occur as a result of failure 

and slump in the underlying materials. 

Movement of unconsolidated materials by flowage is 

likely to produce the most damaging landslides. Existing flows and slides (figs. 46 and 113) commonly 

have a "hummocky" surface. Landslide scars, where 

soils and vegetation have been stripped off the bedrock, may be easily recognized on aerial photographs 

by the light photographic tone of the bedrock in· contrast to darker tones of surrounding areas. Areas of 

potential flow of unconsolidated materials may be revealed by tension cracks or crevices, or suggested by 

the presence of seepage zones or springs, shown by 

dark tones of dense or luxuriant vegetation or by dark 

tones of the soil. 

Ritchie (1958, p. 50) warned, however, that although 

aerial photographs are of significant help in developing the setting for detailed studies, they seldom contain the detail needed by the engineer to carry out 

preventative or remedial measures. The prime application of aerial photographs in landslide studies appears to be in interpreting ground conditions to determine areas that should be avoided in highway route 

layout and similar problems. 

BEACH EROSION 

In a shoreline study in California, Munk and Traylor (1947) showed that va~iation .in wave height and 

refraction of waves along the_si1.ore was controlled in 

part by sea-floor topography. Where refraction causes 

a convergence of waves the rate of shoreline erosion 

may increase. l{rumbein stated ( 1950, p. 203) that 

"Refraction diagrams are becoming an essential part 

of any study concerned with shore processes, and 

graphic methods have been· developed for preparing 

them from hydrographic charts or aerial photographs." 

Photographs taken periodically of shoreline areas 

would also provide information, both qualitative and 

quantitative, in studies of erosion and deposition, 

which might thus be useful in planning remedial action 

against the eroding currents. In a study of shoreline 

erosion by the U.S. Army Corps of Engineers (U.S. 

Beach E~osion Board 1946, p. 13) aerial photographs 

taken in 1941 were compared with land surveys of 

1836 to obtain rates of erosion along the shore of Lake 

Michigan. The rate of erosion plays a significant part 

in planning corrective measures for eroding currents. 

BEDROCK 

GEOLOGIC STRUCTURE AND TYPE OF ROCK 

Because of the influence that faults may have on 

construction design and costs, it is important that 

faults be located and studied in the beginning stages 

of an engineering project. Aerial photographs may 

provide significant information in this regard as some 

faults are recognized from subtle expressions on photographs but are identified only with difficulty on the 

ground. The usefulness of photographs in mapping a 

proposed damsite in southeastern Alaska is shown in a 
,• 

40 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

study of the Swan Lake area (fig. 114) where faults 

stand out in aerial view as conspicuous lineations, 

which are primarily reflections of rectilinear depressions on the ground. In a damsite study in Malaya, 

Alexander and . Proctor ( 1955) used large-scale aerial 

photographs taken 750 feet above the ground. These 

photographs showed clearly the attitude and distribution of faults and joints in granitic country rock. 

In connection with the San Jacinto tunnel project 

in California, Henderson ( 1939) noted that aerial photographs revealed indications of unsuspected faults 

and corroborative evidence of those previously sus.-

pected. Fault locations underground wer.e found to 

be close to that predicted from study of photographs. 

In addition to showing faults, aerial photographs 

commonly shown dipping beds that are important in 

some foundation studies for pier and abutment location. Or dipping beds may indicate potential rockslide conditions where the dip is in the same direction 

as the hill slope. 

Lithologic characteristics of bedrock materials are 

also significant in many engineering problems, as in 

foundation work, or in locating road metal sourees. 

Criteria useful for interpreting the general lithologic 

character of rock types from aerial photographs are 

discussed on pages 16-19. 

INTERPR.ETATION OF AERIAL PHOTOGRAPHS IN 

HYDROLOGIC STUDIES 

LOCATING POTENTIAL GROUND-WATER SOURCES 

Little has been written on the interpretation and 

uses of aerial photographs specifically in hydrologic 

studies, although photogeologic techniques offer both 

practical and research applications in this field. Practical application of aerial photographs in hydrologic 

study are at present confined largely to assisting the 

geologic mapping in ground-water investigations, particularly in areas covered by surficial materials. As 

an aid in ground-water mapping, elements of the soil 

pattern are evaluated in terms of ground conditions, 

just as in many engineering geology studies (see p. 

34-37). Thus, for example, coarse-textured drainage 

or absence of drainage may signify highly permeable 

materials (figs. 52, 66, 88, 104, and 105B; and p. 34-35) 

or the landform (figs. 52 and 104) may suggest the 

kinds of materials that compose it, which permits an 

evaluation of permeability and porosity and potential 

as a water reservoir. In addition, aerial photographs 

may reveal information directly suggesting the presence of water, such as a preferential distribution of 

vegetation at the margins of a gravel cap (see figs. 

104 and 108). The type of vegetation in turn may 

permit inferences with regard to the general quality 

of water. For example, salt cedar is tolerant of relatively high content of salt in water, whereas cottonwood trees are intolerant. Howe (1958) described 

briefly the use of aerial photographs with specific reference to locating possible water-bearing formations. 

WATER-LOSS STUDIES 

In a study of water loss ·caused by phreatophytic 

plants and trees, which take their water from the 

water table, Turner and Skibitzke (1952) used aerial 

mosaics and contact prints for delineating areas of 

different densities of phreatophytes. The areas were 

then measured by planimeter and used in conjunction 

with vertical foliage density, determined by field 

methods, to arrive at volume density to which water 

loss by transpiration for specific species could be 

equated. Data pertaining to tree species and heights 

were determined by ground methods. Turner and 

Skibitzke (1952, p. 67) stated that "The comparison 

between results obtained by air and by ground mapping led to the conclusion that the former offers a 

much faster and more accurate method, particularly 

in areas of dense growth." Further use 'of aerial photographs in collecting basic data for studies of water 

loss by phreatophytes could probably be made. For 

example, it may be possible to determine tree or plant 

species, especially when large-scale black-and-white 

photographs are available or where color photographs 

have been taken. In addition, for many areas where 

the ground surface can be observed through the trees, 

photogrammetric measurements may provide rapidly 

those height measurements needed in water-loss 

stud!es. For areal measurements of differing vegetation types or densities a simple dot-templet method 

may be useful (vVilson, 1949). Like many other 

studies, however, where vegetation and ground features must be observed, measured, and plotted, maximum use of aerial photographs will result from combined use of field and photogeologic methods. 

OTHER APPLICATIONS 

Aerial· photographs as a tool in hydrologic studies 

provide a medium for obtaining basic data, such as 

cross-section profiles, linear, areal, slope, and volume 

measurements. Some of the suggested uses of photogrammetric techniques in hydrologic studies are: (a) 

for definition of channel size and shape of cross section to be used in studies of the effect of channel 

geometry in flood characteristics of streams; (b) for 

determining channel storage capacity used in floodfrequency correlations; (c) for definition of the threedimensional sinuosity of natural channels to be used 

in studies of the rate of energy loss; (d) for definition 

of the geometry of bed roughness of alluvial channel 
KINDS OF INSTRUMENTS 41 

models (Thompson, 1958) ; (e) for measuring topographic characteristics of drainage basins such as 

stream slopes and drainage density; and (f) for determining a.reas of lakes and swamps in water-storage 

studies. 

INSTRUMENTATION 

Instruments in photogeologic study serve three objectives, namely, interpretation, measurement, and 

plotting of data. The instruments used thus may be 

photogrammetric or nonphotogrammetric-photogrammetry is the art of making reliable measu~·ements 

from photographs. Usually different instruments are 

used f~r attaining each of the three objectives, as, for 

example, in using a stereoscope, parallax bar, and radial planimetric plotter, respectively. But all three operations may be combined in a single piece of equipment, such as the J(elsh plotter (fig. 18). "Vhether 

interpretation is done with a simple stereoscope and 

paper prints or with a precision stereoplotting iD:strument usino- o-lass-plate diapositives, the criteria of rec- oo . 

ognition, described in the first part of this pap~r, remain the same except insofar as scale and resolutiOn of 

photographic details are involved. Both meas~rement and plotting are here considered as mechanical 

aids that provide quantitative information to be further used in geologic interpretation of an area, and instruments are discussed primarily with this objective 

in mind. 

1\{easurement involves (a) the determination f~om 

aerial photographs of vertical and horizontal distances, 

which in turn are used to compute stratigraphic thicknesses, angles of dip, and other quantitative data of 

geologic use; (b) the direct determination of inclined 

distances, commonly the stratigraphic thickness of a 

formation; (c) the direct determination of angles of 

slopes or beds; and (d) other quantitative determinations of possible significance in geologic interpretation, 

such as light-transmission measurements. 

Plotting primarily involves the orthographic positioning of geologic data from aerial photographs to 

base maps or compilation sheets. It also involves 

direct plotting of geologic data in cross-sectional views, 

and may include positioning data such as geologic 

contacts, which are not directly recognizable in the 

stereoscopic model, by use of the floating dot or spatial 

reference mark that is found in many photogrammetric 

. in strum en ts. 

KINDS OF INSTRUMENTS 

Many different photogrammetric instruments are 

availaple for making measurements and plotting geologic data to base sheets or cross-sectional profiles (see 

Fischer, 1955; Ray, 1956; Pillmore, 1957; and Hemphill, 1958a). Certain instruments permit on~y the 

measurement of altitude differences, others permit only 

the orthographic positioning of geologic d~ta, but 

many instruments are used for both . measur~ng and 

plotting. Most measuring and plotting ~eviCes are 

designed to accommodate standard 9- by 9-Inc~ photographs; enlarged photographs can be used ':Ith on~y 

a few instruments, such as the overhead proJector, In 

transferring data to a base sheet. 

MEASURING DEVICES FOR USE WITH PAPER PRINTS 

One of the most commonly used and widely available types of instrument for determining altitudes 

from paper prints of aerial photographs is the stereometer or parallax bar. Several varieties of stereometers are available for use with either the lens or 

mirror-type stereoscope (see fig. 10); they consist of 

two small plates, usually of glass or plastic, that have 

inscribed dots, or other targets, that can be centered 

over conjugate image points in a stereoscopic pair. of 

photographs. The plates are attached to a s~pportlng 

bar along which they may be separated horizont~Ily. 

The supporting bar generally has graduated readings 

in millimeters or inches and a slow-motion adjustment 

drum that permits readings of hundredths of millimeters or thousandths of inches. All stereometers are 

based on the "floating-dot" principle, where two target 

dots one seen with each eye, are fused stereoscopically 

into' a single dot that appears to float in space within 

the stereoscopic model. The apparent height of the 

sin o-le fused dot, used as a reference mark in determi~in 0' altitude differences, is related to the horizontal 

separ~ion of the individual dots being viewed. Thus 

by measuring the horizontal separation between individual dots when the fused dot in the stereoscopic 

model is placed at the top of an object (such as a hill 

or cliff) whose height is to be determined, and subtracting from it the measurement of the horizontal 

separation between individual dots when the fused dot 

is placed at the bottom of that object, a measure of the 

hei o-ht called differential parallax, is obtained. Dif- o ' . ferential parallax is eonverted to feet by simple mathematical · calculations (see p. 53) . Stereometers are 

thus merely devices that permit reliable measurement 

of differences of horizontal distances between two or 

more pairs of conjugate image points as seen on two 

photographs that form a stereoscopic pair. . . 

Parallax ladders also may be used for determining 

differences in altitudes from aerial photographs. Like 

the stereometer, the parallax ladder is based on the 

floating-dot or floating-line principle. The instrume~t 

may consist of two diverging rows of dots on plastic 

or glass arms; thus, a series of pairs of dots exist with 
42 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

different horizontal separations, and this in turn results in several floating dots all at different altitudes 

within the stereoscopic model (see fig. 12). Or the 

instrument may consist of two diverging lines with 

intercepts of specific horizontal separations ticked off; 

thus a single plui1ging line would be seen in stereoscopic view. The instrument, oriented at. right angles 

to the flight line, is used by sliding it over the stereoscopic model until a pair of dots or line interceptsseen stereoscopically as a single dot or cross in spaceappears to fall on the baseof the object whose height 

is to be determined. A reading of the horizontal 

separation of the two dots or intercepts is then made. 

The parallax ladder is moved· until a different pair of 

dots or intercepts-also seen stereoscopically as a 

single dot or cross in space-appears to fall on the top 

of the object whose height is to be determined. Again 

a reading of the horizontal separation of dots or inter:- · 

cepts is made. The difference in readings obtained is 

then used in a simple formula to determine the difference in altitude of the two points (see p. 53). 

A direct-reading parallax· ladder that eliminates the 

need for most of the mathe.matical computation has 

also been devised. The differences in horizontal separation of pairs of dots has been calculated by the 

manufacturer in terms of heights in feet for different 

flying heights and photobases, and this permits a direct reading of relative altitudes within the stereoscopic model. 

Also for use with paper prints is a stereo slope 

meter~ designed primarily for determining degrees or 

percentage of slope. The instrument consists of two 

transparent disks each _scribed with eccentric circles of 

specific diameters· (fig. 11). These disks are mounted 

in a frame so as to be movable horizontally in a fashion similar to the dots of the stereometer. In stereoscopic view the two sets of eccentric circles appear as 

a cone that is divided into several zones. By proper 

hori7.ontaJ spacing of the two disks, the resulting cone 

seen stereoscopically can be raised or lo,vered so that 

some two circles will rest on the slope or grade to be 

determined. By appropriate simple calculations the 

slope can be determined. Center dots present on each 

of the transparent disks also permit relative altitudes 

to be determined just as with a stereometer. 

PLOTTING DEVICES FOR USE WITH PAPER PRINTS 

Instruments have been designed specifically for 

plotting information from paper prints to base maps 

or control sheets. An instrument widely employed in 

the past is the sketchmaster (fig. 13), which uses the 

camera-lucida principle in transferring data from the 

photographs to the base sheet. The sketchmaster 

allows the operator to view a single photograph image 

superposed on the base map. Adjustments for scale 

cluuiges in the perspective photograph permit coincidence or near coincidence of photograph control points 

and base-map control points. Geologic detail is 

sketched directly on the base map. This instrument 

can be adjusted to remove small amounts of tilt inherent in some vertical photographs, but like any 

direct-reflecting projector, large amounts of radial displacement due to relief cannot be effectively removed. 

The radial planimetric plotter has been devised especially for transferring photographic detail to a base 

map or control sheet. Unlike the sketchmaker, this 

instrument is designed to eliminate displacement due 

to relief. The plotter consists of a mirror stereoscope 

mounted above two photograph tables (fig. 14). A 

transparent plastic arm with a centrally scribed line 

extends from and pivots around the center of each 

table. These arms are linked to a pantograph attachment. Because the plastic arms radiate from different 

centers they cross each other in stereoscopic view to 

form the so-called plotting cross. 

In operating the radial planimetric plotter a pair of 

vertical photographs is oriented on the photograph 

tables for proper stereoscopic viewing, and control 

points on the photographs are oriented to control 

points on the base manuscript by means of the radial 

arms and pantograph. ~fovement of the pantograph 

attachment moves the plotting cross over the stereoscopic model and permits tracing of photograph detail 

on the base manuscript. Inasmuch as the radial arms 

that intersect a terrain feature on each photograph 

represent azimuth lines from known points on the base 

manuscript, the intersection of these two arms will 

represent the true map position of that feature, just 

as will the intersection of two azimuth lines shot from 

different instrument stations in the field. Thus the 

relief displacement of a feature on a photograph is 

effectively removed; this is one of the chief advantages 

in using the radial planimetric plotter. However, 

the radial lines do not intersect along the principal 

line-or flight-line direction-between photographs, 

and the photograph tables must be shifted to their alternate centers before the central area of ste.reoscopie 

model can be delineated. Because the photograph 

tables are mounted in a horizontal position, the instrument does not permit removal of tilt that may be present in the photographs, but tilt is usually small in present-day photography and generally does not cause 

significant errors in horizontal positioning of geologic 

data. 

The multiscope is a combination of mirror stereoscope and camera lucida that has received limited use 
KINDS OF INSTRUMENTS 43 

u 

.,:I 

FH; L-RE 1 O. Stcn•ometers. FIGURE 11.- Ster!'o slope meter. 

F!cL·nt; 12.- Paralla x laddPr. 

YIEASURING INSTRUMENTS FOR USE WITH PAPER PRINTS 

553966 0 -60- 3 
44 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

! ' 

Fw u RE 13. Skrtchma~ter. FJGIJRE J4 Radial planinwtric plotter. 

PLOTTING INSTRUMENTS FOR USE WITH PAPER PRINTS 

F1c t· HE 15.- Stt•reotope. 

COMBINED MEASURI.KG AND PLOTTING I~STRU.\1ENT FOR USE WITH PAPER PRINTS 
KINDS OF INSTRUMENTS 45 

in plotting geologic data front photographs to base 

mnps or control sheets. Photographs are mounted on 

tnovable photograph plates that permit small adjustments for tilt. The viewing assembly is constructed 

to allow the insertion of one or two half-silvered mirrot·s in the eyepiece, so that in operation either the 

irnage of a. single photograph or the stereoscopic model 

rnay be superposed optically on the map or control 

bnse. Although the stereoscopic model may be seen at 

all tirnes, use of only one half-silvered mirror permits 

the eye to see only the image. of one photograph superposed on the rnn p base; the resulting plot is similar 

to that from the sket:clnnaster-or reflecting projector 

-no etTors clue to relief displacement are removed. 

If, however, two half-silvered mirrors n.re used, appropriate rnnnipulntions permit true orthographic plotting 

of det:t il from the stereoscopic model. Scale adjustments between photographs and map base are made 

by int:erchangi11g special lenses in the viewing assetnbly together with changing manually the distance 

o:f the viewing assembly above the ma.p base. The 

i nstnnnent is said to permit measurement of altitudes 

when coupled rotary prisms are inserted under the 

half-silvered mirrors~ or when a device similar to the 

tnultiplex tracing table is placed in the field of stereoscopic view (Spurt· and Brown, 1!)45, p. 177). 

1\H~ASUHING AND PLOTTING DEVICES FOR USE WITH 

P All"ER PRINTS 

Some instnnnents have been designed both for measuring altitudes and for plotting data from paper prints 

of aeria 1 photographs. These instruments are commonly tel'llled "paper-print plotters" and include the 

KEl( plotter, t·he ~1ahan plotter, and the stereotope. 

The KEJ( plotter consists ·of a stereoscope, two 

photograph tables: floating-dot assembly, and drawing 

attachment. The plotting cross of the radial planirnetric plotter is replaced i 11 the J(EJ( plotter by the 

fused dot~ floating in space. By raising or low~ring 

t"IH~ photograph plates the fused dot is positioned on 

the ground in the stereoscopic model. Vertical motion 

of t"lw photograph plntes .is linked to a drum scale on 

which relative altitudes can be rend directly in feet. 

·Movernent of the pantograph drawing attachment 

allows geologic detail to be sketched directly on the 

tllnp base: but cluring this sketching the fused clot must 

he held on the g•·ouncl in the stereoscopic model by 

si multn neous vert·ica 1 t11ovement of the photograph 

pla t·es. The photop:ra ph plates may be tilted to make 

an npproxirnat"e correction for tilt that may be inherent" i 11 t-he photogm phy. Been use the floating-dot assembly lies above and is physically separated from the 

phof·ogra ph tn bl.es, sma 11 amounts of extraneous para.llax and significant amounts of horizontal shift in the 

map position of a point may result by moving one's 

head in viewing the stereoscopic model. 

The ~Iaha.n plotter is generally similar in principle 

to the J(EJ( plotter but differs slightly· in operation. 

The floating dot is positioned on the ground in the 

stereoscopic model by vertical motion of the disks on 

which the dots are scribed, whereas in the l(EJ( plotter 

the position of the disks containing the scribed dots is 

fixed and the photograph plates are moved vertically 

in order to position the floating dot at a particular 

level in the stereoscopic model. The stereoscope of the 

l\fahan plotter is adjustable, which permits very nearly 

the recovery of the perspective from photography 

ranging in focal length from about 8.25 to 12 inches. 

As with the J(EJ( plotter, a shift of the viewer's head 

when v.iewing the model may cause horizontal positioning errors and the presence of small amounts of 

extraneous pa.rnJlax. The stereoscopic model of the 

KEI( and l\1ahan plotters may be effectively leveled, 

in absence of vertical control, by visual reference to 

certain physiographic features of the terrain (see p. 

55). 

A somewhat differeut paper-print plotter is the 

stereot.ope. The instrument consists of a stereoscope 

with binoculars of X 4 magnification mounted over a 

photoholding assembly that contains a parallax bar 

and attachments for pantograph hookup, as well as a 

mechanism, when vertical control is available, for 

effectively leveling tilted models, for correcting errors 

of horizontal position, and for orthographic plotting 

of geologic data (fig. 15) .. The stereo tope is an elaborate instrument that corrects for tilt by mechanical 

linkages "·ithin the photoholding assembly. No physical tilting of the photoholders bikes place and thus the 

instrument differs significantly from the J(EK and 

l\1ahan plotters. In addition, the targets of the stereometer attachment for altitude determinations are in 

contact with the paper prints, and no extraneous 

parallax can be introduced by movement of the operator's viewing position. In the absence of vertical 

control, which permits tilt correction through mechanical linlmges, the stereotope as a measuring device must 

be used as a simple p<trallax bar. 

MEASURING AND PLOTTING DEVICES FOR USE WITH 

GLASS-PLATE DIAPOSITIVES 

The Kelsh plotter, multiplex, and ER-55 plotter, a.ll 

of which require glass-plate diapositives, are used in 

the United States both to measure quantitative geologie data and to plot geologic detail to base ~naps or 

control sheets (figs. 16-18). These double-projection 

precision-plotting instruments are designed to accom-
46 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

FIGURE 16. - Multiplcx. FIGURE 17. - ER-55 plotter. 

FwuRE 18.- l\:Plsh plottN. 

COl\1Bil\'ED MEASURil\'G Ai\'D PLOTTil\'G INSTRU:\IEJ\'TS FOR USE WITH GLASS-PLATE 

DIAPOSITIVES 
KINDS OF INSTRUMENTS 47 

1110dnte photography of speci fie focal l~ngths, usually 

fi o•· 8.25 inches. All utilize the anaglyph principle of 

projedion of light of complementary colors, red and 

blue-green, tlH·ough glass-plate diapositives to create 

the third dimension. The stereoscopic model is viewed 

through glasses of the same color as the filters used in 

the 1 ight-source projectors. Because double-projection, 

ot· anaglyphic-type, stereoplott.ing instruments simulate in .miniature·the spatial relations of the camera 

stations at the time the photographs were taken, all 

features of the terrain are optically re-created in the 

stereoscopic tnodel in essentially true relation with 

respect to the tneasuring mark or floating dot. 'Vhere 

suflieient, vertical control is available any tilt inherent 

in the photography can be removed so that accurate 

tnensurements of altitudes can be made and true orthogt·nphie positioning of detail obtained. 'Vithout vertical cm~trol the stereoscopic rnodel of many terrains 

commonly can be leveled within 1° of the horizontal 

datum by visual reference to certain physiographic 

chat·acteristics of the tena.in (see p. 55) ; under these 

conditions approxima.te orthographic positioning may 

l>e obtained, but significant errors in vertical measurements may result (seep. 69-75). 

The stereoscopic model is usually viewed on a small 

white-surfaced table called a platen, which may be 

raised and lowered so that an illuminated floating dot 

in its center is kept in contact with the surface of the 

ground, as seen in stereoscopic view. Vertical motion 

of the platen is transrnitted to a scale reading in millimeters of vertical measurement, or, on some instrutnents, t:o a scale that converts readings directly to 

heights in meters or feet. Geologic features are traced 

orthographically on the base map by a. pencil located 

directly beneath the illtnninnted dot on the platen, or 

by a reduction pantograph attached to the tracing 

table. Interpretation, measuring, and plotting can be 

carried out in one continuous operation. 

The 1\.elsh plotter is designed to accommodate glassplate diapositives the same scale ns the original phot·og-ra phy. For many geologic problems film positives 

mny be substituted for the more expensive glass-plate 

diapositives. The scale of the projected stereoscopic 

tnodel is u~ually about [) times that of the original 

phot-ography. Only that pa.rt of the stereoscopic 

n1odel n ppea ring on -the platen is illuminated; this 

re::;uHs in a. concent.mtion of light and a. brightly 

i llurn ina ted model but prevents viewing of the eritire 

111odel at one time. Because the glass-plate diaposit·i ves n t·e the same sen le a~ the original photography 

the resolution of the stereoscopic image is excellent. 

The multiplex uses glass-plate diapositives on which 

the original H- by D-.inch negative is reduced about 

:) times. The small dia.positive image is then enlarged 

approximately 12 times in projection of the stereoscopic model. The projected stereoscopic model is 

about. 2.5 times the original photography scale. As a 

result. of the large amount of reduction of the original 

photogruph negative, some of the photographic detail 

is lost and fine details of terrain, important in geologic 

interpretation, may not be visible. In multiplex projection the entire model area is illuminated, and if the 

terra in being viewed has only low or moderate relief 

the stereoscopic model may be observed in its entirety 

by substituting a large white surface for the platen. 

This overall view of the stereoscopic model is often of 

considerable use in geologic interpretation, because of 

the association of geologic features that C<tn be seen 

at one time. The overall view is generally accomplished by projecting the stereoscopic model to the 

mnlt.iplex table slate; no plotting or measuring can be 

done when the model is viewed on this surface. 

'I'he ER-55 plotter uses glass-plate dia positives on 

which the original 9- by !:>-inch negative is reduced 

about 2.8 times; this amount of reduction does not 

seriously affeet image resolution. The projected stereoscopic model is about 3.5 or 5 times the scale of the 

original photographs: depending Oll the projector 

model. Ellipsoidal reflector-type projec.t.ors resu It in a 

brightly illuminated model in which tet:i:ain detail has 

a high degree of resolution. Like the multiplex, the 

ER--55 plotter permits viewing the entire stereoscopic 

model at one time. In addition to accommodating 

vertical photography the ER-55 projectors are adaptable to twin low-oblique photography taken with the 

camera axis inclined 20° from the vertical position. 

Geologic study of stereoscopic models in doubleprojection plotters has numerous advantages over the 

study of paper prints. The ability to interpret, measure, and plot in one continuous operation has already 

been cited. In addition, the geologist works with an 

enlarged stereoscopic model that ranges from about 

2.5 times to :) times the original photography scale, 

depending on the instrument used; thus small-scale 

photography commonly may be used. A higher degree 

of image resolution is maintained on glass diapositives 

than on paper prints, and results in more detail that 

can be observed in the double-projection stereoscopic 

models, except for the multiplex, in which some resolution needed in qualitative and quantitative interpretation is lost in the projected model. Because all 

features o:f the terrain are optically re-created in essentin lly true relation, reliable measurements for structure 

contouring, for drawing isopach lines, for conq)uting 

strikes and clips, and for determining fault displacements can be obtained for many areas with stereo-
48 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

scopic plotters; measurements can be made rapidly and 

hence economically (see Pillmore, 1957). 

OTHER INSTRUMENTS 

Other photogrammetric instruments such as the 

· stereoplanigraph and autograph permit precise measurements from aerial photographs, but these heavy 

plotters are generally unavailable to the geologist. 

Beside permitting precision of measurement, these instnu,11ents are designed to accommodate different sizes 

of cliapositives and photography taken with different 

foeal-length lenses. 

Certain modifications or additions to existing instruments have been made specifically for geologic purposes; and some new instruments, described below, 

have been designed expressly for geologic study. 

EXAGGERATED-PROFILE PLOTTER 

The exaggerated-profile plotter is a device attached 

to the tracing table of double-projection plotters, such 

as the 1\.:elsh, multiplex, and ER-55 plotters, for drawing terrain profiles. As the tracing table is moved 

along the line of profile and as the platen of the tracing table is moved vertically a pencil attachment 

records the profile on a vertical tracing board. Profiles 

exaggerated as much as 5 times may be plotted. An 

arm on the tracing table slides in a groove along the 

base of the tracing board so that a selected crosssection direction is maintained. Two models of the 

exaggerated-profile plotter have been developed. One 

is based on the lever and f~1lcrum principle (fig. 19) ; 

the other is based on the pantograph principle (fig. 

20). The exaggerated-profile plotter was first developed to aiel in solving a specific geologic problem in 

correlating thin and closely spaced intraformational 

sandstone beds in rocks of Cretaceous and .Jurassic age 

in Wyoming (Pillmore, C. L., oral communication, 

1958). The plotter may be very useful also in quantitative geomorphic studies or in studies involving the 

qualitative correlation of shape characteristics of 

terrain. 

INTERVAL-MEASURING DEVICE 

A floating-dot instrument that permits the maintenanGe of a desired vertical·interval between two floating dots in a stereoscopic model from paper prints, as 

well as serving as a eonventional stereometer, also has 

been developed in the course of photogeologic study 

(IIaekman, R .• J., oral comnnmicntion, 1959). Two 

floating dots, representing different altitudes in the 

stereoscopic model, may be useful for example in positioning an obscured formation contact when other contacts are readily observed and formation thicknesses are 

known. The instrument consists of two transparent 

cirenlar disks mounted on a frame with two worm 

screws, one of which permits separation of the disks 

in the w direction, and the other of which permits both 

disks to be rotated equally but in opposite eli rections 

(see fig. 21). A series of dots, evenly spaced, :u-e arranged in a straight line so as to pass through the 

center of each disk. One dot coincides with the center of each disk and can be moved only in the w direction, as in most stereometer-type instruments. The 

other dots may also be moved in the plane of the disk 

by rotating the disk, thus providing a pa.rallax ladder 

as seen stereoscopically. By proper adjustment a pair 

of dots, one on each disk, can be separated by rotation 

of the disks to provide a direct-reading parallax ladder, or the dot separation may be referred to millimeters of parallax and differences in altitude calculated 

in the usual manner (see p. 53). The center dots 

of each disk can be floated in the stereoscopic model 

at a desired point and the distance above or below this 

point measured merely by rotation of the disks. Scales 

graduated to record hundredths of millimeters of parallax permit readings of separation of the center dots 

of the disks as well as reading of total w parallax resulting from rotation or x motion of the disks. 

UNIVERSAL TRACING TABLE 

Some research in development of instruments carried out to date has centered ltround the modification 

of existing instruments. Important in this group is 

the universal tracing table (fig. 22), which is a nntltiple.x tracing table that has been modified in design by 

R. H. :Morris and C. L. Pillmore of the Geologic Survey. It is used for direct measurement of inclined 

distances, such as stratigraphic thickness of dipping 

beds, in stereoscopic models from double-projection 

plotters. The platen and underlying assembly can be 

tilted as much as 45°. A separate worm-screw drive 

on the tilted assembly distinct from the conventional 

worm-screw drive for vertical movement, permits 

movement of the platen in an inclined direction perpendicular to the surface of the platen. A scale on 

this auxilliary worm-screw-drive assembly permits the 

rending o:f measured intervals in hundredths of millimeters; conversion to feet. is made in the usual manner 

for double-projection stereoscopic models (see p. 

54-55). A series of holes drilled through the platen 

to the underlying light source provides a plane of 

floating dots in the stereoscopic model. 

MEASUREMENT 

PRINCIPLES OF VERTICAL. MEASUREMENT 

Vertical measurements of altitude differences provide by far the greatest amount of quantitative infor)nation in geologic interpretation from aerial photographs. Differences of altitud'es are generally deter-
MEASUREMENT 49 

FwuRE I !J. - Lever and fulcrum type of exaggerated-profile FIGURE 20. - Pantograph type of exaggerated-profile plotter. 

plott!'r. 

I 

FIGURE 21. - Interval-mcasuring instrument. FIGURE 22. - Universal tracing table. 

INSTRUMENTS DESIGNED OR ~ODIFIED FOR GEOLOGIC USE 
50 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

m.ined from overlapping aerial photographs, although 

they may under special circumstances be determined 

from a single photograph. vVhen overlapping prints 

are. used, the altitude difference between any two points 

is determined by measuring the horizontal linear 

parallax difference between the two points and relating 

it to appropriate geometry of the stereoscopic model. 

Altitude determinations from glass plates in doubleprojection plotters require the measurement of actual 

vertical distances within the stereoscopic model. In 

the uncommon circumstance when a single photograph 

may be used, altitudes are determined by measuring 

relief displacement and relating it to appropriate 

geometry of the aerial photograph. 

DETERMINATION OF AL~ITUDE DIFFERENCES BY THE 

PARALLAX METHOD 

"\Vhen an object is viewed or photographed from two 

different positions~ as on two overlapping vertical 

aerial photographs, an apparent shift in the position 

of that object takes place, which is known as parallactic displacement. On overlapping aerial photographs this is a measurable linear distance that is 

directly related to the height of the object. In photogr~unmetric terms parallax of an image point appearing on two overlapping photographs is called absolute 

stereoscopic parallax and is represented by the algebraic difference, parallel to the flight line, of the distances of the conjugate image points from their respective principal points (see fig. 23). The difference 

in absolute stereoscopic parallaxes between two differerent image points is a measure of the distance one 

point is above the other. On overlapping paper prints 

the para1lax difference may be measured reliably by 

using a stereometer-type instrument, based on the 

floating-dot principle; parallax difference m~ty also be 

measured with a ruler, although this procedure is 

rarely used because of inaccuracy of measurement. 

In double-projection stereoplotting instruments 

(Kelsh, multiplex, and ER-55 plotters) an actual vertical measurement of altitude difference is made within 

R,' 

--------- y ---~ -rp-<-2 _______ , ___ -t--- B ~--r--

A ----------~-- X'-- . 

fj' - ------- X - ----..4----------f----t--- p/ 

Principal point, right photograph\~ 

Flight line '\ 

0 0' 

FIGURE 2:~.-·Diagram showing relation between absolute stereoscopic parallaxes and horizontal distances actually measured 

with stereometer-type instruments in determining differences in altitude from paper prints. 

Let images Pt'-JV and Pt'-JV represent tiH:; photographic 

expression of a pole on the left and right photographs of a 

stereoscopic pair. The absolute stereoscopic parallax of the 

base of the pole is x- ( -x') = (x+x'). The absolute stereoscopic parallax of the top of the pole is y- (- y') = (y + y'). 

(Distances measured to the right of the principal point are 

positive, to the left, negative). The parallax difference 

bet.ween the top and bottom of the pole then is (y+y')-

(x+x'). 

From the figure: 

u+y'+B=x+x'+A 

Y+Y' -x-x'=A- B 

(y+y')- (x+x') =A.- B. 

Thus the difference in absolute stereoscopie parallaxes is 

equal to A -13. The distances A and 13 are t:.he actual 

distances measured with stereometer-type instruments, generally in hundredth::; of millimeter~, in determining altitude 

differences from paper prints. 
MEASUREMENT 51 

the stereoscopic mode]; the llnear paralhtx recorded 

on the photograph is translated into n. vertical disht nee as a result of angular parallax in the viewing 

a 1'1'a.ngement. Figures 23 and 24 show dia.grnmmatically the distances thnt are actually measured when 

~h!reo.meter-type instruments and double-projection 

instruments are used respective.ly in altitude deterrninntions. 

Light source 

USE OF STEIU<;OMB'TEit-'l'YPE INSTRUMENTS 

'Vhen paper prints of aerial photographs are used 

in conjunction with a stereometer-type instrument and 

a. stereoscope, two linear horizontal distances. must be 

measured to obtain the parallax difference between two 

different image points. These distances are simply A 

and B as shown on figure 23. They can be measured 

with a finely graduated ruler but more commonly a 

Light source 

li'ruuJtl~ 24.-Diugrnm showing relation between ah&)lute stereoscopic parallaxes and vel'tieal distance measured with doubleprojection type instrmuents in determining differences in altitude from gl.ass plntel';. 

The nh:-;olut:~! st:ereoscOJ)iC parallaxes of the top and bot~ 

tom of a JWle re}lresented on the left J)liOtograph by 

P,-P2 n.nd on lihe '>right photograph by P1'-P2' h.re 

(JJ + u') IIIHl (:c + :c'), ns in tignre 20, and dift'.:•rentiul 

pu•·ullax h~!tween the top and bottom of the pole is 

(!I + 11') - ( m + m') or A. - B. With donble-rn·ojection 

iustTumtmt:s the vertical distance CJJ is actually measur~(l, 

I'HI"IH!t' thun the horizon.tal distance:-; that are measured on 

pa{)er p1·ints. The distance CD. measured genei'Hlly in 

tenths of millimeters, is converted to feet by multipl~'ing 

hy the J( facto•·, whieh represents the number of feet on 

the ground per 0.1 millimeter in the stereoscopic model. 

Points C and D t·epresent the intersections of those light 

nt~·s that pass tlu·ongh the top arHl hottom of the pole 

respectively. 
52 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

stereometer-type instrument or parallax bar is used 

because it permits more consistent readings of smaller 

increments of length··than a ruler. Stereometers are 

generally constructed with a principal scale graduated 

in millimeters and a subordinate drum scale calibrated 

in hundredths of millimeters for recording parallax 

measurements; a few instruments are designed to 

record· in inches. 

To make parallax measurements from aerial photographs it is first necessary to orient the photographs 

properly for stereoscopic viewing. This is accomplished by alining the photograph centers and conjugate centers along a straight line,.-the equivalent of 

the flight-line direction-and separating the photographs for comfortable viewing (fig. 25). Photograph 

centers are located by marking the intersections of 

lines drawn between fiducial marks at the opposite 

sides of each print. Each r~nter is then transferred 

stereoscopically to the other photograph of the stereoscopic pair and its conjugate image point marked. 

Parallax measurements are then made in the so-called 

w direction, parallel to the flight line. Measurement is 

accomplished by adjusting the separation of the dots 

of the parallax bar until a single fused dot, seen stereoscopically, appears to rest on the ground at the first 

point selected. The. instrument reading is recorded 

and the procedure is repeated for the second point 

selected. The difference. ii}· readings is the parallax 

difference between the tw~ 'p'o'lnts. In making parallax 

measurements the fused dot will be seen readily when 

it floats above the apparent ground surface of the 

stereoscopic model but will appear to split into its two 

component dots as it is lowered below the ground surface. If the fused dot, as it is being lowered, appears 

to split at some point just ~hove the ground surface, 

a slight change in the separation of the photographs 

may be necessary to permit simultaneous viewing of 

the floating dot and the stereoscopic model. 

Conjugate image of left photograph center 

Conjugate image of 

right photograph center 

Left photograph center 

FIGURE 25.-Sketch showing correct orientation of photographs for stereoscopic viewing. 
MEASUREMENT 53 

'.l.'o obtain the height of an object or difference in 

nJtitude between two points the difference in parallax 

must be related to the geometr·y of t.he stereoscopic. 

model. ~..,or measuring heights from paper prints this 

relation is best expressed by a formula which, in its 

simplest terms, is 

H h=- ·.1z) b . (1) 

where h= height of the object, in feet, or difference in 

altitude being determined; 

l-l=height of airplane, in feet, above mean 

terrain (determined from specifications of 

photographic mission or relation l-l=f/8, 

see below); 

b=photobase (commonly determined by averaging the distances between the center and 

conjugate center of each photograph of the 

stereoscopic pair); units of measure may be 

millimeters or inches but must be the same 

as .1p; 

and ~p=parallax difference, in millimeters or inches, 

. as determined with stereometer-type instrument (distance A-13 of figure 23). 

It will be noted that tts l-l increases, the measure of 

~p for any given vertical interval will decrease; however, the ·absolute value for any one unit of .1p will 

increase correspondingly. On the other hand, if focal 

length is increased and flying height remains constant. 

the measur·e of ~p for a given vertical interval will 

increase but the absolute value for any one unit of .1p 

will decrease correspondingly. 

The above formula (1) may be used without any 

npprecinble error if relief in an area is low; ~p will be 

small. If relief in an area is high one of the following 

two formulu.s, (2) or (3), should be used: 

l-1' 

h ab+~p . .1JJ (2) 

where h= height of the object, in feet, or difference ii1 

altitude being determined; 

l-1' =height of airplane, in feet, above the lower 

of the two points whose parallax difference 

hns been measured; . 

nb=photobase adjusted to the lower of the two 

points whose parallax difference has been 

measured (commonly determined by mea.suring the distance between photograph centers-0-0' of fig. 23-and subtracting from 

it the distance between conjugate image 

points at the lower altitude-distance A of 

fig. 23); units of measure may be millimeters 

or inches but must be the sam'e as ~p; · · 

and .1p=parallax difference, in millimeters or inches, 

measured with stereometer-type instrument 

(distance A-B of figure 23); 

or h H' ·.1 ab-.1p p (3) 

where h=height of the object, in feet, or difference in 

in altitude being determined; 

H' =height of airplane, in feet, above the upper 

of the two points whose parallax difference 

has been measured; 

ab=photobase adjusted to the upper of the two 

points whose parallax difference has been 

measured (commonly determined by measuring the distance between photograph centers-0-0' of fig. 23-and subtracting from 

it the distance between conjugate image 

points at the upper altitude-distance B of 

fig. 23); units of measure may be millimeters 

or inches but must be the same as .1p; 

and ~p=parallax difference, in millimeters or inches, 

measured with stereometer-type instrument 

(distanee A-B of figure 23). 

Formula (2) is used when H' and ab are determined 

by referring to the lower of the two points whose_ altitude difference is desired, as in measuring upward 

from the base to the top of a cliff. Formula (3) is used 

when H' and ab are referred to the upper point, as in 

measuring downward from the top to the bottom of 

a cliff. Most commonly the lower of two points is 

selected as the reference point and measurements are 

made from tha.t point to some higher point in the 

stereoscopic model; thus formula (2) is more widely 

used than formula (3). . . . Parallax differences genera1ly can be read within 

small tolerances (see p. 73), and errors in computing differences in altitude result mainly in t~e determination of flying height and photobase, assum1ng photographs have little or no tilt. It is therefore gen.erally 

desirable to determine flying height (II') and adJusted 

photo base ( ab) carefully because a given percentage 

error in one of them may cause a similar percentage 

error in the computation of- altitude difference .. In 

addition failure to use formula (2) or (3) when pa.ra.llax diffe~·ence ( ~p) is large will also cause a considerable error in the final result even though the adjusted 

photo base ( ab) has been carefully -determined. 

For calculation of absolute heights as is necessary, 

for example, in determining stratigraphic thickness or 

displacement on faults, the factor II' may be determined from a base map of known scale or from other 

control· using the relation Il' = · f IS.- If the altitude 

difference between some two points, A and B, is desired 
54 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

and if neither A nor B falls in a convenient map position for scaling a distance and determinnig II' for one 

of them, it may be necessary to make parallax measurements with respect to some selected third point, 0, 

for which the flying height (I/') can be readily calculated. 

Where altitudes are to be used for computing angles 

of dip it is not necessary to know the absolute height of 

H'; the tangent relation of angles 

Tan angle of dip vertical distance 

horizontal distance (4) 

involves only relative horizontal distances and relative 

differences of altitudes, and thus any value may be 

assumed for H' when measuring parallax difference 

and determining the scale of the photograph to be used 

in computing dips. In photogrammetric terms this 

relation may be expressed as 

Tan angle of dip 

or Tan angle of dip 

Since 

is also equal to 

flying height (feet) X parallax 

difference 

horizontal distance on ground 

(feet) X adjusted photobase 

H'·~p 

d· (ab ±~p) 

flying height (feet) 

(5) 

(6) 

horizontal distance on ground (feet) 

focal length (inches) 

horozontal distance on photograph 

(inches) 

the following formula may also be used to calculate 

angles of dip when flying height, H', is not known or 

cannot be determined: 

Tan angle of dip 

focal length X parallax 

difference 

horizontal distance on photo-~ 

graph X adjusted photobase 

(7) 

The tangent of dip, in photogrammetric terms, has 

been described in more detail by Desjardins (1943b, 

p. 1536-1538) and Elliott (1952, p. 8). When angles 

of dip are determined from enlarged photographs and 

formula (7) is used, the effect of photographic enlargement on focal length must be considered; effective 

focal length is increased in direct proportion to the 

amount of photographic enlragement ·(see p. 4). 

Hemphill (1958a; p. 43) has devised a chart that 

shows the number of feet represented by each millimeter of parallax change as H' and ( ab + tip) of formula (2) vary. The factor determined from the chart 

is then multiplied by tip to arrive at h, in foot, provided formula (2.) is used. A simple circular photogrammetric computer has also been designed that permits easy computation of ·the number of feet represented by each millimeter or hundredth millimeter of 

parallax as flying height and photobase vary. In addition, the computer allows tnultiplication of this factor 

by the parallax change to give altitude differences .in 

feet. The number of feet represented by each hundredth millimeter of parallax measured on paper 

prints may also be determined fi'om the ratio 

F.lying height (feet) 

Photobase (millimeters) X 100 

This relation is further discussed on page 73. 

Need joT coT1'ect m'ientation of photog1·aphs.-"A'lisorientation of a stereopair of photographs can be considered from the standpoint of rotation around the center of one or both of the photographs. This rotation 

results in misalinement of the stereopair and flight line. 

Parallax differences are not then measured in the true 

x direction, or flight-line direction, but at a slight angle 

to it; an error in the adjusted photobase also results. 

The measurement of parallax difference is not normally 

significantly affected but the adjusted photobase on a 

ste.reopair misoriented by a few degrees, even though 

the human eyes can tolerate a certain amount of error 

and yet see a well-defined stereoscopic image, may be 

in error by as .much as 5 to 10 percent. This will cause 

a similar error in differences in altitudes determined 

by the parallax method when paper prints are used. 

USE 01<' f)()URU;-l'RO.H~CTION INSTltU .MEN1'S 

vVhen double-projection instruments are used to determine altitude difference between two points, only 

one distance need be measured. This is a vertical distance within the stereoscopic model that results from 

the translation of linear parallax recorded on the glassplate diapositives to angular parallax in the viewing 

arrangement (fig. 24). The vertical position of intersecting light rays to the ]ower point to be measured is 

determined by placing the floating dot on the apparent 

ground in the stereoscopic model and recording the 

instnunent reading. The vertical position of the 

upper point is recorded in a similar manner. The 

difference in readings is a measure of the parallactic 

displacement and is related to the difference in altitude of the two points. 'Vith double-projection instruments measurements are normn11y made in increments of tenths of millimeters, although some instruments are so designed that selection of proper gear 

trai1~s permits readings to be converted directly to 

heig·hts in meters or feet. 

In stereoscopic models formed in double.-projection 

instruments the vertica 1 seale rema:ins constant with 
MEASUREMEN'l" 55 

respect:. to the horizontal scale and a sirnple rnult.iplication factor-ca.llecl the [( factor-fol' relati11g increments o:f verticaJ rneasurerne.nt to absolute feet can be 

det:er111 i ned by converting the standard fractional scale 

(representative fraction) of the model to feet per unit 

oJ vertical measurement, that is, per 0.1 milli.meter. 

This is most rapidly a.ccornplished using the formula 

J( model scale denominator 

\. :.\,04:8 (number of tenths of millirneters in 1 foot)" 

Th liS a. stereoscopic model .with u. scale of 1 :4,000 

would have a /( factor of 4:000./3,048 or 1.31 feet per 

0.1 anillimet:er of vertical aneasurement. Increments 

of vertical meaS\H'eanent are rnerely multiplied by the 

appropriate}( factor to obtain differences in altitude 

in :feet. A constant :factor for con ve1ting increments 

of vert.ical rneasurernent to feet is appropr.iate because 

the horizontal a.ncl verticnJ scales are equal and constant throughout the stereoscopic model, but when 

paper prints are used, H1e horizontal scale is not constant:. th rougho11t the model and hence different multiplicn.tion factors must be determined for different 

levels in the anodel. 

Because the horizontal and vertic:tl scales .in stereo-

:;copie rnodels fron1 double-projection instruments are 

equn.l and conshtnt throughout the model, inclined distances, such as stratigr:tphic thicknesses of dipping 

beds, anay also ·be anea.~ured. A special device for 

rnea.stu·ing dist:wces in inclined directions was developed by R. lT. ~{orris and C. L. Pillmore of the Geological Survey; it consists of a multiplex tracing table 

modified so that the platen, platen socket, and light 

source ca..n be tilted n.s a unit (fig. 22). An :tdditional 

woa·m-screw drive, separate from the conventional 

worm-screw da·ive, allows the platen to move in an inclined direction perpendicul:tr to the surface of the 

platen. The resulting measured distance within the 

model is convel'ted to feet in the usual manner, and a 

tneasure of H1e inclined dista.nce is obtained. 

TJLT 

Tilt in present-day vertical photography is generally 

sanall-under good flying and photographic conditions 

fiO percent of the photographs taken for domestic 

mapping are reported to l;e tilted less than 1 o and 90 

pet·cent tilted less tha.n 2° (see Tewinkel, 1952, p. 319). 

But t·ilt rnay be si~niflcant in the photogrammetric 

calculntion of rnany g:eologic measurements, and care 

n•ust therefore be exercised in determining altitudes 

from par:tllux rnensurement's. The signiflcance. of tilt 

on vertical measurements used in geologie interpretation depends in lar~re {)ILJ't on t:he O"eoloo·ic problem. ~ b b 

'V'here vertical measurements nn1st be referred to a 

horizontal datum, as in structure contouring, tilt nmy 

cause significant errors, but where relative vertical 

n1easurements between pairs of points are desired, as in 

constructing isopach lines, tilt may have relatively little 

effect especially where points of any one pair are separated by only a small horizontal distance (see p. 

73). If only simple streometers are available for 

parallax determinations it may be necessary to entirely 

discard some photograph prints unless (a) vertical 

control is available and the geologist carries out the 

tedious task of constructing a correction graph (Desjardins, 1950, p. 2:304-2305; ~1cNeil, 1952, p. 610~615; 

and Visser, 1954, p. 849-853), or (b) points of measurement are selected so that the effect of tilt will be 

minimized (Hemphi.ll, 1958a, p. 46-47, 49). It is also 

possible to partially correct for tilt locally on planar 

orientations, such as the strike and dip of beds, by 

stereographic projection and appropriate rotations in 

stereograph ic constructions. 

If photogrnmmetric instruments are available that 

allow photographs to be tilted, it is generally possible to eliminate any large amount of tilt (greater 

than 1°) by careful inspection of physiographic features of the stereoscopic model, thereby permitting 

reliable parallax measurements to be made even 

though vertical control is not available for leveling 

the model. For example, lakes or other standing 

bodies of water may appea . .r tilted, or certain streams 

or stream meanders may appear to flow uphill (fig. 

10n). Tilt is also often detected by observing that 

headwater tributaries in subdued divide areas where 

gradients are very low appear to flow uphill. Adjustment of the instrumen-t so that the lakes appear flat 

and streams have a normal gradient will minimize 

tilt, and parallax measurem'?nts can then be made. 

For certain sensitive measurements, such as measurement of low stream gradients or measurements used to 

determine the strike of low-dipping beds, it is essential that vertical control for leveling models be available and that a precision plotting instrument, such 

as the I\:elsh plotter, be used. Because a small arrwunt 

of tilt may seriously affect the measure of stream 

gradient the economic advantage of photogrammetric 

techniques may be lost where the measure of gradients is the primary photogrammetric objective, inasmuch as a considerable amount of vertical control 

must be obtained by ground survey to level the stereoscopic models. · I-Iowever, for many geologic measurements, particularly where relative intervals are to be 

measured, ground control, although desirable, is not 

essential for making satisfactory parallax determinations, either from paper prints or glass-plate dinpositives. 
56 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

DETERMINATION OF ALTITUDE DIFFERENCES FROM A SINGLE 

PHOTOGRAPH 

Under certain conditions altitude differences between 

two points may be determined by making .appropriate 

measurements of radial displacement on a smgle photograph and substituting them in the formula 

h=mH r 

where h=difference in altitude desired; 

m=relief displacement of upper image point 

with respect to the lower image point; 

H=height of airplane above lower_ image point; 

and r=radial distance from principal point of photograph to lower image poirit:-· 

The distance m and r must be mea$Ur~.d.-:iri· the same 

units. The height, h, will then be ~.n ~terms of th~ units 

chosen for H. · 

This procedure is only applicable to vertical objects 

where the upper point is known to be directly above 

the lower point. Under these restricted conditions, 

the length of the image of an object, such as a tree, 

or a cliff face at 'right angles to a radial line from 

the _principa1 point, is a measure of the relief displacement. The radial distance to the lower point can be 

easily measured, and the flying height can be obtained 

from the formula 11 =f/S (see p. 53-54) when base 

maps are available, or from the specifications of the 

photographic mission. Thus, for example, if the .radial displacement of a cliff is 0.03 inch, the flying 

height of the airplane 10,000 feet, and the lower point 

is 3. inches from .the photograph center, the height of 

the cliff will be 

0.03 X 10,000 

3.0 h 

h=100 feet. 

Because relief displacement is generally small, a small 

error in measuring such a distance on a single photograph will result in a relatively large error in the 

height of the object, which further limits the usefulness of the formula for determining vertical intervals. 

GEOLOGIC USES OF PARALLAX MEASUREMENTS 

Parallax measurements always result in the determination of spot altitudes or vertical intervals when 

p:tpers prints are used; and generally result in spot 

altitudes or vertical intervals when glass-plate diapositives are. used in double-projection instruments, 

except when a special tracing table is used that permits measurement of inclined distances (fig. 22). 

These spot altitudes and· differences in altitudes combined with horizontal distances determined from the 

stereoscopic model, or under some circumstances from 

a single photograph, may be used to compile structure-contour maps and to determine dips of beds, 

thicknesses of beds, offsets on faults, gradients of 

streams, and related data. 

Because of relief displacement, however, it may be 

necessary to correctly locate the relative horizontal 

positions of points, whose altitudes ~ave been ~~asured before computing strikes and dips, determining ' . stratiO'raphic thicknesses, or any other geologic meas- o . . 

urements where dips are involved. Because tilt IS 

· usually negligible in present-day photography, hori-

_zontal positions of points generally may be plotted 

satisfactorily with any instrument that removes relief 

displacement, such as the radial planimetric plotter, 

paper-print plotters, and double-projection plotters 

even though the stereoscopic model cannot be leveled. 

Horizontal distances may then be scaled off. However, in the absence of plotters for locating the relative map positions of radially displaced points when 

measurements are made with parallax bar and paper 

prints, an overlay or similar procedure may be used 

to determine corrected horizontal distances, unless orthophotographs (see page 69) are available from 

which correct horizontal distances may be scaled. 

The overlay procedure requires first laying out on 

transparent material a line equal in length to the adjusted photobase. The overlay is then placed over the 

right photograph of the stereoscopic pair so that the 

li~~e drawn is coincident with the flight direction 

and its right end terminates at the photograph center. 

Radial lines are then drawn on the overlay from the 

photograph center through all points whose relative 

horizontal positions are to be determined. The procedure is repeated with the overlay positioned over 

the left photograph, again with the original line coincident with the flight direction and its left end terminating at the photograph center. The intersection 

of a pair of lines through the same image points is 

the corrected horizontal position for that point. 

DETERMINING STRIKES AND DIPS 

vVhere bedding surfaces coincide with topographic 

surfaces it is generally sufficient to measure the altitude difference between only two points, one directly 

downdip from the other in calculating the amount of 

dip. The horizontal distance between these two points, 

to(J'ether with the difference in altitude, gives the re- e 

quirecl information for determining the angle of dip 

from the trigonometric relation 

Vertical distance =-~tangent of angle oip. Horizontal distance 

If relief in an area is low the horizontal distance 

may be scaled directly from a single photograph with-
MEASUREMENT 57 

out significant el'l'Ol.' in computing the dip, but where 

relief .is rnoderate or high a correction for the relief 

displacernent of the upper point with respect to the 

lower point generally should be made. In the unique 

circumstn.nce where the strike is radial from a photogrn.ph center or where the stu·face on which the dip 

to be .measured .is at or near a photograph center, 

there .is little or no relief displacement in the dip 

direction, a.ncl no correction in scaling the horizontal 

distance need be made. I:Iemphill (1958a, p. 53) suggested that the corrected horizontal distance between 

the upper and lower points should be at least 0.2 

inch for computing clip a.ngles. Where clips are 

greater than 50°, a longer horizontal distance is needed 

to determine the clip angle reliably, as the tangent 

increment per degree of clip is significantly greater 

for steep angles of clip than for low angles of dip. 

l''he strike line generally can be determined readily 

by inspection of the stereoscopic model and noting 

two points of equal altitude on a bed. Where clips 

are ]ow, however, tilt in the photographs will affect 

the direction of strike; the ]ower the clip, the greater 

the effect, generally, on the amount of shift in the 

azimuth of the strike line. Graphs may be constructed 

to show the effect of tilt on low-clipping beds. (See 

I-Tem ph ill, 1958a, p. 48.) 

"Vhere a bed crops out in a valley wall it will generally be necessary to determine the altitudes of three 

points on the bed. In areas of moderate or high relief corrections for relief displacement must be made 

before distances between these points can be determined; otherwise distances may be scaled directly from 

the single photograph. Determination of the strike 

direction and amount of dip are then made graphically or graphically and trigonometrically. 

DETERMINING STRATIGRAPHIC THICKNESS 

In areas where beds are horizontal or nearly horizontal the stratigraphic thickness may be determined 

directly by converting to feet the parallax difference 

between the· top and the bottom of the bed. No correction is necessary for relief displacement. However, 

if beds are inclined, the angle of dip must first be 

determined; then corrections must be made for relief 

displacement and for the effect of dip on the stratigraphic thickness. The thickness may be calculated 

by simple tt·igonometry or by graphie solution (fig. 

26). 

Desjardins ( 1950, p. 2308-2309; 1951. p. 829-830) 

suggested that floating lines be employed with paper 

prints for determining stratigrnphic. thicknesses where 

beds do not dip more than 15°. In the "floating line" 

procedure, lines are drnwn on transparent strips of 

F'IC:UlH~ 26.-.Diagmm of gently dipping beds showing relation of stratigraphic thickness to differential parallex determined at 

any two points along dip direction and at the formation contacts. 

Stratigraphic thickness is determined from the formula 

where !=stratigraphic thickness; 

h d l=x+x'=--+-.- cos 8 Sill 8 

h=difference in altitude between some point on the lower contact of the bed and some point, along a line at right 

angles to the strike line, at the upper contact of the bed; altitude his determined from the parallax formula; 

d=corrected horizontal distance between points at lower and upper contacts of the bed; 

and 8=nngle of dip. 
58 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

material that are superposed over each of the photographs. "'Vhen one line over one photograph is posi~ 

tioned parallel to a line over the second photograph, 

a single floating line will appear when viewed streoscop.ically unless the lines are parallel to the flight 

direction. The line will float above or within the 

model and appear to be horizontal; its vertical position in the model will depend on the separation of the 

two individual lines in the flight direction. A series 

of lines will form a grid or reference plane (Smith, 

H. T. U., 1943a, p. 171; Desjardins, 1943a, p. 219; 

Hackman, 1957, p. 593) . vVhen any one pair of lines 

is divergent rather than parallel the floating line in 

the stereoscopic model will appear to plunge. By 

changing the horizontal separation of any two lines 

in the flight direc6on the floating line can be made 

to rise or fall in the model, just as with the floating 

dot of the parallax bar. "'Vhen the floating-line procedure is used in measuring stratigraphic thicknesses, 

a line is first floated preferably in the bedding plane 

at the upper formational contact, and parallax measurements are then made at the lower formational contact and on the floating line at a position in space 

vertically above this point (see fig. 27). If beds are 

horizontal or nearly horizontal the· parallax difference will be approximately a n1easure of the stratigraphic thickness. If the beds dip, the stratigraphic 

thickness will bear a cosine relation to the clip angle 

and parallax measurement (see fig. 27). 

vVhere dips are steep it may be desirable to measure 

a horizontal distance along a line at right angles to 

the strike direction between two formation contacts 

and relate this distance and clip angle to stratigraphic 

thickness. Points must be chosen at approximately 

the same altitude in the stereoscopic model (fig. 28). 

ISOPACH MAPPING 

The use of aerial photographs in studies of certain 

areas in the Colorado Plateau of "'Vestern United 

States has included the photogrammetric compilation 

of isopach maps that show local thickenings of certain 

formations in which uranium minerals are likely to 

occur. These thickenings of formations are cha.nnelfill deposits in stream channels that ·were cut in the 

underlying format.ion. Uranium minerals are locally 

concentrated in these deposits. In the ~tfonument Valley area of southern Utah it has been demonstrated 

that isopach lines, indicating the trends and thicknesses of different channel-fill deposits, could be drawn 

Floating line in stereoscopic rnodel 

FIC:URE 27.-Dia.gram of ge11tly dipping beds showing relation of stratigraphic thickness to differential parallax determined by 

floating-line method. 

Stratigraphic thickness is found from the formula 

where t=stratigraphic thickness; 

h 

t=--

cos 0 

h=difference in altitude between bottom of bed and point on floating line vertically above, as determined from the 

parallax formula; 

and O=angle equivalent to angle of dip. 
MEASUREMENT 59 

0.11 the basis of measurements made on vertical aerial 

photographs. li1 this area uranium minerals are pressent: in channel-fill deposits at the base of the Shinarump rnembe1· of the Chinle formation of Triassic age. 

:However, because the top of the Shinarump member 

is eroded away .in many areas, or because it is gradational with the overlying member of the Chinle 

foi'Jnation, a stratigraphic unit below the Shinarump 

member was chosen as the unit for which isopac;h 

lines were dmwn. A thinning of this underlying ur.1it 

then was interpreted to indicate a thickening of, or 

channel deposits of, the overlying Shinarump member. The unit selected for isopach measurements, the 

Afoenkopi formation, is expressed on aerial photographs as a. dark-toned slope-forming unit that contrasts markedly with the light-toned cliff-forming 

units of both· the underlying De Chelly sandstone 

member of the Cutler formation and the overlying 

Shinarump member of the Chinle formation (fig. 44). 

A series of altitude measurements was then made 

:dong the contact of the ~1oenkopi and Shinarump 

:fornuttions from aerial photographs by means of the 

Kclsh plotter, and the locations of these measureJncnts were plotted on a base sheet. Similarly, a series 

of altitude measurements was made along .the contact 

of the De Chelly and :Moenkopi formations and the 

locations were plotted on the base sheet. ~1easurements along the respective geologic contacts were 

generally spaced at horizontal distances of about 500 

feet. Because the stratigraphic units dip locally it 

was necessary to compl.1te the strike and dip of the 

beds and to correct for the dip angle in determining 

the stratigraphic thicknesses of the ~1oenkopi iriterval. 

Thickness computations were plotted at the points of 

altitude measurements along the contact of the Moenkopi and the Shinarump, and isopach lines were 

drawn at 10-foot intervals on the basis of the distribution of thickness figures. The relatively large number of readings, both for strike and dip of beds as well 

as for altitude measurements, permitted elimination 

of certain computations that were inconsistent with 

regard to the overall mass of statistical data. The 

locations and depths· of channels of Shinarump member as shown by isopach lines based on photogrammetric measurements and plotting were generally in 

close agreement with those determined by field Jnethods. Details of the photogeologic study are described 

by "Titkind, Hemphill, Pillmore, and ~{orris (1960). 

FACII<:S CHANGE 

l\1easurements from aerial photogntphs in conjunction with stratigraphic studies of Cretaceous rocks in 

northern Alaska have demonstrated the rate and direction of "sha.ling," or facies change, of certain formations. In the Utukok-Corwin area resistant rocks of the 

Kukpowruk formation, which is predominantly silty 

shale, siltstone, and sandstone, overlie less resistant 

• o I • 

. . . . . ... ·x· :- . · .. :- · .. ';,··· . .. . . 

~ ~ • • • •. . . • • I 

P1<11IR~~ 28.-Diagmm of ~t.ccply dipping bedlj ljhowing relation of stratigraphic thickness to dip angle and horizontal distance 

between top and botton of bed. 

St.rat;ig•·nphic t.hiekness is determined from the formula 

where l=stmtigmphic thickness; 

t=~ sin 8 

d= horizontal distance at right angles to strike line between points at same altitude on top and bottom of bed; 

aud 0= angle of dip. 
60 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

rocks of the Torok formation, which is largely clay 

shale, claystone, and silty shale. The Torok formation 

commonly forms the lowlands, which are characterized 

by rather uniform and low-dipping topographic slopes. 

The Kukpowruk formation forms high, resistant ridges 

that can be traced many miles. The contact of the two 

formations is marked by a break in topography that 

has been mapped in the field in conjunction with the 

study of aerial photographs. Slopes below the contact 

are generally smooth and unbroken; slopes above the 

contact are marked by conspicuous topographic breaks 

caused by hard, resistant sandstone beds in the basal 

part of the I\:ukpowrtik formation. Vertical exaggerati~n of the stereoscopic model accentuates these topographic breaks in slope. Photogeologic and field studies show that these resistant beds pinch out usually in 

an easterly 1 or northerly direction (see fig. 97). It is 

significant that their topographic expression is lost. 

Ground observations along stream cuts indicate that 

the resistant beds grade lateraJly into shaly sections. 

Quantitative studies from aerial photographs of the 

amount of section affected by facies change in the 

Utukok-Corwin area show that several thousand feet 

of sandy section of the I\:ukpowruk formation grade 

laterally to the east and north into shale of the Torok 

formation. Chapman and Sable (1960) stated that the 

methods used in these studies included tracing on vertical aerial photographs a resistant unit within the 

l{ukpowruk formation around the flanks of a large 

open structure and then measuring the stratigraphic 

intervals between the resistant bed and the contact of 

the I\:ukpowruk and Torok formations to determine 

the new relative stratigraphic positions of the contact 

in different p~~rts of the area. Photogrammetric measurements were made whenever possible in localities of 

good field control; a few stratigraphic thicknesses were 

determined solely by photogrammetric methods without field control. Locations for measurements were 

chosen along east-west and north-so11th lines. Relative 

stratigraphic positions of the contact of the I\:ukpowruk and Torok formations were then plotted graphically against lateral distances between· points of measurement; this resulted in a line whose gradient showed 

the average rate of rise of the contact between points 

of measurement. An average eastward-rising gradient 

of 58 feet per mile and an average northward-rising 

gradient of 97 feet per mile were calculated. 

It was concluded on the basis of plotting these average gradients as vector quantities that a resultant maximum gradient of 115 feet per mile exists and that 

more than 10,000 feet of sandy beds grade into shaly 

sections over the area studied. The maximum gradient 

direction was further interpreted to lie at right angles 

to the direction of minimum facies change, which is 

believed to indicate the general trend of the old shoreline during the period of deposition of the l(ukpowruk 

and Torok sediments. Thus the photogeologic measurement of stratigraphic intervals not only permitted 

determination of the directions and amounts of facies 

change, but further allowed inferences concerning 

the geologic history and environment, which may be 

significant in the search for petroleum in that area. 

STRUCTURE CONTOURING 

Structure contours are usually constructed on the 

basis of a series of sf>Ot altitudes, some of which may 

be photogrammetricaJly determined at the top of the 

horizon to be contoured, but many of which will be 

measured at formation contacts above or below that 

horizon. Photogeologic procedure is similar to field 

procedure and requires adjusting the altitudes of aJl 

points not on the horizon to be contoured by making 

appropriate considerations of strike and dip and stratigraphic thickness. Structure-contour lines are then 

adjusted to the resultant series of altitudes projected 

to the same formation contact. 

NO'l'Ol\f-15 QUAI>ltANGU: 

In photogeologic interpretation and mn,pping of t.he 

Notom-15 quadrangle, Utah, (Hackman and Tolbert, 

1955) altitudes for structure contouring \Yere measured 

using a I\:elsh plotter. No previous geologic or topographic mapping had been done in the area; geologic 

data and altitude measurements were plotted to a control net established by photogrammetric methods. The 

Notom-15 quadrangle is well-exposed canyon country 

typical of the Colorado Plateau of Utah. Gently 

folded sedimentary rocks of Triassic and ,Jurassic age 

underlie the area. Canyons transecting the general 

structure trend made the application of photogeologic 

procedures ideal. A series of altitude measurements 

were made along the contact of the 'Vingate sandstone 

of Triassic age and the overlying I\:ayenta formation 

of Triassic ( ~) age. The wide exposure of this contact 

throughout the area permitted sufficient measurements 

to control structure contouring. Structure contours 

were then drawn on the top of the vVingate sandstone. 

Subsequent fieldwork corroborated the general structure as contoured by I\:elsh plotter from aerial photographs and also corroborated the distributions of 

rock formation as interpreted and mapped from photographs. Positioning of other planimetric data., such 

as streams, was shown to be in excellent agreement 

with the subsequently compiled st.anda,rd topographic 

map of the quadrangle. The positioning of data and 

altitude determinations by I\:elsh plotter thus resulted 

in a highly reliable geologic map of the area. 
MEASUREMENT 61 

JHSOOVERY ANTICLINE 

Procedures used in contouring the Discovery anticline in northern Alaska included stereometer measurement of altitudes combined with simple trigonometric computations and graphic constructions to 

compile a generalized structure-contour map (~1arshall 

n.nd Rosendn le, 1953). These procedures contrast 

mn.rkedly with the J(elsh-plotter compilation of the 

Notom-15 quadrangle, but demonstrate that simple 

methods ma.y be useful in obtaining structural information of a reconnaissance nature fron1 aerial photographs. 

Discovery anticline, a gently folded structura'I feature a.pproximately 25 miles long and 7 miles wide, is 

underlain primarily by Cretaceous rocks. The area 

is generally covered with tundra grasses, but resistant 

beds within the straJigrn.phic section are expressed 

topographically as ridges, or breaks in slope, that can 

be traced for ma.ny miles; some beds are expressed by 

photographic tone due to differences in vegetation .(see 

fig. 40). Dip slopes rarely coincide with topogntphic 

surfaces. 

Procedures for obtaining and positioning data for 

st.ructure contouring involved the arbitra.ry selection 

of cross-section lines normal to the general structural 

trend. Strikes and dips of beds were then determined 

by simple stereometer methods at. numerous localities 

a.nd projected to the nearest cross-section line. '~There 

the strike line did not intersect the line of section at 

right :mgles the apparent dip was determined tind 

plotted on the section 1 in e. This procedure was followed for all strikes and dips, and resulted in crosssection lines along which the different dips were 

plotted. A marker bed was then selected and its profile along the line of section graphicaJly reconstructed 

on the basis of the positions and amounts of dip previously plotted (fig. 29). It was assumed that bedding 

throughout the stratigraphic section was parallel. 

Structure contours were then positioned along each 

line of section by further graphic constructions (fig. 

30). I-Iowever, the datum for each line of section 

commonly was different, depending on which part of 

the stratigraphic section was expressed in any one 

local area, and it was necessary to obtain stereometer 

measurements of altitudes between cross-section lines 

so that structure contours, as positioned along each 

line of section, could be tied together (fig. 31). The 

resulting reconnaissance map, plotted on an uncontrolled photomosaic, indicated the general magnitude 

and closure of the anticline and the attitude and steepness of bedding. Figures 29-31 show graphic constructions in reconnaissance structure contouring from 

measurements made with stereometer-type instruments. 

DETERMINING DISPLACEMENTS ON FAULTS 

The vertical component of displacement on. $Ome 

faults may be readily determined from parallax measurements made on mn.rker beds on opposite sides of 

the fault by simple conversion of parallax difference 

to feet or other appropriate unit of measurement. If 

the fault plane dips, relative altitudes- on marker beds 

may be related to the down-dip component of displacement by trigonometric or graphic solution. 

Top of marker bed 

Line of section 

l!'IGUlt~: 29.-Diagram showing graphic reconstruction of marker bed along line of section. 

I )ip~ 1letel'lnined ft·om aerial photographs are first plotted alonl! line of section. 'rhe marker bed is then reconstntct:ed hy extending hell upward ft·om point n at 12o 

di11 fot· half the distance between a, and b. l!"'rom this 

point the hell is extended nt 15° dip to a point half the 

distance between b atHl c. l!"'rom this point the bed is 

again extended at 20° dip to a point half the distance between c and d, and so on. A smooth curve is then clra wn 

to represent the marker heel. 
62 

_J 

oct: 

AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

~ 400' r-------------------------------------------------------------------------------~ w 

fz 

~ 300'r-------------------------------~------------------------------~/, 

0 

fz 

0 

(.) 

I 

w 

0:: 

::::> 

1-

(.) 

::::> 

0:: 

fen 

HORIZONTAL LINE OF SECTION 

FIGURE 30.-Diagram showing graphic construction in positioning structure-contour lines on top of marker bed and the 

projecting of these lines to their relative horizontal positions along the line of section. 

A 8 

300' 

1650' 

1400' 

0 

0 

1350' 

FIGURE 31.-Diagram showing relation of structure-contour positions along two lines of section at different altitudes. 

Relative positions of 50-foot structure contours along B lie at 1,350 and 1,400 feet, respective!;\', it is necessary 

each line of section are shown to the right of each to connect the 50-foot contour po~ition on line A. with the 

line. Absolute altitudes of structure-contour positions are zero contour position on line B, in compiling the strucshown to the left of each line of section. Because the zero ture-contour map. 

points on the reconstructed marker bed along lines A and 
MEASUREMENT 63 

DETERMINING STREAM GRADIENTS 

Stream gradients may be calculated from parallax 

measurements under certain circumstances. Because 

many stream gradients are very low-a few feet or 

tens of feet per mile-an instrument should be used 

that will permit correction for tilt in the photographs. 

It will also be desirable to have vertical control for 

the removal of tilt, as a small amount of tilt not removed from the stereoscopic model could seriously 

affect the computation of stream gradient. If a stream 

has a · meanderi1i.g course, then the horizontal distance 

along the stream bet\Yeen points of measured parallax 

must be determined by a chartometer or similar instrument for measuring distances along a curved path. 

DIRECT DETERMINATION OF SLOPE 

DIPPING PLATEN 

In addition to the indirect determination of slope 

by the use of parallax measurements in conjunction 

''"ith trigonometry or graphic solution, direct measurements of some slopes may be made, both from glassplate dispositives in double-projection instruments and 

from paper prints. The simplest procedure is with 

double-projection instruments and involves the use of 

a modified platen that can be tilted to coincide with the 

dip of a topographic surface or bedding plane (fig. 

32). Because all features of the terrain are optically 

re-created in the stereoscopic model of double-projection instruments in essentially true relation, the dip of 

the platen when made to coincide with the ground surface, or to intersect three or more points on a bedding 

plane, will be a true measure of the dip of that surface. The dip can be measured with a clinometer atFIGURE 32.~Dipping platen. 

tached to the platen or by an accessory device such as 

a "devil level" or Brunton compass. Dips below 40° 

can be readily obtained by this method, provided topographic expression or rock outcrops permit positioning 

of the platen. Dips greater than 40° are difficult to 

determine owing to the difficulty of positioning the 

dipping platen in the plane of a steeply dipping surface; steep dips should be computed using the parallax 

methods described above. 

STEREO SLOPE COMPARATOR 

A unique method for determining angles of slope 

from paper prints is employed with the stereo slope 

comparator (fig. 33), which was developed by the U.S. 

Geological Survey (Hackman, 1956b, p. 893-898) . The 

instrument consists of two gear housings each containing a horizontal shaft on which small targets are 

mounted that can be fused stereoscopically into a single 

target. Swing of the horizontal shafts permits positioning the targets in any direction of strike. The gear 

housings are mounted on sliding tubes that permit the 

targets to be separated horizontally so that the fused 

target appears to rise or fall in space, just as the dot 

in stereometer-type instruments. However, the dip is 

determined by physical tilting of the target in space. 

The instrument is unique in that it measures the 

exaggerated dip of the imaginary slope created by 

stereoscopic viewing. With vertical photographs of 

normal 60 percent overlap, most individuals see the 

terrain exaggerated in height from 2.5 to 3.5 times. 

The exaggerated dip, recorded on a protractor attachment on the stereo slope comparator, must be reduced 

to a true dip. This is accomplished by reference to 

FIGURE 33.;-Stereo slope comparator. 

. . 

INSTRUMENTS FOR DIRECT DETERMINATION OF SLOPE 
64 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

a, slope-conversion graph made up of intersecting lines 

that represent true angle of slope, exaggerated angle 

of slope, and exaggeration factor (fig. 34). Construction of the graph is based on the relation 

Tangent true angle of dip 

tangent exaggerated angle of dip 

exaggeration factor 

In determining dips from a pair of paper prints 

it is necessary to know the exaggeration factor of the 

interpreter for that stereoscopic model. This factor 

varies slightly \Yith different individuals and is determined by reference to a supplementary slope chart on 

which a dipping surface of specific dip has been constructed by methods described by Hackman (1956a, 

p. 387-391). By measuring the exaggerated dip of 

this constructed surface, known to represent a given 

true angle of dip, an exaggeration factor is obtained; 

it is then possible to make measurements on a specific 

pair of photographs and to convert any exaggerated 

dip to a true dip. Because of the difficulty of placing 

the tilted target in a steeply dipping plane, the instrument is most reliably used where true dips are 20° or 

less. 

DIP ESTIMATION 

From paper prints an experienced interpreter can 

estimate dips, particularly in the range of 1° to 5'0 , 

with considerable reliability, as a result of vertical 

exaggeration in the stereoscopic model. Because of 

vertical exaggeration, angles of slope in terms of their 

tangent functions are exaggerated in the stereoscopic 

_models as much as 2.5 to 3.5 times, with the result that 

low-dipping .surfaces, which are in places difficult to 

observe in the field, are readily interpreted (figs. 45, 

88, 90, 91, and 97). For example, a true dip of 1° may 

appear to be a dip of 3° or 4° in the stereoscopic model, 

and a dip of 5° may appear to be 15° to 20°. For true 

dips below 20° an error of a few degrees in estimating 

the exaggerated dip has only a small effect on the true 

dip (see fig. 35), and an interpreter is able to make reliable visual determinations of these dips from most 

stereoscopic models. But steeply dipping surfaces are 

also exaggerated and made more difficult to evaluate. 

Any small error in determining the angle of exaggerated dip of steep slopes results in a relatively larger 

error in the true dip calculation for that slope; Brundall and Harder ( 1953, p. 150) stated that "the accuracy of individual estimation is generally inversely proportional to the steepness of dip." In estimating dips 

a correction obviously must be made for the oversteepened slopes as they appear in the stereoscopic 

model (see figs. 34 and 35) . 

Factors affecting vertical exaggeration in stereoscopic models have been. discussed by several writers 

(Treece, 1955; Goodale, 1953; Thurrell, 1953; ~{iller, 

1953; Aschenbrenner, 1952; and Stone, 1951), but con1-

plete agreement has not been reached concerning the 

effects of specific factors such as viewing distance and 

image separation. However, once an interpreter has 

correlated amounts of dip, determined photogrammetrically or by field procedure, with a given set of 

conditions such as type of stereoscope, focal length of 

photography, photo base, and image separation, reliable 

estimates of true dip can be made over wide areas 

without reference to known dips. 

OTHER METHODS OF DETERMINING ANGLES OF SLOPES 

Another method of determining slopes in stereoscopic models is based on the floating-line princi pie. 

Brundall and Harder (1953, p. 150-151) briefly mentioned a device consisting of two lines scribed on plastic 

plates. The lines, seen stereoscopically as a single line 

floating within the model, can be made to converge 

so that the floating line appears to plunge in space. 

The plunging floating line is placed in the component 

of dip direction at right angles to the flight line. The 

angular rotation of the scribed lines required to obtain a given amount of steepness of plunge of the floating line is then related to true dip by means of a 

graph. As this method wQuld result in true dip only 

for beds striking parallel to the flight line, the correction graph must also consider the azimuth of the 

dip component observed with respect to the strike of 

the beds. 

Wallace (1950) has used angular relations in the 

plane of the aerial photograph as the basis for a 

method of determining strikes and dips from aerial 

photographs. The method is best applied where erosion has exposed bedding trace lines as contrasted to 

broad topographic surfaces that coincide with bedding. 

No parallax measurements are required. It is necessary to measure the angular relation of two bedding 

trace lines with respect to a known or assumed azimuth 

line on each photograph of a stereoscopic pair and to 

determine the plunge of the line of sight from the 

camera position to the outcrop, or bedding-trace line. 

These data are then plotted orr a stereographic net to 

determine the strike and dip of the bed C\;Vn.llace, 1950, 

p. 275). 

OTHER METHODS OF DETERMINING QUANTITATIVE 

DATA 

~{easurement of geologic data, other than altitudes 

and direct determinations of slope angles, are only infrequently made from aerial photographs. I:Iowever, 

the possible applications of light-transmission measurements, from both black.,.and-whit.e and color pho-
EXAGGERATED SLOPE c9o. ~. <9~. c9"• <Pv. cSI"'· GJ.. ~ 

TRUE SLOPE 

FIGURE 34.-Slope-conversion chart showing relation of exaggeration factor, exaggerated slope, and true slope. 

:3 

·7.0 

6.5 

6.0 

55 

5.0 

4.5 

4.0 

3.5 

3.0 

2.5 

a:: 

l:".l 

> UJ 

~ 

~ 

l:".l 

a:: 

l:".l 

~ 

~ 

c.n 
66 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

0. 

0 

UJ 

::J 

ex: 

1-

EXAGGERATED DIP 

FIGURE 35.-Diagram, based on tangent functions, showing relation of true dips to exaggerated dips seen in stereoscopic 

models. 

tography, light-reflectance measurements, measurements related to heat-retention characteristics of terrain 

photographed, infrared absorption, size, frequency, and 

orientation measurements of physiographic features, 

and perhaps others, offer challenging opportunities for 

future study (see Ray and Fischer, 1960). 

PLOTTING 

Map compilation-planimetric positioning of datais basic to most geologic studies and generally requires 

methods of plotting data orthographically. Thus, 

data plotted or interpreted on vertical aerial photographs, most of which are perspective in view, must be 

replotted on an orthographic base. The orthographic 

base may be planimetric or topographic. Where. no 

base map exists perspective aerial photographs provide 

a medium for constructing a control layout on which 

geologic and other data may be plotted o~thographically. 
PLOTTING 67 

CONSTRUCTION OF CONTRO•L LAYOUT 

A control layout is an orthographic plot of a series 

of points selected on aerial photographs that cover a 

given area. These points are used to control the positioning of data plotted from the perspective photogntph. Plotting, or transfer, of data from the photogt·aph may be done with simple overhead projectors 

or sketchmasters under some circumstances, or with 

stereoplotting instruments described above. The accuracy of the finnl plot will depend primarily upon the 

control net u.nd the instrument used in plotting (see 

p. 74.-75). 

Bn.sically u. control layout or control net is established by triangulation from the aerial photographs in 

much the same manner as field triangulation is carried out. This nerial triangulation requires mechanical 

construction of templets which are assembled together 

into the trinngula.tion network. There are two general 

methods of constructing templets. One method involves rnditt.l triangulation using the principal point 

of the single photograph as a principal triangulation 

station to construct radial templets; the other method 

involves rndinl triangulntion using some orthographically positioned pass point from the stereoscopic model 

as a principal triangulation station to construct stereotemplets. 

Where single photographs ttre used for triangulation 

it is generally assumed that the photographs are truly 

vertical and have no tilt; relief displacement is radial 

from the photograph centers. However, a tilt of 1° 

ot· 2° does not significantly affect horizontal positioning over small areas. Templets from single photographs may be constructed by the hand-templet 

method, the radial-arm ("spider"-templet) method or 

the slotted-templet method. Photographs are fi'rst prepared by marking the centers, conjugate centers, and 

selected pass points, usually six in number, along the 

margins of a photograph; pass points are selected 

which are common to adjoining flight strips and common to succeeding and preceding photographs within 

the flight strip. From each photograph, azimuth lines 

are then constructed from the photograph center 

through all other selected points to form a t~mplet for 

that photogt·aph. Azimuth lines may be drawn in 

pencil on an overlay sheet (hand-templet method) ; 

they may be constructed of metal arms ("spider"-

templet, method), or they may be cut as slots in a 

piece of cardboard (slotted-templet method). After 

templets have been constructed for all photographs 

they are assembled into a triangulation network in 

which the intersections of azimuth lines locate the 

orthographic positions of the points selected. The 

result .is a base sheet with a series of points located 

553966 0-60-4 

in their correct relative horizontal positions. Where 

horizontal control points are available, the distance 

between plotted positions of any two of them determines the scale of the control layout. Details of constructing and assembling these templets are described 

by Kelsh ( 1952, p. 419-429). 

Construction of stereotemplets requires the use of a 

stereoplotting device to plot selected points common to 

adj-oining flight strips and within the flight str!p. The 

templet is actually a double templet of the area of the 

stereoscopic model rather than the single photograph. 

Azimuth lines are constructed for each part of the 

templet using one of the pass points as a principal 

triangulation station for one part of the templet, and 

using a different, generally diagonally opposite, pass 

point as the principal triangulation station for the second part of the templet. These two parts are joined 

together to form the completed templet for ea,ch 

stereoscopic model. Details of constructing and assembling the stereotemplets have been described by 

Scher ( 1955). Because . of the longer base line of 

stereotemplets the resulting triangulation network is 

said to have a stronger scale solution than can be obtained from radial templets constructed from single 

photographs. Other photogrammetric advantages are 

. mentioned by Scher ( 1955). 

ORTHOGRAPHIC POSITIONING 

PLOTTIN·G ON ORTHOGRAPHIC BASE MAPS OR CONTROL 

LAYOUTS 

The main objective in orthographic plotting of 

geologic data is to show the true planimetric relations 

of geologic features by removing relief and tilt displacement that may be present in perspective vertical 

aerial photographs. Relief displacement is commonly 

large, but tilt displacement is generally small and is 

usually disregarded when 'positioning geologic data on 

a planimetric base sheet. When positioning geologic 

data to a topographic base both relief and tilt displacement may be significant and require plotting from 

a leveled stereoscopic model. For horizontal positioning of data to a planimetric base in areas of low relief 

the relief displacement will be small;· overhead projectors may be used to transfer data from the photograph 

to base map or control net. For example, with common photography of 1:20,000 scale taken with a 6~inchfocal-length lens, radial displacement near the edg~ o~ 

the photograph will only amount tQ about 0.1 inch 

where topographic relief does not exceed 300 feet. 

Basically relief displacement of a give~ amount of 

relief is a function of flying height and is independent. 

of focal length and scale (seep. 74.-75). 

Where relief displacement is excessive it is· possible 
68 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

to change the photograph scale by racking the overhead projector up and down and, provided base map 

control is sufficiently dense, to scale small areas of the 

photograph to the base, thus compensating for much 

of the radial displacement of the perspective photograph. The reliability of the final plot will be in 

part a function of the density of control points on the 

base. The suitability of the final plot will depend 

on the geologic objective. On most radial triangulation networks it is common to find eight control points, 

exclusive of the photograph center, for each photograph; this number of points will not normally be 

sufficient to give a reliable plot of the geologic detail 

in areas of moderate or high relief unless a stereoplotting instrument is used. The vertical sketchmaster, in which the image of a single photograph 

is seen superposed on the base map or control sheet, 

generally is used in the same way that overhead projectors are used in plotting. The same general limitations of the overhead projector apply to the sketchmaster; that is, radial displacement due to moderate 

or high relief cannot be effectively removed. 

Orthographic plotting may also be accomplished 

with the radial planimetric plotter (fig. 14), or with 

the more complex paper-print plotters, such as the 

ICEK or Mahan plotters. (See p. 42, 45). The radial planimetric plotter is the simplest to operate. 

It is easily scaled to control points on the base map 

or triangulation network and effectively corrects for 

the radial displacement of vertical aerial photographs. 

Fine detail, such as the configuration of reentrants in 

contact lines, may be lost in plotting, however. Tilt 

inherent in photographs cannot be eliminated, but 

horizontal positioning due to small amounts of tilt 

found in present-day photography do not significantly 

effect horizontal positioning of photographic detail 

for most geologic interpretations. The ICEK and 

Mahan plotters may also be used to plot geologic 

data from paper prints. These instruments permit 

compensation for any inherent tilt that may be present in the photographs but are more difficult to operate than the planimetric plotter. 

Whenever· paper prints are used in plotting or 

transferring geologic data to base maps or control 

sheets it may be desirable to annotate, or mark, the 

geologic features to be transferred. This is particularly desirable when overhead projectors are used and 

is usually a necessity for all instruments if someone 

other than the interpreter is to do the plotting. 

Double-projection instruments afford a high degree 

of reliability in plotting geologic data from aerial 

photographs. The ability to correct for tilt and 

relief displacement combined with an enlarged view 

of the stereoscopic model when the l(elsh, n1ultiplex, 

or ER-55 plotters are used permits a highly reliable 

compilation of geologic data. Even when vertical 

control is lacking, it is possible to remove most of 

any tilt inherent in the stereoscopic model by careful 

reference to physiographic manifestations of tilt such 

as oversteepened or reverse stream gradients, tilted 

meanders of a stream, or tilted bodies of standing 

water (see p. 55). 

True orthographic positioning is not always attained, 

however, in geologic compilations from aerial photographs. Not only is there a question at times of the 

reliability of interpretation because of the subjective 

nature of much photogeologic ·study, but the various 

instruments themselves have differing degrees of accuracy inherent in their construction. Positioning of 

clearly identified geologic features by overhead projector, which does not permit adjustments for tilt or 

effective removal of relief displacement, would be far 

less reliable under most circumstances than positioning by double-projection instruments. However, the 

geologic product sought should determine in part the 

procedure to be used in plotting. Normally there is 

no reason to use an instrument that will plot detail 

more accurately than is needed to satisfy the geologic requirement. For example, in small-scale reconnaissance mapping, it is hardly necessary to use a precision plottit1g instrument for horizontal positioning. 

The problem of instrument requirement in relation to 

the geologic problem is further considered under a discussion of interpretation and plotting systems (see 

p. 69-75). 

Only where a significant saving in time and money 

can be. shown is it justifiable to use an instrument 

whose capability exceeds the minimum requirements 

of the geologic problem. Such a condition may exist 

for example where small-scale photographs of an area 

are available. Much of the United States is now 

covered by photographs at a scale of approximately 

1 :60,000. The use of stich small-scale photographs in 

an instrument such as the ICelsh plotter may reduce 

by as much as 90 percent the number of stereoscopic 

models that would normally be oriented for interpretation, measuring, and plotting if 1 :20,000-scale photographs were used. Even when 1 :20,000-scale photographs are used with simple instruments the common 

practice is to separate the operations; that is, the 

stereoscope is used to interpret geologic detail and 

another instrument, such as the stereometer, is used to 

determine vertical measurement. Thus, by using a 

precision plotter, which combines interpretation, measuring, and plotting, a significant saving in time and 

money may be expected. 
INTERPRETATION, MEASURING, AND PLOTTING SYSTEMS 69 

PLOTTING ON ORTHOPHOTOGRAPHB 

An orthophotograph is a photograph that has the 

position and scale qualities of a map. In addition 

it hns the abundant imagery of the conventional perspective photograph and hence a wealth of photographic deta.il (fig. 47). Theoretically the orthophotogrnph, with tilt and radial displacement eliminated 

in prepnration (Bean and Thompson, 1957), has the 

desirable detail of a photograph and the geometric 

characteristics of an accurate planimetric map. The 

method oJ constructing the orthophotograph from 

double-projection stereoscopic models results in a small 

loss of photographic resolution, but horizontal positioning accuracy is comparable to that of the standard topographic map of the same scale. 

The orthophotograph, when viewed with an alternate perspective aerial photograph, forms a stereoscopic model, although with only a small amount of 

stereoscopic relief as contrasted to viewing two perspective photographs. In viewing an orthophotograph 

with a perspective photograph it is generally best to 

use the same perspective photography as was used in 

the preparation of the orthophotograph. Combining 

the orthophotograph with a perspective photograph 

permits plotting of geologic detail from the perspective photograph to the. orthophotograph and gives a 

reliable photomap. Geologic data on the orthophotograph then can be transferred by direct projection to 

a topographic or other base, if desired, or merely 

traced off to give a reliable planimetric map of areas 

where no base maps exist. As an aid to field geologic 

mapping, in which a daily plot of observed data is 

desirable, the orthophotograph may be especially helpful. 

The range of potential applications of orthophotography has been summarized by Bean and Thompson (1957, p. 178) who stated that "* * * whenever 

the.re is IUl advantage in being able to determine accurately on an aerial photograph the positions of points, 

the lengths or directions of lines, the courses of curved 

lines, or the shapes or areas of given tracts, the orthophotograph offers a potential means of accomplishing 

the desired end." In addition, as the basis for constructing mosaics the orthophotograph has a significnnt application, especially in areas of moderate or 

high re.lief. 

PLOTTING ON GRIDDED BASE MAPS 

Gridded base maps mny be especially valuable for 

field plotting of geologic data without the assistance 

of photogrammetric instruments. In constructing a 

gridded base map a grid made up of small squares, 

usually 1 centimeter on a side, is first superposed 

photographically onto the perspective aerial photograph. The same grid is then orthographically transferred to the base map by a photogrammetric technician using stereoplotting instruments (Landen, 

David, oral communication, 1958). The grid lines are 

straight as superposed on the perspective photograph 

but are curved on the base map (figs. 115 ·and 116). 

By dividing the photograph into many small grid 

squares the effects of tilt and relief displacement become so small as to be negligible within any one 

square. Information sketched within grid squares on 

the perspective photograph is transferred visually to 

the base map by matching grid squares on the photograph with those on the map. The method may be 

especially useful in the field where it is desirable to 

keep a compilation of each day's work. 

VERTICAL POSITIONING IN CROSS SECTIONS 

In addition to the orthographic positioning of photogeologic data on base maps it is commonly desirable to plot geologic cross sections. The third dimension of the stereoscopic model is then added in part to 

the normal two-dimensional planimetric presentation 

of geologic information. Vertical positioning of geologic data, as shown in cross section, may be helpful 

to the geologist interpreting the aerial photographs 

of an area, as well as to the map reader who may be 

less familiar with the geology of the area. 

Cross sections are easily and rapidly constructed 

with a profile plotter from streoscopic models in 

double-projection plotters. Modification of the tracing table permits profiles to be drawn in any direction 

within the stereoscopic model (figs. 19 and 20). Sections may be drawn at a horizontal-vertical ratio of 

1:1 or they may be exaggerated as much as 5 times. 

When a 1 :1 ratio is used, dips of beds or components 

of dip, stream gradients, thickness of beds, and similar data may be obtained directly from the section. 

Exaggerated profiles may be desirable where relief of 

terrain is low, or, for example, when interpreting the 

significance of vertical position of thin stratigraphic 

units in correlation studies. Vertical positioning in 

sections may also show information suggesting the 

presence of faults. 

INTERPRETATION, MEASURING, AND PLOTTING 

SYSTEMS 

An interpretation, measuring, and plotting system 

(see fig. 36) may be defined as a combination of photographs and instruments that will permit attaining 

specific qualitative and quantitative geologic information from stereoscopic models. The optimum system is generally achieved by combining the minimum 
70 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

ORIGINAL MATERIALS INTERPRETATION MEASURING 

Paper-print plotters 

(Stereotope, KEK, Mahan plotters) 

Double-projection plotters 

(Kelsh, multiplex, ER-55 plotters) 

PLOTIING 

Orthophotograph 

Radial planimetric plotter 

Overhead projector 

Sketch master 

FIGURE 36.-Photogrammetric systems for geologists. 

scale photographs and minimum instrument capability that will prov~de the desired geologic measurements within given accuracy requirements. That is, 

the scale of photographs should be the smallest that 

will permit differentiation of photographic detail 

needed for qualitative interpretation, as well as the 

smallest that will permit photogrammetric measurements within specified limits. Fundamentally, the geologic objective of a particular study is the C{)ntrolling 

factor in the choice of the optimum system of interpretation, measuring, and plotting. The geologic objective may permit the use of existing instruments and 

photograph scales or it may necessitate new photography or instruments to attain given accuracy requirements. 

The type of geologic map arid its intended usethe geologic objective-can be considered on the basis 

of accuracy requirements or limits of error, both in 

obtaining vertical measurements a.nd in plan positioning of geologic features, provided photographic detail 

is adequate for qualitative interpretation. For example, a detailed structure-contour map may require 

an accuracy of vertical measurement within ± 5 feet 

of altitude and a horizontal position of geologic 

features within 1:/25 inch of true map position. In 

contrast a small-scale structure-reconnaissance map 

for planning purposes may permit a vertical tolerance 

of ±50 feet and have only general horizontal positioning requirements. Thus the choice of an interpretation, measuring, and plotting system may be reduced 

to certaih mathematical requirements that can be satisfied by combining instrument capability with appropriate photography; or the geologic objective or tolerances of measurement may be modified to permit the 

use of existing photography and instruments. 

However, geologic maps, unlike topographic maps, 

have no standard accuracy limits except those imposed by the individual geologist, with the one exception in the Geological Survey that solid lines, 

representing faults, contacts, or similar geologic features, should generally be within 1/25 inch of their 

correct map positions for general geologic maps at a 

scale of 1:63,360 or larger. Horizontal positioning of 

data within 1/25 inch of true map position may be 

accomplished with almost any plotting instrument or 

plotting system, especially if relief of the area. is low 

and excessive tilt is not present in the photographs. 

Hence, the vertical-accuracy requirement specified by 

the geologist for a given geologic problem is ideally 

the determining factor in choosing an _interpretation, 

measuring, and plotting system. This accuracy requirement, as well as the maximum range of measurements expectable for any system, may be equated 

against instrument capability and photograph scale as 

follows: 

Geologic requirement of verti- Instrument capacal-measurement accuracy bility in terms of K 

Expec~ed maxi~um rang.e of ~fin~ 3i factors (see 

vertical-measurement error g ) · 

Figure 37 is an elaboration of this equation and 

shows the general relations between the geologic requirements of vertical accuracy or expected maximum 

error in measuring spot altitudes or vertical intervals 
INTERPRETATION, MEASURING, AND PLOTTING SYSTEMS 71 GEOLOGIC REQUIREMENT OF VERTICAL- MEASUREMENT 

ACCURACY OR EXPECTED MAXIMUM RANGE OF VERTICALMEASUREMENT ERROR, IN TERMS OF INSTRUMENT CAPABILITY 

Datum leveled to vertical control 

SPOT MEASUREMENTS ±2K 

(for constructing structure·contour lines) 

INTERVAL MEASUREMENTS :!:3K 

(for constructing isopach lines 

determining offsets on faults.) 

SPOT MEASUREMENTS ±4V 

INTERVAL MEASUREMENTS ±6V 

Datum not leveled to vertical control. Model can be leveled 

visually to within 1° of horizontal datum 

SPOT MEASUREMENTS 

±2K ::!: error due to tilt 

INTERVAL MEASUREMENTS 

:!:3 K .:!: error due to tilt 

SPOT MEASUREMENTS 

±4V ± error due to tilt 

INTERVAL MEASUREMENTS 

±6V ± error due to tilt 

Model cannot be leveled 

SPOT MEASUREMENTS 

±4 v ± error due to tilt 

INTERVAL MEASUREMENTS 

±6V ± error due to tilt 

GEOLOGIC REQUIREMENT OF HORIZONTAL POSITIONING 

AT PHOTOGRAPH SCALE OR SMALLER 

Generally within 1/25 inch of true map position 

Generally not within 1/25 inch of true map position 

PHOTOGRAMMETRIC INSTRUMENTS THAT MAY BE USED 

Kelsh, multiplex, ER-55 plotters 

KEK, Mahan, stereotope 

Kelsh, multiplex, ER-55 plotters 

KEK, Mahan, stereotope 

Parallax bar, stereotope 

Kelsh, multiplex, ER-55, paper. print 

plotters. radial planimetric plotter I 

Any plotting instrument or system I 

Any plotting instrument or system I 

FwunE 37.-Photogmmmetric measuring and plotting systems for geologists showing relation of different instruments to verticalmeasurement and horizontal-positioning errors. 

where K factor=difference in altitude, in feet, on the ground 

per unit of vertical measurement in an 

anaglyphic-type model, that is, per 0.1 

millimeter 

model-scale denominator 2 

3,048 

where V factor= difference in altitude, in feet, on the ground 

per unit of measurement from a parallax 

bar or a paper-print plotter, that is, per 0.01 

millimeter 

flying height (feet) 

photobase (millimeters) X 100 

1 No excessive tllt assumed to be present for paper-print models that cannot be leveled. 

1 Model scale for Kelsh plotter ls usually about 5 times original photography scale; for ER-55usually about 3.5 or 5 times, depending on the projector model; and 

for multiplex usually about 2.5 times. 
72 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

in terms of instrument capability on one hand and 

photogrammetric instruments that may be used for 

measuring and plotting on the other hand. The determination of spot altitudes or relative vertical intervals 

satisfies most geologic requirements of altitude measurements from stereoscopic models and permits construction of structure-contour lines, and isopach lines, 

and the determination of clips of slopes, displacements 

on faults, and simHar data. Figure 37 can be used in 

choosing the best selection from existing photograph 

scales and instruments for measuring vertical distances, 

or it can be used to determine what combination of 

photograph scale and instruments would be optimum 

in n1aking vertical measurements within certain specified tolentnces. For economic reasons the geologist 

more often than not will be concerned with the best 

selection of existing photographs and instruments but 

this selection may not be the optimum interpretation, 

measuring, and plotting system. This may require 

modification of his geologic objective, at least in reporting the limits of measurement. 

INSTRUMENT CAP ABILITY 

Instrument capability, selected as the principal basis 

for the systems shown in figure 37, is an empirically 

derived factor generally considered with respect to 

topographic mapping systems as the minimum contour 

interval-stated in terms of a fraction of the flying 

height-that can be plotted to fulfill certain accuracy 

requirements. For geologic measurements it is desirable to consider instrument capability as the minimum 

increment of vertical measurement that generally can 

be determined by the geologist from the stereoscopic 

model. This permits a simple evaluation of expected 

maximum errors iiL geologic measurements when double-projection stereoscopic models are used, or when 

stereoscopic models from paper prints are used. It 

also allows the geologist to determine what photograph 

scale and instrument combinations will give him a desired accuracy of measurement. Because the minimum in~rement of vertical measurement is empirically 

derived, the limits of error for vertical measurements 

described below may be subject to modification. 

To determine the expected maximum error for a 

given combination of photograph scale and instrument three possible individual errors must be considered. These are (a) instrument capability in terms 

of the geologist's ability to measure horizontal linear 

parallaxes and increments of vertical measurement, 

(b) curvature of the datum within a stereoscopic 

model when double-projection instruments are used, or 

paper and lens distortions when paper prints are used, 

and (c) tilt. Consideration of the cumulative effect of 

all three possible errors is necessary in determining 

the expected maximum error for a particular measuring and plotting system, except for leveled models 

where tilt has been eliminated. In measuring spot altitudes or relative intervals these errors may be compensating rather than cumulative and therefore many 

measurements will be well within the expected maximum error determined for a particular combination of 

photograph scale and instrument. 

Experience has shown that for double-projection 

plotters, it is generally possible for the geologist to 

determine any one reading to one unit of vertical 

measurement. One unit of vertical measurement is 

equal to 0.1 millimeter. vVith care the increment of 

measurement may be read closer than one unit of vertical measurement. The amount of altitude diff~rence, 

in feet, on the ground represented by one unit of vertical measurement in the model-called the f{ factordepends solely on the scale of the projected stereoscopic model, and is determined by the ratio 

l\1odel -scale denominator 

3,048 

This factor is constant throughout the stereoscopic 

model. Hence spot altitude readings generally may 

be read within an amount equal to ±K, that is, the 

geologist may read a point one unit ( 0.1 millimeter) 

too high or one unit (0.1 millimeter) too low. In 

addition, an error of ± J{ is added for curvature of 

the model datum. Thus the maximum error for spot 

altitudes can be expected to be on the order of ± 21{ 

for models leveled to vertical control. 'Vhere errors 

due to reading and to curvature of the datum are compensating a minimum error approaching zero can be 

obtained. For interval measurements the altitudes of 

two image points in the stereoscopic model of course 

must be measured. Each point may be read within 

an amount equal to ±[{~or a total reading error ±2[{. 

Curvature of the model datum may affect the reading 

of each point a similar amount and can be disregarded 

when relative intervals are measured on points close 

together in plan view; for points close together the expected maximum error generally will be similar to that 

for spot altitudes. But if points of measurement are 

not selected close together an added error of ±K due 

to datum curvature may be introduced. Thus the 

maximum error for relative interval measurements is 

on the order of ± 3[{ (figs. 37 and 38). 

'Vhen vertical control is absent many double-projection stereoscopic models and some models from 

paper prints may be leveled visually to within 1° of a 

horizontal datum by observing certain physiographic 
INTERPRETATION, MEASURING, AND PLOTTING SYSTEMS 73 

A K 

AB+3K AB A:t;3-3K 

\ 

B'----

---- \ 

l!,IOUiti<~ 38.-Diagrnm showing dntum-curvnture reLation to maximum expected erro·r in measuring vertical intervals from 

leveled double-projection stereoscopic model. 

Let A IJ equal the interval tlesiretl. If the upper point is 

mensured nt A.' nn error equul to -1\., or one unit of measurement., mny result fr·om dntum curvature; when point B' 

is measUI'ed an el'l'or of +J( nuty result. If points A and 

W nre chosen for mensuring the interval and point A is 

mensut·ed one unit too high (equivnlent to +K> and point 

features in the model (see p. 55) and mn..king appropriate instrument adjustments. For double-projection stereoscopic rnodels that are leveled visually 

some error due to tilt thus may be present in addition to possible errors in reading and curvature of the 

tnodel datum; the order of maximum error for spot 

altitudes becomes ± 2[{ ± error due to tilt, and for 

relative interval measurements becomes ± 3[{ ± error 

due to tilt. '\Then relative stratigraphic thicknesses, 

displacements on :faults, or similar geologic measurements are being made, errors due to tilt or from curvature of the model datum may be minimized by measuring points close together in the stereoscopic model. 

For points spaced far apart the error due to tilt can 

exceed the error in reading intervals. 

Limits can also be placed on reading units of measurement with paper-print plotters or parallax bars, 

except t.hat a T1 factor rather than a [{ factor is employed. 1\. differentiation of factors is necessary as the 

V factor, unlike the [{factor, is not constant through-

?ut the model, but varies slightly due to dift'erences 

.111 scale at different nJtit.udes in models formed from 

paper prints. For practical purposes, TT is the ratio: 

Flying height (feet) 

Photobnse ( rn ill i meters) X 100 .. 

l'he V fact-or represents the difference in altitude in 

:feet on the ground per unit. of measurement from a 

parallax ba.r or paper-print plotter, t.hat is per 0.01 

millimeter. Experience ha~ shown that parallax measJJ' is measured one unit too low (also equivalent to +K) 

a maximum expected error, including. datum-cun:ature er.-

. ror, of +3H may result. Likewise if points A' and B are 

chosen for mensuring the · intet·vitl a maxiinum expected 

error, including dattim-curvature error, of -3K may result. 

urements from paper prints may be read wit.hin about 

±0.02 millimeter or ±2F. Considering possible errors 

due to paper or lens distortion and due to tilt,. total 

expected errors for spot altitudes a.nd relative intervals 

may be determined as shown on figure 37. (See also 

. p. 74.) Errors resulting from pa.per and lens distortions are considered to be equivalent to about two units 

of measurement or about 2TT. 

Thus two different factors are considered in relating 

geologic accuracy requirements or expected maximum 

vertical measurement errors to photograph scale and 

instrument capability. These factors are the [{ factor 

for use with stereoscopic models from glass plates iJl 

double-projection plotters, and t_he TT factor for use 

with stereoscopic models formed from paper prints. 

For any given system of measurement it is likely that 

many measurements will fall well within the expected 

maximum errors as determined from figure 37, especially if high-resolution photographs are available that 

may permit reading units of vertical measurement less 

than [( or TT, and if points of measurement are carefully chosen. Also, errors of reading and datum 

curvature or tilt may be compensating rather than 

cumulative, as stated above. In a. recent test of determining altitudes of gravity stations with multiplex 

leveled to vertical control, ICinoshita and !Cent ( 1960) 

found that 96 per cent of the points measured fell 

approximately within the limits of ±[{ and that 71 

percent of the points measured fell approximately 

within the limits of ±l{/2. Results of this special 

test do not represent the general order of vertical ac-
74 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

. curacy that geologists may expect to obtain by photogrammetric methods, but they do suggest that many 

measurements made will be well within the limits 

shown in figure 37. 

RELATION BETWEEN INSTRUMENT C!.A.PABILITY, 

SCALE, AND VERTICAL MEASUREMENTS 

Let it be desired to determine the accuracy of spot 

altitudes for compiling a structure-contour map of an 

area covered by recent, high-quality 1 :20,000-scale 

photography flown at 10,000 feet. Adequate vertical 

control is available for leveling stereoscopic models 

and a Kelsh plotter is available for measurement and 

compilation. The expected maximum error shown in 

figure 37 for spot altitudes for leveled models is ± 2K. 

Because the scale of the l{elsh model is approximately 

5 times that of the original 1 :20,000-scale photography 

(fig. 37), a projected stereoscopic n1odel at a scale of 

1 :4,000 would result. Thus the K factor for this model 

would be 4,000/3,048 or 1.31 feet (fig. 37). Spot altitude measurements could be expected to be correct 

within ± 2K or ± 2.62 feet for most points in the 

model. 

Many geologists will have only paper 'prints and 

simple measuring devices to use so as a second example let it be desired to determine the expected 

maximum error in measuring a stratigraphic interval 

with a parallax bar and paper prints already available. Photographs at a scale of 1:20,000 and taken at 

10,000 feet are available. Inspection of physiographic 

characteristics of the terrain indicates that very little · 

· tilt, estimated not to exceed 1°, is present. Rock units 

are clearly expressed, the formations are essentially 

flat lying, and points of measurement are within 14 

inch on photographs or a distance of about 400 feet 

on the ground. The photobase distance is 90 millimeters. Figure 37 .shows that with parallax bar the 

expected range of error for relative interva]s is ±6V 

± error due to tilt, 

h r 1o,ooo 1 (fi w ere / = 90 X 100 = .1 g. 3 ) . 7 

The maximum error due to tilt, however, could amount 

to 14 feet in this example and thus a total expected 

error of .20.6 feet could result. But the reading error 

plus paper and lens distortion error may well be less 

than ±617 and the points of measurement may well 

lie in a direction other than the maximum tilt direction; thus, a measurement error of much less than 

20.6 feet will probably result. Figure 37 is constructed 

to show only the order of maximum error. The percentage of error will depend on the total thickness of 

the stratigraphic interval; the percentage of error becomes smaller as the thickness of the interval increases. 

The geologist will be concerned generally with the 

expected maximum error of measurement that photographs and instruments already at his disposal will 

give, and figure 37 is readily used to determine expected maximum errors of measurement for a given 

photograph scale and instrument combination. However, to determine what photograph scale and instrument combination would be needed to attain specified 

accuracy requirements is not so easy, as · the photograph scale and instrument may be varied and yet 

give the same required tolerances of vertical n1easurements. For practical purposes it may be best to 

equate the vertical-accuracy requirement against instrument capabilities, and to determine from this the 

scale of photographs that would be· needed. Thus, if 

a requirement for Ineasuring vertical intervals of ± 5 

feet . is specified it ean be seen, for leveled doubleprojection stereoscopic models, for example, that ± 3K, 

the maximun: expected error for relative intervals, 

must equal 5 feet or K must equal 1.66 approximately. 

It follows (see fig. 37) that the projected model-scale 

denominator must equal 1.66 X 3,048 or about 1 :5,000. 

The scale of photographs required to provide a projected stereoscopic model of 1 :5,000 depends on the 

double-projection plotter used. For the Kelsh plotter, 

the original photography sca]e would be approximately 1:25,000; for the ER-55 plotter, approximately 

1 :17,000 or 1 :25,000 depending on the projector model; 

and for the multiplex, approximately 1 :12,000 (fig. 

37). Similar computations can be made for other 

instrument capabilities and requirements of vertical 

measurements. ''There two different combinations of 

instruments and photograph scales will provide the desired accuracy of vertical measurement, the optimum 

interpretation, measuring, and plotting system will be 

determined by the smallest scale of photography that 

will allow recognition and qualitative interpretation 

of geologic features pertinent to the given geologic 

objective. 

HORIZONTAL POSITIONING 

Horizontal positioning within 1/25 inch of true n1ap 

position generally can be accomplished at photograph 

~cale or smaller with any of the plotting instruments 

that removes relief and tilt displacement. Such instruments include the double-projection plotters nnd 

some of the paper-print plotters. The small amount of 

tilt inherent in present-day verticnJ photography is 

generally of little significance in horizonttil positioning 

of geologic features, however, and the radial pln.nimetrie 

plotter may also be used to plot geologic features within 

1/25 inch of true map position. Orthophotographs may 

also be used for reliable horizontal positioning of data 

(see p. 69). In addition, geologic features in areas of 
INTERPRETATION, MEASURING, AND PLOTTING SYSTEMS 75 

low relief Intty be plotted within 1/25 inch of true map 

position by sketchmaster or overhe~td reflecting projector if tilt is negligible. Where relief is low, radial 

disphtemnent (n1so ettlled relief disphteement) of image 

points is commonly less than }~5 inch. For example, 

on vertic1tl photogntphs tttken from a flying height of 

10,000 feet, d.isplttcement where the relief is 100 feet 

mnounts to l/25 inch nt a point 4 inches from the center 

of the photogntph. N en.rer the center of the photogritph the displacement is less. Thus, geologic dattt on 

such photogntphs can be plotted generally within 1/25 

inch 1Lti photogntph settle or smttller with simple overhmtd reflecting projectors, or can even be tnteed directly 

ft·om the photogntphs imd still be within the 1 /25-inch 

tolerance for genentl geologic mn.ps. The amount of 

ntdi1tl displacement, m in inches, is determined from 

the formul11 

rh 

m=-H 

where r=radial distance on the photograph, in inches, 

from principal point to base of image displaced; 

h=height of object displaced, in feet above dn.tum 

(height of terrain); 

and H=flying height, in feet, above dat~m; 

( h and l:l must be referred to the same datum and be 

measured in the same units. The amount of displacement, 1n, is then in the same units of measure as 

chosen for r). Thus, the limit of relief that can be 

adequately plotted within the 1/25-inch tolerance for 

general geologic maps is basically a function of flying 

height and not of focal length. Displacement due to 

relief varies inversely with flying height. 
j 
AERIAL PHOTOGRAPHS 77 

-..... 

' ~ ,.,.. " ' 

Stereoscopic pair 

Approximately 1 mile 

FIGURE 39.-POORLY RESISTANT PYROCLASTIC ROCKS OVERLAIN BY RESISTANT 

CAPPING FORMATION (COLORADO). 

[Approximate scale 1: 2{),000. Photographs by Soil Conservation Sen-ice] 

Coar:<e pyroclastic rock:<, A, are highly dissected in contra:;t to the more resistant and protecting welded tuff capr ock, B, in this a rea of flat-lying formations. Deep dissection of pyroclastic rocks, occurring here preferentially 

on one slopP, permit>< delineation of contacts that can be 

tntef'cl into nearby areas where strong dissection has not 

taken placP. The u><e of the 11oating-<lot reference mark of 

tnan~· photograwmetric instrument" ma~· be especiall.l' helpful in t racing a geologic contact from an area where thP 

contact i~ well expres,.ed to an a rea where the contaet is 

ohscurerl. Sandstone and shale below the pyroclastic rocks 

are in l arge part poor!~· exposPd heeause of the slope wa:<h 

thflt coYPr>< the deposit:<. 
78 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

Stereoscopic pair 

Approximately 1 mile 

FwunE 40.-AREA UNDERLAIN PREDOMINANTLY BY GENTLY FOLDED SANDSTONE, 

SHALE, CONGLOMERATE, AND GRAYWACKE (NORTHERN ALASKA). 

(Approximate scale 1: 20,000. 

In some areas where outcrops are sparse or lacking bedding may be traced on the basis of topography and vegetation.' Resistant beds of sandstone and conglomerate cause 

small topographic rises, in some areas as little as 2 feet, 

which are easily discerned in the stereoscopic model because of vertical exaggeration present in viewing most 

stereoscopic pairs. The steepened slope caused by the re-

~istant bed is commouly populated with a different mossy 

and grassy vegeta.tion from that of surrounding areas, and 

bedding is revealed as wide, strongly curved bands of vegetation that are recognized on photographs by their lighter 

or darker photographic tone than surrounding areas. Such 

Photographs by U.S. Na,·.'"] 

bands of vegetation, as at ..1, are eommonly seen clearly on 

aerial photographs or from a distance on the ground, but 

are not always readily distinguished when standing directly on them on the ground. 

Permafrost is shown by the presence of beaded drainage, indicated by isolated pools of water connected by 

short, incised watercourses along main stream. Striped 

slopes, B, are typical but not unique to permafrost areas 

(contrast fig. 53). Note high-center polygons, C, that exhibit raised centers surrounded by depressed dark-toned 

edges. Angular drainage channels, D, are related to melting out of ice wedges ancl are common in permafrost areas. 
AERIAL PHOTOGRAPHS 79 

Stereoscopic pair 

Approximately 1 mi le 

FIGuRE 41.-EXTRUSIVE VOLCANIC ROCKS AND ASSOCIATED CINDER CONE (UTAH). 

[Approximate scale 1: 20,000. Photographs by Commodity Stabilization Sen·ice] 

Cinder cone, A, is readily identified on the basis of shape 

of the constructional landform. Associated flows, B, are 

<lark toned, irregular in ground plan, and marked by very 

uneven topographic surfaces. Many Java flows lack surface drainage rills, which suggests permeable material. 

Note especially the lack of surface drainage channels on 

flanks of cinder cone. 
80 AERIAL PHOTOGRAPHS IN GEOLOGIC INTli'-qPRETATION AND MAPPING 

Stereoscopic pair 

Approximately 1 mile 

FIGURE 42.-FLAT-LYING SANDSTONE (SOUTHERN UTAH). 

[Approximate scale 1: 20,000. Photographs by U.S. Geological Survey] 

Prominent wide-spaced joints a re shown by many short 

lineations. Where beds are essentially flat lying, joints 

commonly form in a right-angle pattern and give the terrain a blocky appearance locally, although one direction 

of jointing may dominate and give a conspicuous linearity 

to the topographic grain. The light photographic tone and 

coarse-textured drainage is typical of coar e sedimentary 

rocks. Major drainage is dendritic; this pattern is commonly characteristic of flat-lying rocks. Minor drainage is 

controlled by joints. 
AERIAL PHOTOGRAPHS 81 

Stereoscopic pair 

Approximately 1 mile 

FIGURE 43.-SANDSTONE AND CONGLOMERATE BEDS INTRUDED BY DIORITE PORPHYRY 

LACCOLITH (UTAH) . 

[Approximate scale 1: 20,000. Photographs by Commodity Stabilization Service] 

Conspicuous contrast in vegetation results from different li thologic types. Sandstone and conglomerate beds, 

A, are covered largely with scattered tall jack, ·ellow, ancl 

ponderosa pine trees. Diorite porphyry, B , is covered with 

scrub oak as much as 10 feet high. '!.'his dense growth of 

scrub oak forms a photographic texture that contrasts with 

areas covered with pines. Note serrated edges of bluffs, C, 

which are expressions of joints in sandstone and conglomerate beds. 
82 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRE'l'ATION AND MAPPING 

Stereoscopic pair 

Approximately 1 mile 

FwunE 44.-GENTLY DIPPING BEDS OF SANDSTONE, SILTSTONE, ~\.XD CO~GLOl\IER~\TE 

TH \T LOCALLY CONTAIN CHANNEL-FILL DEPOSITS (ARIZONA). 

[ .-\pproximn te scale 1 : 20,000. l'hotog-raphs by U.R. Geolog-ical Snrn•)·] 

Light photographic tone uf rocks, .-1, is typical of massive crossbedded sandstone in this area. Dark-tone(! rocks, 

H, are · reddish thin-bedclecl shaly siltstone, shale, and fin('-

~rainecl sandstone. Light-toned consp icuously jointed rock,;, 

C, are uranium-bParing sanclstune, conglomerate, and mudstun!'. Locally the dark-toned rocks have been eroded by 

cha nnel >:<cour, and the channels have bPen till ed with sandstone and conglomerate that now lie unconformably on the 

older rocks. Erosion of dark-toned rocks along line D. 

which represents the line of un conforwity an(l orientation 

of a channel in this area, is conspicuous by the photographic tone change. At FJ the dark-toned rocks have het>n 

cornpletel_,. erodecl away and light-toned jointed ><andstone 

and conglomerate rest on the light-toned wassh·e cro~,.;­

hedded sandstone. At F the dark-toneLl rock~ again lweome conspicuous where overlying rocks thin. Difference~ 

in stratigraphic thickne;;;;e,.; of thp !Ia t'k-toned rocks, which 

rel1ect a thickening of overlying rock" that were llepo~itt>(l 

in old rivt>r channels, can be wea;;nrt>(l photogrammetrican~-. Isopach lines ma~· be drawn to indicate the orientation of the channel and tht> general rtPpth of thP 

channel. 
AERIAL PHOTOGRAPHS 83 

Stereoscopic pair 

Approximately 1 mile 

Frmnm 45.-GNEISS, SCHIST, AND GRANITE FLANKED BY DIPPING SEDIMENTARY ROCKS 

TRUNCATED BY PEDIMENT GRAVEL (COLORADO). 

[Approximate scale 1: 21,000. 

'l'opographicallr high, vegetated area, A, is underlain b.r 

gneiss, schist, ancl granite. Light-toned sedimentary rocks, 

B. lying unconformably . on the igneous and metamorphic 

rock:;, are primarily arkose and sanclstone. Trace of unconformity between sedimentary rocks and igneous and 

metamorphic rocks is indicated by break in slope, 0, which 

i>< ocall~· accentuated by contrast of trees on steep slope 

Photographs by U.S . .-\.ir Force] 

uph ill from the trace of the unconformity with grassy vegetation on the gentler slope below. Note effect on clip of 

vertical exaggeration in the stereoscopic model. The arkose 

anll sandstone unit clips about 25°. The grass-covered pediment gravel, D, which truncates the underlying seclimentan· 

rocks, clips 2° to 3°. 
84 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

Stereoscopic pair 

Approx imately 1 mile 

FIGURE 46.-LANDSLIDE AREA (COLORADO) . 

[Approximate scale 1: 37,000. Photographs by U.S. Geological Survey] 

Landslides are indicated primarily by landform. rote 

the characteristi c hummocky topography of slide material 

promin,ent at 1l. Minor ridges of the slide area are commonly parall el to the contour of the slope; clumps of vegetation locaJJ~· are oriented parallel to the contour of the 

slope also. Poorly resistant sedimentary rocks underlie the 

landslide debris; resistant volcanic flows (not shown) cap 

the sedimentary rocks. In many areas where poorly resistant rocks underlie a resistant caprock, conditions for 

landsliding are favorable. Highly fractured rocks in canyon are schist and gneiss. 
AERIAL PHOTOGRAPHS 

FIGURE 47.-0RTHOPHOTOGRAPH AND PERSPECTIVE PHOTOGRAPH 

OF SAME AREA. 

( Left) Part of perspective aerial photograph s howing distortions in powerline ca used by 

r elief displacement of image points. (Right) Orthophotograph of the same area showing 

t hat reli ef clii<placeutent has been eliminated in prepm·ing orthophotograph . :1\ote that the 

pO\\'erline is straight. 

85 
AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 48.-AREA SHOWING RELATION OF PHOTOGRAPHIC TONE, LAND-USE PATTERN, AND DRAINAGE TO 

RESIDUAL SOIL, ROCK TYPE, AND GEOLOGIC STRUCTURE (SOUTH CAROLINA). 

[Approximate scale 1: 60,000. Photographs by Army l\Iap Service] 

Aerial photographs commonly __ show an association of feature,<:; that is 

meaningful to the interpreter. Here photographic tones, laud-use pattern, 

and drainage pattern are associated with specific soils, rock types, and geologic structure. Note that the small scale of the photographs is a significant 

factor in showing the striking land-use pattern and drainage pattern (see 

also fig. 4). The cireular land-use pattern of the farmed area generally 

bounded by vegetated areas, as . at A, represents soils developed on underlying mafic rocks. Rocks in the immediately surrounding vegetated areas are 

largely silicic intrusive types. The drainage characteristics of the area also 

suggest a pluglike or domal geologic structure. Streams at B and C form 

a conspicuous annular pattern around the area of mafic rocks and probably 

mark the contact of the mafic rock body. 
AERIAL PHOTOGRAPHS IX GEOLOGIC INTERPRETATIOK AND MAPPING 

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FmuRE 49.-SHALE AND THIN SANDSTONE INTERBEDS CAPPED BY POORLY CONSOLIDATED GRAVEL AND SAND, 

LOCALLY CEMENTED 'VITH CALCAREOlTS ~1ATERIAL (TEXAS). 

[Approximate scale 1: 63,360. Photographs by Army .Map Sen·ice] 

Laek of :-;urfaee drainage at .4 indicates high degt·ee of permeabilitr in 

capping material. Note in contrast the striking tine-textured drainage Oil 

the relatively impermeable shale area, B. Broad depressions, C, in cappinggravel and sand are very shallow and show little relief even with vertic-al 

exaggeration found ill the stereoscopic model. These depressions resulted in 

large part from wind action; some solution of local calcareous cementing 

material also may have taken place. Dark areas within depressions represent a darker vegetation growth; light-ton.ed areas represent in part washedin tine materials that were dry and hence light colored at the time the 

photographs were taken. Locally some light-toned evaporite deposits ma.r 

be present. 
90 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATIOK AND MAPPING 

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FIGURE 50.-RECENTLY FAULTED STRAND LINES OF FORMER LAKE (NEVADA). 

[Approximate scale 1: 20,000. Photographs by Soil Conservation Service] 

Strand lines of beach sand, A, are shown strikingly by dark photographic 

tone and slightly higher altitudes than surrounding surficial material. Note 

that strand lines are truncated by a recent fault, B, shown hy narrow hand 

of dark vegetation and by low scarp. Drainage density is relativel.v high 

on the thin alluvial cover, 0, but decreases noticeably on opposite side, D, 

of fault where better drained, thicker deposits of alluvial material are present. Strand line is faulted up about 25 feet at E by recent fault movement. An irregular fault trace is also seen at Ji'. 

~ . . . 
92 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 51.-STEEPLY DIPPING SEDIMENTARY ROCKS INTRUDED BY QUARTZ DIORITE (CALIFORNIA). 

[Approximate scale 1: 20,000. Photographs by U.S. Geological Survey] 

Generally massive dark-toned conglomerate sequence, A, exhibits coarsetextured drainage and erosional characteristics, whereas fine-grained weakly 

foliated quartz diorite, B, has a fine-textured drainage and erosional topography. Widely spaced joints, conspicuous in many intrusive masses, are not 

present in the quartz diorite. Dendritic drainage in the conglomerate sequence is not typical of most dipping sedimentary rocks; the dendritic 

drainage pattern suggests a massive character and uniformity of the conglomerate sequence and genera] lack of bedding which might control the 

drainage. Local light-toned interbeds, 0, indicate strike and dip of the sedimentary rock sequence. 
94 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 52.-AREA SHOWING DRAINAGE CHARACTERISTICS IN SURFICIAL MATERIALS (WYOMING). 

[Approximate scale 1: 28,000. Photographs by U.S. Geological Survey] 

Strong contrast in drainage densities of areas A. and B reflect differences 

in underlying materials. Fine-grained surficial sedimentary deposits, A., that 

form a thin cover on impei·meable shale here exhibit fine-textured drainage. 

The many drainage rills flowing across the fine-grained surficial material 

disappear into sand dune area, B, which is composed of granular ma~erial 

of high permeability. Although partly stabilized and covered by vegetation 

the sand dunes locally show barchan shapes, which aid in their identification. 
96 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 53.-AREA OF BASALT FLOWS (IDAHO). 

[Approximate scale 1: 20,000. Photographs by Commodity Stabilization Service] 

Conspicuous "snakeskin" pattern is commonly found on the undissected 

basalt flows, A; it is not observed on other rock types in this area. The 

pattern consists of white or light-toned spots scattered in a dark field. Note 

that a rounded "shape" is a distinctive part of the pattern. The light-toned 

rounded areas are mounds of clay and silt 20 feet or more across and 2 or 3 

feet high. Grass grows on these areas and causes the light photographic 

tone; the basalt is typically barren or covered with sagebrush and has a 

dark photographic tone. The striped pattern of closely spaced drainage rills, 

B, is also typical of basalt talus slopes in this area; however, similar-appearing stripes are found on a variety of rock types in other areas (see fig. 40). 

Where basalt cappings are underlain by poorly resistant rocks, conditions 

are favorable for landslides, as at 0. 
98 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 54.-RING-DIKE AREA (NORTH· CAROLINA). 

[Approximate scale 1: 58,000. Photographs by Army Map Service] 

Ring dike about 1 mile wide and an area of gabbro-diorite within the 

ring dike can be differentiated from surrounding areas primarily on the 

basis of the lesser tree coverage and greater land use within the ring-dike 

area. A ring structure is also suggested by the concentric pattern of trees, 

A, which may reflect structure within the igneous rocks. The contact of 

syenite of the ring dike and gabbro-diorite of surrounding areas cannot be 

identified in the stereoscopic model, and aerial photographs of this terrain 

serve primarily to differentiate anomalous features which then must be examined in the field. Note excavation in syenite of the ring dike. 
100 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

A Stereoscopic pair 

Approximately 1 mile 

B Stereoscopic pair 

Approximately 1 mile 
AERIAL PHOTOGRAPHS 101 

FrouRE 55.-AREA IN MAINE SHOWING ESI{ER LANDFORM ON PHOTOGRAPHS OF TWO 

DIFFERENT SCALES . . . . : .. ~~ 

[A: Approximate scale 1: 70,000.: · Photographs by Army Map Service] 

[B: Approximate scale 1: 44,000. Photographs by U.S. Geological Survey] 

The serpentine shape in plan view and local topographic relief of the esl{er are readily distinguished in the stereoscopic 

view of figure A. The esker ridge is discontinuous, but the 

vnrious segments are clearly part of the same ridge. 'J:he 

nssoclation of isolated ridge segments is readily made \vhen 

small-scale photographs showing a large area are viewed. 

Landform is the primary recognition feature of esker ridges. 

In figure B the esker landform is not as readily identified as 

in figure A, nnd on commonly available 1 :20,000-scale photographs the serpentine shape would probably not be seen in 

any one stereoscopic model. Esker sands and gravels are 

commonly well drained and are a source of borrow material 

for the· construction engineer. Note old secondary road, a, 

built along the top of the esker, probably because of good 

subgrade and drainage. 
102 AERIAL PHOTOGRAPHS IN GEOLOGIC I TERPRETATION A D MAPPING 

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FIGURE 56.-STABILIZED SAND DUNES (CENTRAL ALASKA). 

[Approximate scale 1: 40,000. Photographs by U.S. Air Force] 

Strong contrasts in photographic tone and photographic texture mark 

sand dunes and interdune areas. Dunes, as at A, are now stabilized and 

covered with trees. Coarse well-drained sands of the dunes support darktoned black spruce. Light-toned interdune areas of frozen silt support 

grasses. Lakes were formed on the original glacial outwash plain, which 

was the source of sand for dune formation. The shape of the dune landform is an important aid to recognition, and the association of trees is significant in the interpretation of the soils and ground conditions. Helief of 

the dunes is vertically exaggerated in the stereoscopic model. 
104 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 57.-MORAINAL DEPOSITS OF TWO GLACIATIONS (CENTRAL ALASKA). 

[A·pproximate scale 1: 40,000. Photographs by U.S. Air Force] 

Arcuate plan-view shape of terminal moraine, A, is marked by a lighter 

photographic tone than that of the surrounding area. This moraine is 

characterized by an abundance of small undrained lakes and ponds and by 

poorly integrated drainage. In older terminal moraine, B, lakes and ponds 

are largely drained, but arcuate plan of this moraine is still preserved. 

Terminal moraine, C, represents an advance only slightly younger than A. 

Note similarity of topographic expression and development of ponds and 

lakes in moraines A and C. 
106 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 58.-MORAINAL DEPOSITS OF CONTINENTAL GLACIATION (SOUTH DAKOTA). 

[Approximate scale 1: 60,000. Photographs by Army Map Service] 

Terminal moraine, A, contains generally few lakes and ponds in contrast 

with older ground moraine, B, in which water-filled kettle holes are numerous. Arcuate plan-view shape and higher relief than surrounding ground 

moraine are typical of the terminal moraine. Note drainage rills developed 

on parts of topographically high terminal moraine, but general lack of welldeveloped surface drainage in ground-moraine areas. Ponding also indicates 

poor drainage in this area. Linear trends parallel to arcuate border of 

terminal moraine--which represent depositional and erosional characteristicsare similar to some linear trends indicative of faults in other areas. 
108 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FrGuRE 59.-GENTLY DIPPING SEDIMENTARY ROCI(S LYING UNCONFORMABLY ON GNEISS-GRANITE-SCHIST 

COMPLEX (UTAH). 

[Approximate scale 1: 20,000. Photogmphs by U.S. Geological Survey] 

Strong photographic tone differences mark the gneiss-granite-schist area, 

A., and the sedimentary rocks, B. Relatively even dip-slope surfaces of sedimentary rocks contrast strongly with hummocky erosional topography of underlying crystalline rocks. The fine-textured dendritic drainage on the crystalline rocks stands out, although such fine-textured drainage is not necessaril~' diagnostic or typical of igneous and metamorphic areas. The gneissg.-anite-schist area shows no heclding, hut hedding is m:u·ke•l in the ove1·Iying sedimentary rocks by topographic breaks in slope. Light photographic 

tone and vertical cliffs are characteristic of thick sandstone formation in 

this area. Large talus blocks, at base of slope, C, indicate massive and 

resistant character of overlying rocks. Shale and silty shale unit below 

sandstone has a lesser angle of slope, but this slope is masked in part by 

dehris from overlying sandstone. 
110 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 60.-GENTLY DIPPING SEDIMENTARY ROCI\:S CUT BY NUJ\1EROUS NEAR-VERTICAL FAULTS (UTAH). 

[Approximate scale 1: 63,360. Photographs by Army Map Service] 

Sedimentary rocks are indicated .particularly .by. topogrnphlG ex.pr.essi.on .. 

.Jointed, resistant sandsto11e beds stand in bold cliffs, A. Soft rocks have 

lesser angles of slope. Strong photographic tone differences mark contact~ 

of some rock units, as at B; locally vegetation changes are suggestive of 

different rock types, C. 

Lava flow, D, cuts discordantly acros~ formational contacts. Remnant 

la.\\a flow.. at. E appeat:s .. to hav.e, been continuous . with .,.flow at D. ..Small 

cinder cone, F, demonstrates use of shape as a recognition element of constructional landforms. 

Faults are commonly marked by conspicuous topographic alinements (G, 

H). Throw on fault G is about 100 feet; on H the throw is about 150 feet. 

Note narrow alinements of vegetation marking faults, J(. 
112 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 61.-FLAT-LYING ~EDS OF SANDSTONE, LIMESTONE, AND SHALE (TEXAS). 

[Approximate scale 1 : 20,000. Photographs by Commodity Stabilization Service] 

Limestone caprock, A, shows a concentration of oak trees along rim but 

. not across capping bench, which is covered by a veneer of cherty conglomerate. Sandstone break at B also has oak trees. Certain oaks have an 

affinity for limestone soils and some for sandy soils, but oak types are not· 

distinguishable on conventional black-and-white photographs at a scale of 

1 :20,000. Shale valleys are covered with mesquite and locally have stock 

tanks, 0, which suggest underlying impermeable materials. Note dendritic 

drainage and distribution of vegetation bands along the topographic contour, which in places form closed loops and suggest fiat-lying or nearly fiatlying beds. 
114 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 62.-AREA OF FAULTED, GENTLY DIPPING SEDIMENTARY ROCKS (UTAH). 

[Approximate scale 1: 37,000. Photographs by U.S. Geological Survey] 

Strong contrasts in drainage densities and in erosional features of the 

terrain result from different sedimentary rock types. Coarse-textured drainage of little-dissected area, A, marks a persistent limestone bed that locally 

forms bluffs. These bluffs overlie fine-grained sandstone and shale, B, which 

have a lesser angle of slope than the limestone, although the difference in 

slope angle is masked by the vertical exaggeration of the stereoscopic model. 

Note that top of limestone and topographic surface coincide permitting ready 

estimate of the amount of dip. Dark-toned shale and fine-grained sandstone 

and sandy shale at C show fine-textured drainage and erosional topography. 

Note closely spaced joints in fine-grained sandstone beds at D. Vertical offsets of resistant limestone bed at E are conspicuous. Drainage along trace 

of faults is prominent at F and G. A lmver sandstone, H, contrasts locally 

with overlying dark sandstone and shale as a result of its light photographic 

tone and cover of scattered small trees. 
Ste

reoscopic pair 

Approximately 1 mile 

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FIGURE 63.-SHALE IN SEMIARID CLIMATIC AREA (UTAH). 

[Approximate scale 1: 20,000. Photogra,phs by U.S. Geological Survey] 

Fine-textured drainage is typical of shale and other fine-grained clastic 

rock types. Relief is commonly subdued and divide areas gently rounded. 

Note that rounding of some divide areas may not be conspicuous because 

of vertical exaggeration of the stel'eoscopic model. For example, the relief 

at B is only about 70 feet. The fine-textured drainage and erosional topography suggest plastic soils. The numerous angular crenulations in minor 

tributaries probably reflect closely spaced joints. Thin light-toned bands, A, 

are expressions of bedding. 
118 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 64.-SHALE AND LIMESTONE IN HUMID CLIMATIC AREA (VIRGINIA). 

[Approximate scale 1: 43,000. Photographs by U.S. Geological Survey] 

· Gently dipping shale, A, shows a fine-textured drainage. Main streams 

draining shale flow almost directly in the downdip direction. Limestone 

underlies anticlinal ridge, B. Sinkholes are present locally, C, but are not 

conspicuous within the area. In general the limestone areas are less dissected than shale areas because of better internal drainage in limestone. 

Valley, D, is underlain by shale. Note discordance of strike of steeply dipping beds, E, with scarp, F, which suggests presence of a fault along this 

topographic break in slope. Thin resistant bed of sandstone, G, crops out 

conspicuously on flank of anticlinal ridge. Trellis drainage in area D is 

typical of folded sedimentary rocks. 
120 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 65.-AREA OF GENTLY DIPPING LIMESTONE AND SHALE (PENNSYLVANIA). 

[Approximate scale 1: 20,000. Photographs by Commodity Stabilization Service] 

Impermeable shale, A., has a greater resistance to erosion than the limestone, B, and stands topographically high in this area. The contact of shale 

and limestone is marked by a pronounced topographic break, as at 0. The 

shale has been noticeably dissected by stream erosion in contrast to the 

limestone in which drainage is in large part internal. Note orchards in 

well-drained limestone areas. Vegetation along stream valleys in the shale 

accentuates the drainage contrast of shale and limestone areas. Soil mottling, D, in limestone areas occurs where soil cover is thin and allows differences in moisture conditions, believed to result from drainage channels 

in the limestone bedrock, to stand out. Despite good drainage in the .limestone area residual soils developed on limestone may react to engineering 

tests as a plastic soil and not as a granular soil. Limestone quarry is at E. 
122 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 66.-AREA MANTLED BY GLACIAL DEPOSITS (SOUTH DAKOTA). 

[Approximate scale 1: 60,000. 

~'opographically high area, .t1, with typical knob-and-kettle topography is 

covered by morainal material; ponds are common but surface drainage channels are conspicuously absent. Rim of high ground has been noticeably dissected by deep channels. The distribution of these channels, accentuated by 

dark-toned vegetation, forms a radial pattern similar to that over some subPhotographs by Army l\Iap Service] 

surface structures in other areas. Drainage disappears in permeable material, B, surrounding the high area. Contrast lack of surface drainage rills 

in moraine at A, where drainage is poor, as indicated by ponds, with lack 

of surface drainage rills in outwash at B, where internal drainage is good. 

Well-drained area, 0, probably also represents glacial outwash. 
124 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 67.-GLACIATED AREA IN SOUTHEASTERN ALASKA. 

[Approximate scale 1: 40,000. 

Conspicuous linear trends, .A, represent glacial grooving. These lineations, 

except for the fanning out in the direction of ice movement, may appear on 

photographs similar to lineations that represent foliation in metamorphic 

rocks. Drumlinoid features that give rise to this iinear pattern are generally 

found in broad, flat, and topographically low areas, such as the floors of 

broad valleys. Commonly the drumlinoid features are elongated parallel or 

Photographs by U.S. Navy] 

subparallel to the valley or general topographic grain. Lineations representing foliation on the other hand may transect or parallel the topographic 

grain; unlike drumlinoid features they aFe not restricted to any one topographic level. Pock-marked area, B, is glacial outwash and probably some 

glacial lake deposits. 
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FIGURE 68.-APPROXIMATELY FLAT-LYING LIMESTONE AND INTERBEDDED SANDSTONE AND MARLSTONE 

BEDS COVERED LOCALLY BY ALLUVIUM AND WINDBWWN DEPOSITS (TEXAS). 

[Approximate scale 1: 63,360. Photogra.phs by Army Map Service] 

Rounded dark-toned depressions are sink holes that mark area of lime- In contrast to the limestone area, surface drainage is conspi~uously develstone. Drainage is primarily internal and few surface drainage channels oped in the interbedded sandstone and marlstone, B, but becomes less well 

may be seen; a few short surface rills drain into the sink holes, as at A. developed in permeable alluvium and windblown deposits, 0. 
128 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 69.-AREA OF BASALT FLOWS (IDAHO). 

[Approximate scale 1: 20,000. Photographs by Commodity Stabilization Service] 

The oldest basalt, A, shows subdued rounded topography and well-developed surface drainage. A younger basalt, B, is interbedded with light-toned 

sedimentary rocks. Some surface drainage has developed on the sedimentary rocks. The youngest basalt, C, shows a "lizard skin" appearance on 

aerial photographs; locally a collapse structure may be present as at D. 

Almost no integrated surface drainage is present on the youngest basalt. 

Stream E flows in a fault valley. The fault is younger than, and separates, 

basalts A and B ; basalt A has been upthrown. 
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FIGURE 70.-AREA OF GRANITE (SOUTH DAKOTA). 

[Approximate scale 1: 24,000. Photographs by U.S. Geological Survey] 

Dendritic drainage, seen best in stereoscopic view, is typical of many terrains underlain by intrusive igneous rocks or other generally homogeneous 

rocks. The irregularly spaced joints in several different orientations are 

characteristic of granite and other intrusive rocks; extremely irregular topography is also common in such terrain. Banding, suggestive of sedimentary 

or metamorphic rocks, is absent. The occurrence of trees predominantly of 

one variety and the lack of conspicuous preferred orientation of vegetation 

is suggestive of uniform composition of bedrock. 
132 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FrGURE 71.-AREA OF GRANITIC INTRUSIVE ROCKS (WYOMING). 

[Approximate scale 1: 28,000. Photographs by U.S. Geological Survey] 

Profuse, irregularly spaced joints in several different orientations are typical, although not necessarUy diagnostic, of intrusive igneous rocks. The 

massive character of the rocks and apparent lack of bedding in turn suggest 

igneous intrusive rocks, although locally the rocks shown appear similar to 

some highly fractured massive sandstone. Conspicuous rectilinear depressions, as at A, marked by dark-toned vegetation, are shear zones that are 

more easily eroded than surrounding rock areas, and probably have a higher 

concentration of moisture that supports vegetation growth. In places, as at 

B, older shear zones have been offset by younger fractures. Note strong 

control of drainage by faults and joints. 
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FrGURE 72.-AREA SHOWING VEGETATION DIFFERENCES IN IGNEOUS-METAMORPHIC TERRAIN 

(SOUTHEASTERN ALASKA). 

[Approximate scale 1: 40,000. 

Dense stand of coniferous trees, A_, is typical of massive marble and related carbonate rocks in this area. Diorite, B, has only patchy tree -growth 

but shows typical crisscross joint pattern of short lineations common in many 

intrusive rocks. Area C is schist. The surface expression of the schist is 

Photographs by U.S. Navy] 

similar to that of the diorite in patchy tree cover and irregular topography 

but the schist has a dominant lineation direction not present in the intrusive rocks. Vegetation bands, D, may cover thin beds of marble. Note that 

gently cuned vegetation bands suggest steeply dipping rock units. 
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FIGURE 73.-AREA UNDERLAIN BY SILICIC VOLCANIC TUFFS AND FLOWS AND BEDDED ARGILLITE OR 

SLATE (NORTH CAROLINA). 

[Approximate scale 1 : 58,000. 

Major streams, A., are developed along the regional cross-joint direction. 

Roads extending ·generally parallel to the major streams accentuate the 

cross-joint direction. Minor tributary streams flow predominantly parallel 

to the regional structural trend and at right angles to the cross joints. 

Hills are commonly elongate in the direction of the regional strike, which 

may represent foliation or flow cleavage rather than bedding. In such lowPhotographs by Army Map •Service] 

relief areas the extensive land-use pattern tends to obscure structural trends; 

commonly the structural trends can be better observed from a mosaic composed of several photographs. Any one stereoscopic pair can only show a 

small part of the regional geologic structure. Structural interpretation may 

also be facilitated by study of separate drainage maps, as in figure 8. 
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FIGURE 74.-AREA OF FOLDED, FAULTED, AND HIGHLY METAMORPHOSED ROCKS (NORTH CAROLINA). 

[Approximate scale 1 : 57,000. 

This pair of photographs illustrates a use of drainage characteristics in 

the interpretation of geologic structure. Where dips are gentle, tributary 

streams developed on the updip side of a strike valley commonly will be 

longer than tributary streams developed on the downdip side of the valley, 

as at A.. The direction of dip of strata is thus indicated. Length relations of 

streams are commonly best studied on a separate drainage overlay (see fig. 7) 

where vegetation and other obscuring features in the stereoscopic model are 

not present. In the area shown, a plunging fold is also suggested by the 

Photographs by Army l\Ia.p Service] 

d.istribution of photographic tones and. gentle curvature of streams around. 

the nose, B, of the fold. Arrow indicates c~irection of vlunge. :\1any streams 

in highly metamorphosed terrain have developed in response to, and therefore reflect, the strike and dip of foliation rather than bedding. Prominent cross joints are expressed by linear segments of major streams, as at C, 

and by alinement of vegetation along some tributary streams. Note conspicuous fault intlicated by alinement of stream segments, D; this fault is 

inconspicuous on the ground. 
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.FIGURE 75.-AREA UNDERLAIN BY PHYLLITE AND SLIGHTLY METAMORPHOSED SLATY ROCKS (ALABAMA). 

[Approximate scale 1: 29,000. Photographs by U.S. Geological Survey] 

Poorl~' resistant phyllite and slightly metamorphosed slaty rocks here are 

characterized by fine-textured drainage, commonly found in fine-grained clastic rocks. The area also has a fine-textured erosional topography. Several 

conspicuous linear trends, A, are at right angles to the regional strike of 

bedding. These trends probably represent regional cross joints. In areas of 

metamorphic rocks the prominent regional cross joints commonly are '~idely 

spaced and streams flow along them. The linear trend of the stream at B 

is parallel to the regional strike of bedding. There is no direct surface 

manifestation of bedding. Head\vater tributaries form a generally dendritic 

clrain~ge patterp that suggests uniformity of rock type, but principal streams 

show rectangular drainage probably influenced by bedding and cross joints. 
142 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 76.-HEAVILY VEGETATED TERRAIN UNDERLAIN BY METAMORPHIC AND EXTRUSIVE VOLCANIC 

ROCKS (SOUTHEASTERN ALASKA). 

[Approximate scale 1: 40,000. Photographs by U.S. Geological Survey] 

Numerous subparallel lineations, A., represented primarily by rectilinear 

uepressions that channel the drainage, are expressions of foliation in the 

metamorphic rocks. Conspicuous rectilinear depressions, B, transecting the 

foliation direction, represent faults. The remnant of a recent volcanic cone, 

C, is recognized by its shape. Note lobate patterns of vegetation, D, suggesting areas of flow rocks. Absence of lineations in area E suggests that 

t1ows also are present there. 
144 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FrauRE 77.-FLAT-LYING BEDS OF LIMESTONE AND SHALE (KANSAS). 

[.Approximate scale 1: 20,000. Photographs by Commodity Stabilization Service] 

Strong contrasts in photographic tone result primarily from different lithologic characteristics of underlying formations; dark-toned beds are shale with 

some grassy vegetation ; light-toned beds are limestone. The near-coincidence 

of bedding traces and topographic contours, closed loops of tonal patterns, 

as well as the dendritic drainage pattern are typical of flat-lying beds. 

Small thickness of limestone interbeds is not conducive to development of 

sink holes. In some places individual limestone beds can be distinguished 

on the basis of weathering and erosional characteristics; massive cherty 

limestone benches may show rounded rims, whereas thin platy limestone 

benches may show a sharp break at the rim. Abundant trees and shrubs 

along part of bed :L suggest a plentiful supply of water. Bed A is locally 

an aquifer. 
146 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 
FIGURE 78.-FLAT-LYING BEDS OF SANDSTONE, LIMESTONE, AND SHALE (TEXAS). 

[Approximate scale 1: 20,000. Photographs by Commodity Stabilization Sen-ice] 

Flat-lying beds are indicated by benchlike landform, extension of tonal 

bands along topographic contour, and by dendritic drainage. Dark photographic tone on sandstone caprock, .!1, results from growth of oak trees. 

Benches of limestone, B, exhibit concentration of oak growth along rims but 

not across fiats. Valley areas are underlain by shale, which is covered predominantly by mesquite growth. Stock tanks, 0, are common in impermeable material and where present suggest shale in this region. 
148 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 79.-AREA OF SPARSE OUTCROPS UNDERLAIN LARGELY BY GENTLY FOLDED SANDSTONE, 

CONGLOMERATE, AND SHALE (NORTHERN ALASKA). 

[Approximate scale 1 : 20,000. 

Dip slopes are characterized by few streams in contrast to back slopes on 

which a system of numerous subparallel streams has developed. Drainage 

in the dip-slope direction is generally coarse textured except for profuse 

channels developed at heads of tributaries. Drainage on the back slopes is 

tine textured. The textural difference, or drainage density difference, rather 

\._: 

Photographs by U.S. Navy] 

than any specific drainage pattern, stands out. The back slope, with its tinetextured drainage, is predominantly a shale section in contrast to the sandstone and conglomerate of the dip slope. Note horizontal offset of lighttoned sandstone beds by near-vertical faults, illustrated at A, transverse to 

the strike. 
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FrGURE 80.-GENTLY TO STEEPLY DIPPING SEDIMENTARY ROCKS OVERLYING SCHIST-GNEISS-GRANITE 

COMPLEX (WYOMING). 

[Approximate scale 1 : 59,000. 

Striking discordance in dip of beds can be. seen between beds at .tl and B 

and between near-vertical beds at C and flat-lying beds at D. Such a discordance in dip may represent tightly folded structures in some areas but 

here it represents faulting. Note also the conspicuous discordance in strike 

between beds at A and C and between beds at B and C. The loeations of 

fault traces may be inferred from the positions of discordant beds, but 

Photographs by Army Map Service] 

details of fault structure and positions of formation contacts cannot be seen 

clearly on the photographs of many parts of this complex area. Dark-toned 

rocks, E, lacking bedding or banding are schist-gneiss-granite complex. 

Light-toned jointed rocks, Ji', are predominantly limestone and dolomite. 

""here light-toned rocks are cut by a stream, as at G, the outcrop pattern 

form::; a V in the direction of clip. 

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FIGURE 81.-GENTLY FOLDED SEDIMENTARY ROCKS (CENTRAL UTAH). 

[.Approximate scale 1 : 20,000. Photographs by Soil Conservation Service] / 

Contrasting photographic tones, representing different soil mid rock types, 

strongly outline plunging anticline. Plunge of anticline is also indicated by 

stream curvature at A and B. Note that major stream has generally consistent direction of flow until it reaches anticlinal nose at 0, at which point 

it is deflected in the directiDn of plunge of the structure. Outcrop pattern 

. 1 ... 

of contact between light- ancl dark-toned ro<.:ks, JJ, along flank of the anticline forms pronounced V's, which point in the uirection of dip. Local intrusions of basalt dikes, E, stand in topographic relief abov·e softer surrounding rocks. 
154 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 82.-RING DII{E IN NE'¥ HAMPSHIRE. 

[Approximate scale 1: 34,000. Photographs by U.S. Geological Survey] 

The ring dike, composeu of several types of igneous rock, is the most conspicuous feature of the terrain. Different rock types in places are reflected 

in the topographic expression of terrain. An outer ring and partial inner 

ring, .4_, composed of monzonite stand in conspicuous topographic relief. 

Hornblende diorite, in places foliated, forms the low areas, B. Some geologic 

contacts may be located on the basis of topography. However, specific placement of contacts cannot be made from the stereoscopic model; but in igneous 

terrain, where one rock type may grade into another, contacts are difficult 

to select in the field. The ring dike is cut by several faults, 0, indic-ated by 

alinements of scarps, depressions, and vegetation. 
156 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 83.-GENTLY DIPPING SEDIMENTARY ROCKS OFFSET BY NEAR-VERTICAL FAULT (UTAH). 

[Approximate scale 1: 20,000. PhotogrSJphs by U.IS. Geological Survey] "'d 

~ 

Line separating areas of contrasting photographic tones is trace of fault. 

The straightness of the fault trace indicates that the fault has approximately a vertical clip. Note vertical offset of clark-toned shale bed, A, and 

difference in erosional character of topography on opposite sides of fault at 

B. Faint trace of fault in alluvium at C is lqcally marked by differences in 

drainage on opposite sides of the fault. Light-toned sandstone interbeds, D, ~ 

cap local areas and exhibit serrated margins as a result of closely spaced 8 

joints. Fine-textured. drainage is typical of the poorly resistant shale and ~ 

thin sandstone interbeds. ~ 

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158 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 84.-FAULTED, GENTLY DIPPING SEDIMENTARY ROCKS (UTAH). 

[Approximate scale 1: 20,000. Photographs by U.IS. Geological Survey] 

Conspicuous high-angle faults offset beds in many places, as at A, where 

limestone caprock is down-faulted about 125 feet. Note the effect of vertical exaggeration in the stereoscopic model on the amount of offset. Fault 

is indicated by scarp, B, and its alinement with stream at 0. Note rightangle bei1d in stream at D along trace of fault. Another fault is indicated by alinement of stream E and break in vegetation F. Several other 

faults with only small displacements are shown by alinements at G. Resistant limestone caprock stands in vertical bluffs overlying less resistant 

shale and shaly siltstone that have a lesser angle of slope. 
160 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 85.-VEGETATED TERRAIN UNDERLAIN BY IGNEOUS AND METAMORPHIC ROCKS LOCALLY COVERED BY 

BASALT FLOWS (SOUTHEASTERN ALASKA). 

[Approximate scale 1: 40,000. 

A significant contrast between the igneous and metamorphic complex and 

younger volcanic flows is shown in the distribution of faults and vegetation. 

Faults, indicated primarily by rectilinear depressions, A, are common in the 

igneous and metamorphic rocks, but are absent from the area of lava flows, B. 

Note that certain faults, as at C, may be traced up to the lava flows and 

that continuations of these faults may be picked up on the opposite side of 

the flow area. Some of the shorter lineations, as at D, probably represent 

wide-spaced joints. Lobate patterns of dark-toned vegetation, E, commonly 

Photographs by U.S. Navy] 

delineate the edges of individual lava flows. Small cone at F is suggested 

by shape of constructional landform. Relatively flat, muskeg-covered flows 

are generally poorly drained and locally contain numerous shallow ponds, 

G, that lack any integrated drainage. The muskeg areas have a relatively 

light photographic tone because of the mossy vegetation and do not appear 

dark despite the abundance of moisture. The photographic texture of the 

muskeg areas is also distinctive. 
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FIGURE 86.-AREA OF CRYSTALLINE ROCKS (WYOMING). 

[Approximate scale 1 : 60,000. 

Numerous widely spaced joints, typical of many intrusive masses, are conspicuous, although widely varying orientations of joints, found in most plutonic rocks, are not prominent. Drainage is markedly controlled by faults 

and joints; the dendritic drainage that is normally characteristic of crystalline masses is thus subdued. Although the rocks shown here are locally 

Photographs by Army Map <Service] 

gneissic, evidence of bedding of original sediments or gneiSSIC structure is 

generally lacking. Major fault, A, marked by stream alinement, is offset by 

younger fault, B, also delineated by stream alinement. Note alinement of 

streams on opposite side of divide, 0. 
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FIGURE 87.-LAVA FLOW LYING UNCONFORMABLY ON GENTLY DIPPING SEDIME.NTARY ROCKS (NEW MEXICO). 

[Approximate scale 1: 54,000. 

Resistant sandstone beds, A, dipping 15° are truncated by overlying lava 

flow, B. Bedding stands out where topographic and bedding surfaces coincide. Hogback landform is also prominent where resistant sandstone beds 

and intervening shale areas have been etched by erosion. Note how vertical 

exaggeration of dip causes unconformable relation between sedimentary beds 

and overlying lava flow to stand out. Surface of lava flow has mottled 

Photographs by Army Map Sm.·vice] 

appearance that is typical of basalt flows in many areas. Basaltic dikes, 0, 

stand in topographic relief above soft shale shown by fine-textured drainage at D. Some landsliding has taken place at E where resistant lava overlies less resistant rocks. Road alinement F is subject to potential destructive 

action of additional slides. 
166 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 88.-GENTLY DIPPING SEDIMENTARY ROCKS OVERLAIN IN PART BY RIVER GRAVELS (UTAH). 

[Approximate scale 1: 20,000. Photographs by Commodity Stabilization Service] 

Flat-lying gravel cap, A, truncates underlying sedimentary rocks. Traces 

of bedding below granular material of gravel cap are indicated by minor 

breaks in slope, B, accentuated by shadows. Surface drainage is conspicuously absent from well-drained broad gravel flat, D, which represents a 

younger, lower and less dissected terrace than A. Gullies in silty valleyfill, E, exhibit near-vertical slopes where recent stream dissection has taken 

place. Note effect of vertical exaggeration in the stereoscopic model. True 

dip of beds at C is about 6°. 
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FIGURE 89.-GENTLY FOLDED SEDIMENTARY ROCKS IN HEAVILY VEGETATED AREA 

(NORTHERN GUATEMALA). 

[Approximate scale 1: 65,000. Phot()gra.phs by Army Map Service; published with permission of the Guatemala G()vernment] 

Anticlinal structural feature in heavily vegetated jungle area is indicated 

by erosional characteristics of terrain and by drainage. Core of anticline, A, 

is topographically higher than surrounding area and is somewhat more dissected. Bedding and direction of dip are suggested by the topographically 

high bands of ground at B ; note that tree crowns appear to fall in a dipping plane. Stream, 0, has been deflected around the nose of the anticline; 

the convex side of the curve indicates the direction of plunge, but the major stream of the area has maintained its course across the anticline. 
170 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 90.-PLUNGING ANTICLINE IN GENTLY FOLDED ROCKS (SOUTHERN UTAH). 

[Approximate scale 1: 37,900. Photographs· by--u.s, Geological Survey] 

Distribution of rock types and characteristics of drainage clearly outline 

plunging anticline. Note that major stream curves around nose of the anticline and the convex side of the curve indicates the direction of plunge. 

Lithologic differences between formations are conspicuously shown by contrasts in photographic tone and erosional characteristics of terrain. Darktoned formation, A, is an interbedded sequence predominantly of red sandstone and some mudstone and shale. Light-toned formation, B, is made up 

of a rather uniform weakly cemented massively bedded sandstone showing a 

fine-textured dendritic drainage. Drainage rills in formation A and 0 are 

controlled in part by hard, resistant sandstone interbeds that locally channel the drainage along the interbeds. Formation 0 largely comprises a 

series of red to brown silty sandstone and shale. Rocks at D include predominantly thin-bedded red sandstone and limestone interbeds. Bedding is 

prominently shown because topographic surfaces and bedding surfaces coincide in many places. Dip of sedimentary beds is exaggerated as in most 

stereoscopic models. True dip at E is 5°. Dark-toned resistant basalt dike, 

F, stands in relief above light-toned rocks. 
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FIGURE 91.-STRATIGRAPIDC SECTION ACROSS GENTLY FOLDED ANTICLINE (UTAH). 

[Approximate scale 1: 20,000. Photographs by U.S. Geological Survey] 

Breaching of anticline by major river has exposed cross section of the fold. 

Bedding. is clearly represented at A where bedding surfaces coincide with 

topographic surfaces. Effect of vertical exaggeration on dip is shown on 

both flanks of the anticline. True dip at A is about 20°; at B the beds are 

horizontal; at C the dip is about 4°. Shale, siltstone, and fine-grained sandstone, D, exhibit dark photographic tones; relatively closely spaced joints 

may be seen locally. Coarse sandstone with some conglomerate, E, has light 

photographic tone; widely spaced joints are present as at F. The coarsegrained rocks characteristically form cliffs. Where beds are nearly horizontal the fine-grained rocks have slopes that are less steep than those of t~e 

coarse-grained rocks. Where beds are dipping, the softer rocks are in many 

places etched out by erosion leaving hogbacks of resistant rock types. 
174 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 92.-GENTLY DIPPING SEDIMENTARY ROCKS ON EAST SIDE OF SAN RAFAEL SWELL (UTAH). 

[Approximate scale 1: 20,000. Photograrlhs by U.'S. Geolo;.!ical Survey] ~ 

~ 

Bedding on flank of fold is clearly shown by photographic tone differences 

and hogback landform where dip surfaces and topographic surfaces coincide. 

Note effect of vertical exaggeration on dip in the stereoscopic model; beds 

at .;1 have true dip of 6°. Massive crossbedded sandstone, B, has light photographic tone and widely spaced joints oriented predominantly in one direction; locally its appearance on aerial photographs is similar to some 

igneous rocks. Dark-toned resistant beds, A, are red mudstone and lenticu- ~ 

larly bedded medium-grained sandstone. Poorly resistant interbedded red 8 

limy sandstone, mudstone, and limestone, C, is easily eroded and has fine- ~ 

textured drainage. Beds at C are flat lying. Windblown sand, D, is marked ~ ~ by ahsence of surface drainage channels. rJJ 
176 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 
FroURE 93.-UNCONTROLLED MOSAIC OF UMIAT ANTICLINE AREA, NORTHERN ALASKA, UNDERLAIN BY 

SHALE, SANDSTONE, CONGLOMERATE, AND GRAYVVACKE. 

[Photographs by U.S. Navy] 

~ 

s 

Curvature of streams, A, has developed on flank of anticline where hard 

and soft interbeds are exposed. Anticline has been breached into soft shale 

permitting streams developed on the more resistant dip slope to curve along 

8 

0 soft interbeds and not across the axis of the fold. Contrast figure 81 where o 

stream curvature indicates position of structural axis and direction of struc- ~ 

tural plunge. ~ 

r:JJ 
178 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

Stereoscopic pair 

Approximately 1 mile 
AERIAL PHOTOGRAPHS 179 

FwunE 94.-SUBSUR.FACE STRUCTURE REFLECTED IN SURFACE DR.AINAGE 

CHARACTERISTICS (TEXAS). 

[Approximate scale 1: 63,360. Photographs by Army Map Service] 

Subsurface structures at A. and B are suggested by drainage characteristics of the area. Streams flanking the structure at A. curve around the structure in their headwater portions. ~Ct·ibutnry streams drain radially off the structure (see 

fi!?ures 9 and 95). In addition, the cruclely circular pattern of 

more highly dissected ground and the pattern of dark photographic tone, 0, which results from a concentration of vegetation, suggest subsurface structure. Compressed meanders 

are present over a subsurface structure at B. Also note stream 

def1ection downstream from D which is a further spggestion 

of subsurface structure in area B. 
180 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

Stereoscopic pair 

Approximately 1 mile 
AERIAL PHOTOGRAPHS 

Fwum~ 95.-SAME PI-IOTOGRAPHS AS FIGURE 94, BUT REVERSED TO SIIO'V 

PSEUDOSCOPIC EFFECT. AS AN AID TO INTERPRETATION. 

[Approximate scale 1: 63,360. Photographs by Army l\fap Service) 

181 

Heversal of the positions of two aerial photographs of a 

stereoscopic pair produces the well-known pseudoscopic effect, 

i 11 which stream valleys appear as ridges and hills look like 

depressions. This unnatural view of the terrain commonly 

accentuates stream characteristics, which may be significant 

indicators of subsurface structure, as at A. The pseudoscopic view also commonly facilitates plotting of· drainage. 
182 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

~ 

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I 
FIGURE 96.-UNCONTROLLED MOSAIC OF WAINWRIGHT AREA, NORTHWESTERN ALASKA. 

[Photographs by U.S. Navy] 

The radial-dendritic drainage pattern shown is anomalous to the Arctic 

coastal plain where northward-flowing streams predominate. Local relief 

across the area is only a few feet per mile. The radial drainage pattern, 

suggestive of subsurface structure, is readily seen on the small-scale mosaic 

but could be easily overlooked on single photographs or individual stereoscopic pairs. Note the northward deflection of the Kugrua River; the 

abrupt change in direction of streamflow is believed due to subsurface structure. Seismic work indicates a structural nose plunging southeast in the 

north-central part of the area. Outcrops are sparse in the area and surface 

indications of structure, other than drainage pattern, are generally lacking. 

The drainage pattern of this area is similar to that of an area to the southeast where gas was found in a subsurface structure that had been defined 

by geophysical methods. 
184 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 97.-AREA UNDERLAIN PREDOMINANTLY BY GENTLY DIPPING SANDSTONE AND SHALE 

(NORTHERN ALASKA). 

[Approximate scale 1: 20,000. 

Sandstone beds stand out because of photographic tone, relief, and vegetation changes, as at A. As these sandstone beds are .traced across the area 

they lose their topographic expression, as at B, and are replaced by shale 

of the underlying formation. The rate of "shaling" or change of facies from 

sandstone to shale can be calculated from quantitative studies of the aerial 

Photographs by U.S. Navy] 

photographs, and the direction of maximum thinning of the sandstone section determined. Note the effect of vertical exaggeration on dip of resistant 

sandstone beds; true dip is 7° to 8° in this area. Effect of vertical exaggeration is also shown at C where minor topographic b!·eaks in slope, accentuated by shadows, reflecting thin resistant ·beds stand out. 
186 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 98.-GENTLY DIPPING SEDIMENTARY ROCKS CUT BY HIGH-ANGLE FAULTS (NEVADA). 

[Approximate scale 1: 20,000. Photographs by Soil Conservation Service] 

Topographic expression of resistant, jointed quartzite bed, A, permits differentiation of quartzite and underlying carbonate sequence of rocks, B. 

High-angle faults, transverse to the strike of bedding are shown at C and D 

by the vertical offset of the quartzite bed. The contact of quartzite. and 

underlying carbonate rocks is the locus of polymetal sulfide veins, but the 

veins cannot be seen in the stereoscopic model. Note mine dumps, E, marking location of crosscuts to base of the quartzite, which dips into the slope 

about 20° to 25°. Direction of dip is clearly indicated by V in outcrop pattern of quartzite bed where major stream cuts through the ridge. Trace of 

a major fault with several thousand feet displacement is only weakly shown 

parallel to slope at F and G where density of vegetation is slightly greater 

on the downhill side of the fault trace. 

··.-...... · 
188 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 99.-COMPLEXLY FOLDED AND FAULTED AREA OF GREENSTONE, SCHIST, LIMESTONE AND MARBLE 

(SOUTHEASTERN ALASKA). 

[Approximate scale 1: 40,000. Photographs by U.S. Navy] 

Relatively dark-toned highly fractured rocks, A, are greenstone and green- rich gold-quartz veins have been found in this area. Note that fault lineastone schist. Light-toned profusely fractured rocks, B, are limestone and tions characteristically are rectilinear depressions. 

marble. Prominent lineations, C, are representative of faults along which 
190 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 100.-IGNEOUS AND METAMORPHIC TERRAJN (SOUTHEASTERN ALASKA). 

[Approximate scale 1: 40,000. Photographs by U.S. Navy] 

In this area lead and silver deposits are found locally along the contact of 

intrusive qua_rtz diorite and· metamorphic rocks. Smooth slopes, A, are underlain by metamorphic rocks. The closely spaced parallel ridges and depressions, B, reflect foliation in the metamorphic rocks. Depressions, 0, 

probably represent solution cavities in carbonate interbeds. Lineations, D, 

transecting the foliation direction are believed to be expressions of faults. 

The uneven surface, E, cut by a crisscross of joints is underlain by quartz 

diorite. This pattern of lines representing joints is typical, but not neces· 

sarily diagnostic, of intrusive rocks. The contact of igneous and metamorphic rocks is in the conspicuous saddle between A and E. 
192 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FroURE 101.-GENTLY TILTED AND FAULTED LAVA FLOWS (OREGON). 

[Approximate scale 1: 54,000. 

Perched remnants of basaltic lava, A, form skeletal dendritic pattern, 

which suggests that lava filled former stream valleys. In some areas perched 

lava remnants are known to cover older auriferous stream gravel. Several 

faults are indicated by alinements of stream segments, vegetation lines, and 

photographic tone contrasts (see alinement of arrows). Dip on flow sur· 

faces, as at B, may be readily estimated or measured from aerial photographs because of the relatively large area examined in a single view, and 

because of vertical exaggeration of the dip slope. Note that the younger 

Photographs by Army Map 'Service] 

flow, A, has a gentler dip than the older flows, B and C. Surfaces B and C 

may be correlative and separated by a fault in the intervening valley. At 

point D the younger perched lava flows are also faulted, but throw is considerably less than where older lava flows are displaced by the same fault. 

In some areas faults displace only the older rocks and do not offset the most 

recent flows. A long period of fault activity is thus suggested. Note that in 

monoscopic view fault alinements, E, accentuated by vegetation, are .striking. 
194 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 102.-AREA OF PEGMATITE DIKES (SOUTH DAKOTA). 

[Approximate scale 1: 24;000. -·Photographs ·by U.S. Geological Survey] 

Pegmatites in the area shown are generally concordant with ·schistosity 

and bedding of quartz-mica schist country rock. In many places the pegmatite dikes stand in topographic relief above the surrounding schist, as at 

A. The massive, jointed character of the larger pegmatite dikes is similar 

to other intrusive rocks and contrasts with the thin-bedded schistose rocks. 

Thin pegmatite dikes commonly cannot be distinguished in the aerial photographs and only general contact areas can be mapped for most of the wide 

pegmatite dikes because of the vegetation cover. Locally steeply dipping 

schist also stands in topographic relief; ·these rocks can be distinguished by 

the sheeted character of the outcrops. General structural trends are reflected in topographic expression of parallel ridges and in photographic tones 

of vegetation in fields, B. Note mining activity in pegmatite at C. 
196 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FrGURE 103.-METAMORPHOSED SEDIMENTARY ROCKS INTRUDED BY QUARTZ MONZONITE (CALIFORNIA). 

[Approximate scale 1: 47,000. Photographs by U.S. Geological Survey] 

Metamorphic rocks here are generally dark toned, as at A, and locally 

show prominent banding; joints are closely spaced and are not conspicuously 

shown on the photographs. Quartz monzonite, B, is relatively light toned 

and lacks banding, but like most intrusive igneous rocks it has a strong 

development of irregularly and widely spaced joints that are readily seen in 

the stereoscopic model. Light-toned streaked, prominent talus slopes, 0, have 

formed where the fine-grained metamorphic rocks are present. In areas of 

igneous rocks the talus slopes are less well developed; locally large individual talus blocks can be observed at the bases of slopes. A fault trace is 

suggested by conspicuous linear feature, D. Note alinement of short straight 

linear segments, E, on opposite sides of divide; .these linear traces may represent the trace of a fault or the prominent development of joints. 
198 AERIAL PHOTOGRAPHS IN GEOLO.GIC INTERPRETATION AND MAPPING 

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FrGURE 104.-DISTINCTIVE LANDFORM OF GRAVEL TERRACES ALONG A MAJOR STREAM 

(NORTHEASTERN UTAH). 

[Approximate scale 1: 2:0,000. Photographs by Commodity Stabilization Service] 

The steplike landform of terraces is conspicuous. Other elements of the 

soil pattern-vegetation, drainage, and erosional characteristics-provide significant information to the photointerpreter. Absence of surface drainage, as 

at A, suggests well-drained materials. At A the material is composed of 

coarse bouldery gravel that has good internal drainage. Vegetation, B, is 

prominent along edges of upper terrace gravel cap where water emerges from 

the gravel. Lower terraces, as at 0, are more continuous and less dissected 

than the older terraces at higher altitudes, although recent streams in conspicuous V-shaped gullies, D, dissect the edge of the terrace. Terrace deposits commonly are favorable reservoirs for ground-water storage; in addition they provide a ready source of some materials used by the construction engineer. 
200 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

Approximately 1 mile 
AERIAL PHOTOGRAPHS 201 

Fiounm 105.-AHEAS SI:IOV\TING DRAINAGE CI-IARACTERISTICS IN SURFICIAL MATERIAL 

TI-IAT IS PREDOl\1INANTLY LOESS. 

[.Approxhnute scale 1: 2,0,000. Photographs by Commodity Stabilization Service] 

:Wigure A. Contrasting drainage characteristics and differences In photographic tones In areas a and b suggest differences In character of the surficial materials. ~rhe modified 

dendt•ltlc clrnlnnge uncl scoop-shaped valley heads are typical 

of. dlssectecl loess areas, a.. Lnck of surface drainage rills is a 

t:eut:ure oe ~nnd <lillie lll'em;, /J, where lllllt"eriall:i are <.:oarsegrnlned, highly permeftble, and drainage is largely internal. 

Note light-and-dark mottled soils in sand dune area. The finegrain sir-e of loess is believed respon~ible for the fine-textured 

d•·nlnage (Iowa). 

trlgure B. As in at·eu shown in figure A, the dissected loess 

hills exhibit a modified dendritic drainage pattern. Recent 

erosion has cut deep narrow and sharply defined channels in 

the bottoms of broad valleys, as at a. Gully walls are essentially vertical. Note characteristic scoop shape of side drainage and serrated ridges along some divides. Light streaks due 

to thin grass cover or to cultivation are typical of many loess 

areas. Broad, flat river flood plain, b, has good ~internal drainage and exhibits a lack of visible surface drainage channels 

(Nebraska). 

Figure C. Deeply dissected loess hills have draiqage pattern similar to areas shown in figures A and B, but erosion 

may have proceeded more rapidly than in other areas. Valley 

walls are sharply defined. Steep valley walls commonly characterir.e loess (Nebraska). 
202 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 106.-AREA OF SURFICIAL DEPOSITS, PRIMARILY WINDBLOWN (CENTRAL NEBRASKA). 

[Approximate scale 1 : 67,000. 

Highly dissected loess hills, A, abut against well-drained alluvial terraces, 

B. Extensive dissection of loess has resulted in a fine texture of erosional 

topography and fine-textured drainage. Bottoms of many valleys in loess are 

notched by recent stream dissection. The modified dendritic drainage pattern is typical of loess area. Note absence of surface drainage rills in .terrace alluvium, B, and elongate windblown sand hills, C, indicating the highly 

Photographs by Army Map Service] 

permeable nature of the underlying materials. Loess is generally considered 

well drained internally and is thus an exception to the general association of 

fine-textured drainage and impermeable soils. The fine-textured drainage in 

loess is believed to be a result of the fine grain size. Contrast appearance 

here of drainage density in loess with drainage density on large-scale photographs of figure 105. 
204 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 107.-MOTTLED SOILS OF DRIFT PLAIN (lOW A). 

[Approximate scale 1: 27,000. Photographs by U.S. Geological Survey] 

Light-toned areas are generally slightly higher than surrounding darktoned areas. High areas of light photographic tone are somewhat better 

drained than the intervening dark areas, and generally contain silty soils 

with a lesser accumulation of organic material than the depressions. Clay 

materials and humus have accumulated in depressions to impart dark photographic tone. Linear pattern of light-toned areas may represent minor recessional moraines. They trend across the direction of movement of ice. 

Near major streams the reworked glacial drift may be better sorted and 

locally an important source of borrow material for the construction engineer. 

.. 
206 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 108.-COASTAL PLAIN UNDERLAIN BY CLAY, SAND, AND GRAVEL (NEW JERSEY). 

[Approximate scale 1: 20,000. Photographs by Commodity Stabilization ·Service] 

Cultivated areas are thin cappings of sand and gravel overlying clay formation. · Lack of surface drainage rills in sand and gravel indicate that the 

capping material is well drained internally. Except for dark-toned crop 

vegetation the cultivated areas also have the general light tones commonly 

associated with well-drained materials. Note borrow pit in granular material at A. Streams flow in clay. The steep walls and fiat bottoms of gullies may not .be typical of plastic clays in humid regions, however. Headward extent of small gullies generally ends abruptly against contact of clay 

with overlying sand and gravel. Vegetation growth is conspicuous along 

steep sides of gullies where trees have not been cleared for cultivation. 

Vegetation growth is locally favored by relatively constant supply of water 

that flows to surface along contact of clay with overlying sand and gravel. 
208 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 109.-POLYGONALLY PATTERNED GROUND IN PERMAFROST AREA ALONG MAJOR STREAM 

(NORTHERN ALASKA). 

[Approximate scale 1: 20,000. 

The susceptibility of low, poorly drained areas to the development of polygonally patterned ground resulting from ice-wedge formation is shown conspicuously along this major stream valley. Development of polygons, primarily low-center polygons, A, shown with relatively dark-toned centers and 

light-toned marginal ridges, has followed old channel scars representative of 

old slip-off slopes. Some high-center .polygons, B, are also present. The 

angular gully pattern, 0, is commonly found where ice wedges are melting 

Photographs by U.S. Navy] 

out of polygonally patterned ground. Also note serrated edges of oxbow 

lake, D, and rim of river terraces were ice wedges in polygonally patterned. 

ground have melted out. Contrast appearance of poorly drained river terraces here with river terraces in nonpermafrost areas where internal drainage is generally good. The conspicuous tilt of the oxbow lake and up-hill 

flow of stream meanders indicate that tilt is present in the aerial photographs. 
210 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 110.-WELL-DEVELOPED PINGO IN AREA OF PERMAFROST (NORTHERN ALASKA). 

[Approximate scale 1: 20,000. 

Partly drained lake basin, A, a particularly favorable area for development of permafrost, is conspicuous. Note old beach lines, B, shown by narrow bands of contrasting photographic tones and along which polygonal 

ground has developed. Low-center polygons having dark-toned centers and 

light-toned marginal ridges are abundant, as at 0. High-center polygons 

Photographs by U.S. Navy] 

with light-toned centers and dark-toned marginal trenches, are also conspicuous in many places, as at D. Pingo, E, with a single crest crack here, 

is almost always diagnostic of underlying permafrost conditions. Also note 

pingolike forms, F, covered with polygonally patterned ground. 
212 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 
FIGURE 111.-AREA OF PERMAFROST (NORTHERN ALASKA). 

[Approximate scale 1 : 20,000. 

Low, wet but drained lake basins, as at A, are favorable areas for the 

development of permafrost. Low-center polygons, B, shown as dark-toned 

depressed centers and light-toned raised edges are common. High-center 

polygons, C, exhibiting high, light-toned centers, and dark-toned marginal 

trenches, are also present. Pingolike forms, D, also covered with polygonally 

Photographs by U.S. Navy) 

patterned ground, are highly indicative of permanently frozen ground conditions. The serrated shoreline, E, of some lakes is typical of the thawing 

of polygonal ground and further suggests the presence of permafrost. Note 

floating ice, F; bottom of lake may be seen at edge of ice, and in places 

depth of water may be measured from the aerial photographs. 
214 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 112.-CONSPICUOUS BEADED DRAINAGE IN AREA OF PERMAFROST (NORTHERN ALASKA). 

[Approximate scale 1 : 20,000. 

Beaded drainage along main stream, shown as rounded pools of water connected by deeply incised watercourses, typically forms in areas underlain by 

permafrost. Note steep banks of pools, as at A, which have resulted from 

thawing of frozen ground and subsequent caving. Interpretation of permafrost conditions is supported by presence of high-center and low-center polygons. High-center polygons, B, generally are shown by relatively light-toned 

centers and dark-toned rims, although locally, as at C, the reverse tonal 

contrasts may be present. Low-center polygons with relatively dark-toned 

Photographs by U.S. Navy] 

depressed centers and light-toned raised rims are shown at D. The wetter 

areas, as in the partly abandoned drainage channel, E, or upper reaches of 

minor drainageways, are especially susceptible to formation of polygons. 

Area F, probably a small alluvial fan area, shows well-developed drainage. 

The angular pattern may indicate incipient polygon formation in contrast 

to the angular pattern of drainage, G, which is typical of areas where melting of ice wedges in polygonal ground is taking place. 
216 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 113.-LANDSLIDE AREA IN VOLCANIC AND SEDIMENTARY TERRAIN (NEW MEXICO). 

[Approximate scale 1: 37,000. Photographs by U.S. Geological Survey] 

Resistant flat-lying basaltic lava, A, caps mesa underlain by weakly resistant mudstone, shale, siltstone, and sandstone, a geologic environment 

particularly susceptible to the occurrence of landslides. Landslides, B, below lava cap are characterized by hummocky topography and local lobate 

form suggestive of flowage. Note buildings, 0, constructed on toe of slide 

areas. Recent lava flows are indicated by the dark photographic tone and 

irregular surfaces, D. The lobate character typical of flows can also be seen. 
218 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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FIGURE 114.-DAMSITE AREA (SOUTHEASTERN ALASKA). 

[Approximate scale 1 : 40,000. 

Conspicuous faults, indicated by linear topographic depressions A, B, C, 

and D, are present. The outlet of the lake itself appears to flow along a 

fault marked by the rectilinear depression E; .this fault is probably the most 

Photographs by U.S. Navy] 

significant geologic structure in the area because a dam would have to straddle it. Highly fractured rocks comprise metamorphic types and intrusive 

quartz diorite. 
220 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

FIGURE 115.-GRIDDED PHOTOGRAPH. 
AERIAL PHOTOGRAPHS 221 

FIGuRE 116.-GRIDDED BASE MAP OF AREA SHOWN IN FIGURE 115. 
222 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

SOURCE AND IDENTIFYING DATA OF AERIAL PHOTOGRAPHS 

The following table gives all data needed to identify 

the aerial photographs and also the agencies from which 

the aerial photographs may be purchased. Note that 

the agency holding the negatives is not necessarily the 

one that originally acquired the photographs. 

Identifying photographic data 

Figure 

Project code Roll 

39 ________ _ DKK______________ 3 40 ________ _ BAR _____ ---.. ______ 309 

4L ____ - __ _ DSA _______________ 2K 

42 ________ _ GS-WL _ _ _ _ _ _ _ _ _ _ _ 12 

43_-------- DIG_______________ IF 

44 ____ ----- GS-RR_ ___________ 3 

45 ________ _ AFTRC/3415TTW/ . ______ _ 

49-10/M-1. 

Exposures 

16-17 

071-072 

19-20 

146-147 

127.:_128 

147-148 

697VV -698VV 

Focal length of lens, in 

inches 

8. 25 

6 

8. 25 

6 

8. 25 

6 

12 

Area. 

Negatives 

held by 1-

"DKK" Gunnison area, Colorado_ _ _ _ _ _ _ _ _ E Alaska _________________________________ A 

Kane County, Utah _____________________ D 

Utah ___________________________________ A 

San Juan County, Utah __________________ D 

Arizona________________________________ A 

Golden, Colorado ________________________ B 

46 _________ GS-VJV___________ 3 150-151 6 Colorado _______________________________ A 

47 _________ -------------------- _________ --------------- __________ ---------------------------------------- N 48 _________ 7L________________ 60 8395-8396 6 South Carolina __________________________ A 

49 _________ 138________________ 3 489-490 5. 2 Texas __________________________________ A 

50--------- DR.K______________ 21 3-4 8. 25 "DRK" Addition to Humboldt River, E 

Nevada. 

5L ________ GS-COL___________ 3 56-57 6 California __ ---------------------------- A 

52 _________ GS-JF_____________ 6 11-12 6 Wyoming _______________________________ A 

53 _________ DLG_______________ 7G 97-98 8. 25 Elmore County, Idaho ___________________ D 

54 _________ 7L________________ 26 3514-3515 6 North Carolina ___________________________ A 

55A _______ 73_________________ 4 117-118 6 Maine _________________________________ A 

B _______ GSM______________ 3 31-32 5. 2 _____ do ___________________ · _______________ A 

.56 _________ Mission 649 __________ -------- 0120-0121 6 Alaska __________________________________ A 

57 _________ 'Mission 648_________ ________ 0161-0162 6 _____ do ____________________ , ____________ A 

58 _________ 113________________ 16 1746-1747 6 South Dakota ___________________________ A 

59 ____ ----- GS-WL ___ - _ _ _ _ _ _ _ 10 102-103 6 Utah .. ___________ .. ________ - __ .. ______ ---_ A 

60 _________ 126________________ 18 1723-1724 6 _____ do _________________________________ A 

6L _____ . ___ CUU______________ 8G 141-142 8. 25 Stephens County, Texas __________________ D 

62 _________ GS-RO____________ I 125-126 6 Utah ___________________________________ A 

63 _________ GS-WL___________ 36 68-69 6 ______ do _________________________________ A 

64 _________ GS-AZ_____________ 2 119-120 5. 2 Virginia ________________________________ A 

65 _________ AQV_______________ ID 55-56 8. 25 Northampton County, Pennsylvania _______ C 

66 _________ 113________________ 21 2600-2601 6 South Dakota ___________________________ A 

67 _________ SEA_______________ 92 019-020 6 Alaska _________________________________ A 

68 _________ 138________________ 10 1919-1920 5. 2 Texas __________________________________ A 

69 _________ CNP_______________ IC 11-12 8. 25 Gooding County, Idaho __________________ D 

70 _________ GS-VE____________ 5 26-27 6 South Dakota ___________________________ A 

7L ________ GS-OMA__________ 32 88-89 5. 2 Wyoming _________________ · ______________ A 

72 _________ SEA_______________ 29 028-029 6 Alaska _________________________________ A 

73 _________ 7L________________ 50 7084-7085 6 North Carolina __________________________ A 

74 _________ 71_________________ 7 820-821 6 _____ do _________________________________ A 

75 _________ GSY ____ ~---------- 1 145-146 5. ~ Alabama_------------------------------ A 76 _________ SEA_______________ 112 181-182 6 Alaska _________________________________ A 

77 _________ ZF________________ 2G 174-175 8. 25 Wabaunsee County, Kansas ______________ D 

78 _________ CUU______________ 8G 152-153 8. 25 Stephens County, Texas __________________ D 

79 _________ BAR_______________ 44 61-62 6 Alaska _________________________________ A 

80 _________ 13L_______________ 11 1711-1712-1713 6 Wyoming _______________________________ A 

8L ________ BPM______________ 11 63-64 6 Muddy River area, Utah _________________ E 

82 _________ G8-PB______________ 1 50-51 6 New Hampshire _________________________ A 

83 _________ GS-WL___________ 5 156-157 6 Utah ____________________________________ A 

84 _________ GS-RR____________ 17 43-44 6 _____ do _________________________________ A 

85 _________ SEA ___________ :____ 112 077-078 6 Alaska _________________________________ A 

86 ____ c_- -- _ 13L ___ ._ _ _ _ _ _ _ _ _ _ _ _ 7 1187-1188 6 Wyoming_______________________________ A 

87 _________ 120________________ 70 10110-10111 6 New Mexico _______________ . _____________ A 

88 _________ DRW______________ IK 94-95 8. 25 Duchesne County, Utah ________________ :.. __ D 

89 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ N 

90 _________ GS-RO____________ 4 167-168 6 Utah ___________________________________ A 

9L ________ GS--RR .. ___________ 2 64-65 6 _____ do _________________________________ A 

92 _________ GS-RR____________ 17 64-65 6 _____ do _________________________________ A 

93 _________ --------------------- ________ ------------------ __________ ---------------------------------------- N 94 _________ 138________________ 5 867-868 6 Texas __________________________________ A 

95 _________ 138________________ 5 867-868 6 _____ do _________________________________ A 

96 _________ -------------------- ________ ---------------- ____ Wainwright, area index quadrangle D-19, A 

97_________ BAR ______ .. _______ _ 98____ __ ___ DRK _____________ _ 

See footnote at end of table. 

44 

21 

139-140 

197-198 

Alaska. o Alaska _________________________________ A 

8. 25 "DRK" Addition to Humboldt River, E 

Nevada. 
SOURCE AND IDENTIFYING DATA OF AERIAL PHOTOGRAPHS 223 

SOURCE AND IDENTIFYING DATA OF AERIAL PHOTOGRAPHS-Continued 

Identifying photographic data 

.Figure 

Project codo 

9!L ______ - _ SEA_-_------------ l 00 ____ - _- _ SEA _______ -_- ____ _ lOJ ________ 109 _______________ _ 

102 ________ GS-VB_ ----------- 1 o3____ _ _ _ _ as..:.QQ ___________ _ l 04_ _ _ _ _ _ _ _ DR W _____________ _ 

l05A __ ---- BKP ______________ _ 13 _ _ _ _ _ _ BN Q _____________ _ 

c ______ BNQ·-------------- :106 _____ , ___ 128 _______________ _ I 07 _ _ _ _ _ _ _ _ GS-D Y ___________ _ 

108 ____ --- _ CMU----- _------- _ 109 ________ BAR ______ . ________ _ JlO ________ BAlL _____________ _ llL _______ BAR ______________ _ 112 ________ BAIL _____________ _ 113 ________ GS-VID __________ _ 

114_____ _ _ _ SEA _____ --- ______ _ 

1 Oodo Is ns follows: 

Roll 

1.33 

1.06 

43 

5 

1 

l.K 

4G 

1.72 

96 

5 

5 

3R 

72 

72 

72 

92 

1. 

114 

Exposures 

01.9-020 

125-1.26 

5283-5284 

112-113 

89-90 

47-48 

14-1.5 

1.5-16 

3Q-31. 

751-752 

4Q-41. 

139-140 

087-088 

098-099 

093-094 

01Q-011 

46-47 

008-009 

A-U.S. Oeologlcul Survey, lVfup Information Ofllco, W~hlngton 25 D. C . .B-U .S. A lr Jforco, Photogmphlc Hocords und Services Division, Office of Research 

mtd Llnlson, USAl!' Aoronuutlcnl Chart und Information Center, Washington 

25, D.O. 

C-Enstorn Lubomtory, Performuncc Division, Commodity Stablllzutlon Service, 

U.S. Dopnrtmont of Agriculture, WfiShington 25, D.C. 

553966 0-60-1 I 

Focal length of lens, in 

inches 

6 

6 

6 

6 

fi 

8. 25 

8. 25 

8. 25 

8. 25 

6 

5. 2 

8. 25 

6 

6 

6 

6 

6 

6 

Area 

Negatives 

held by IAlaska _________________________________ A _____ do _________________________________ A 

Oregon _________________________________ A 

South Dakota ___________________________ A 

California ____________________ .. _ _ _ _ _ _ _ _ _ A 

Duchesne County, Utah__________________ D 

Pottawattamie County, Iowa_____________ C 

Valley County, Nebraska_________________ C 

Valley County, Nebraska _________________ C 

Nebraska _______________________________ A 

Iowa ___________________________________ A 

Burlington County, New Jersey ___________ C Alaska _________________________________ A _____ do _________________________________ A 

_____ dO------------------------~-------- A _____ do _________________________________ A 

New Mexico____________________________ A Alaska _________________________________ A 

D-Western Laboratory, Performunce Division, Commodity Stabilization Service, 

U.S. Department of Agriculture, 2505 Parleys Way, Salt Lake City 9, Utah. 

E-Divlsion of Cartography, Soil Conservation Service, Agricultural Research 

Center, Beltsville, Maryland N-Negatlves not available. 
224 AERIAL PHOTOGRAPHS· IN GEOLOGIC INTERPRETATION AND MAPPING 

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I 

....-.:. 
226 AERIAL PHOTOGRAPHS IN GEOLOGIC INTERPRETATION AND MAPPING 

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beds in a hydraulic test flume: Photogramm. Eng., v. 24, 

no. 3, p. 468-475. 

Thornburn, T. H., 1951, The preparation of soil-engineering 

maps from agricultural reports: Natl. Research Council, 

Highway Research Board Bull. no. 46, p. 87-95. 

Thurrell, R. F., Jr., 1953, Vertical exaggeration in stereoscopic 

models: Photogramm. Eng., v. 19, no. 4, p. 579-588. 

Treece, W. A., 1955, Estimation of vertical exaggeration in 

stereoscopic viewing of aerial photographs : Photogramm. 

Eng., v. 21, no. 4, p. 518-527. 

Turner, F. J., 1952, "GefUgerelief" illustrated by "schist tor" 

topography in central Otago, New Zealand: Am. Jour. 

Sci., v. 250, no. 11, p. 802-807. 

Turner, S. F., and Skibitzke, H. E., 1952, Use of water by 

phreatophytes in 2000-foot channel between Granite Reef 

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Twenhofel, W. S., and Sainsbury, C. L., 1958, Fault patterns 

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79th Gong., 2d sess., House Doc. 526. 

Visser, J., 1954, The construction of the datum-correction 

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REFERENCES CITED 227 

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Wengenl, S. A., 1947, Geologic interpretation of trimetrogon 

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110. 4, p. 586-600. 

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Photogramm. lllng., v. 15, no. 2, p. 225-236. 

Witkind, I. J., Hemphill, W. R., Pillmore, C. L., and Morris, 

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1043-D. (In press) 

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aerial photographs in the correlation between permafrost 

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210-219. 

--- 1934b, Results of experimental aerial survey with special reference to oil prospecting: World Petroleum Cong., 

1st, London 1933, Proc., v. 1, p. 220-229. 

Zonneveld, J. I. S., and Cohen, A., 1952, Geological reconnaissance, in The use of aerial photographs in a tropical 

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151-157. 

r-

A Page 

Accuracy, In horizontal positioning ___________ 74-75 

In vertical mc!ISuremcnt •.•••....••..•.•.. 72-74 

Altitude, determining dlfTerenccs oL ...•.. 53-54,56 

See also Flying height. 

Anaglyph principle ..• ------------------------ 45,47 

Annular drulngc. See Dra:nage, pattern. 

Anomaly, In drainage ........••••...•••.•.... 26-27 

In photogmphlc tone .•... ---------------- 29 

Anticline. See Folds. 

Association of features------------------------ 11-12 

Automatic dodging. See Photographs, printIng of. 

13 

B!ISe-hclght ratio .. ------.•.. -----·----------. 14 

13cnch erosion ••.. -----..••• ---------......... 39 

Bonded drainage.............................. 38 Bedding ___________________________________ 16, 19,26 

Dotnnlcnl guides •..• -------------------------- 32-33 

c 

Cleavage •. ---------------------... __ .. ____ .. _ 24 

Cllm!\te, relation of soils to.------------------ 15 

Color, negative-type film..................... 8 

recognition clement ••... -------------- 8-9,31,36 

transparencies ••••••••.•.•..•... ---------- 8, 31 

Contours. See Structure contouring. 

Contr!ISt. See Photographs, printing of. 

Control net or layout_________________________ 67 

]) 

Dendritic drainage. See Drainage, pattern. 

Density, drainage ••.••. -----------~--------.. 9 

film ••• __ ----•• ----.• --••• -----.------.---

See also Drainage, texture. 

Dlapositivcs •.•••••.••.•• __ ••••.•..•. ___ • ___ . _ 45-47 

Difference In altitude, methods of determinIng .. -------------------------- 50-54,56 

Dipping beds, eflect of vertical exaggeration... 13-

14,64 

estimation of dip .. -----------.. ·-·-------- 64 

mc!ISuro of diP------------------- 54,56-5 7, 63-64 Dipping platen. __________________________ 63; fig. 32 

Displacement, on faults______________________ 61 

&e also Radial displacement. 

Dodging. See Photographs, printing of. 

Double-projection instruments ... ------------ 45-48 

Drainage, anomaly .••.. ----·-··-------------- 26-27 

bonded .. -----------------------.. -------. 38 

pattern .•.•. --------------------- 11, 18,19, 26,27 

texture .. ---------------------- 9, 16, 17, 26,34-35 

tributary-length relations_________________ 19 

E 

Effective fooallongth •.•••••.•...•.•....•.••.• 

Elements or soil pattern ..••••.•.............• 34-37 

Engineering geology, Interpretation of aerial 

photographs In ___________________ 33-40 

Engineering soil maps .• ---------------------- 33 

]!';nlargemcnt, or double-projection stereoscopic 

modeL--------------------------- 47-48 or paper prints ____________________________ 4, 54 

EH.-5.i plotter _________________________ 46, 47; fig. 17 

Exnggemted-proflle plotter ..•.•••..... 48; flgs-.19, 20 

Exaggeration. See Vertical exaggeration. 

Exaggeration factor---------------------- ___ .. 64 

Extrusive rocks. See Igneous rocks. 

INDEX 

F Page 

Facies. _____ .~_ ....... __ --_. __ ._.---....... 30, 59-60 

Faults, me!IS~rlng displacement on___________ 61 

recognition .. --------~------------ ________ 21,29 

Film and filters, effect on photographic image. 4 

panchromatic film-minus blue filter ______ _ 

Flat-lying beds.----------------·------------- 19 

Floating dot.~------------------·-----·----- __ 41,47 

Floating line_: _____________________________ 57-58,64 

Flying height ___________ ·.----------------- 2, 4, 53-54 

relation to radial displacement..__________ 75 

relation to scale ...• ---------------- ______ 2, 4, 53 

relation to vertical exaggeration___________ 14 

Focal length, effective________________________ 4 

relation to radial displacement ___________ _ 

relation to scale and flying height_________ 2, 4 

relation to vertical exaggeration___________ 14 

Folds .•.•. ------------------------ ______ . __ 19, 21,26 

Foliation. See Cleavage. 

Formula.s, for determining differences in alti- tude. _____________________________ 53,56 

Cor determining feet-equivalent of parallax 

me!ISurements ______________ 54,55, 72,73 

for determining radial displacement._____ 75 

Cor determining scale.-------------------- 2, 4 

Cor determining stratigraphic thickness of 

inclined beds _____________ figs. 26. 27,28 

Fractures. See Faults; Joints. 

G 

Gradient. See Stream gradient. 

Granular materials._---------------- ______ 33, 34,35 

Gridded b!ISe-map ______________________ 69; fig. 116. 

Gridded photograph ____________________ 69; fig. 115 

Ground conditions_---------------------- ____ 33-39 

Ground water .............. ------------------ 40 

H 

Height, flying. See Flying height; also Difference In altitude. 

Hor~zontal position, accuracy of plotting __ . ___ 74-75 correction for. ____________________________ 56-57 

Hydrology, photogrammetric applications In._ 40-41 

use of aerial photographs in _______________ 40-41 

Igneous rocks, extrusive______________________ 17 

Intrusive ... ___ -----------------. _________ 17-18 

Instrument capabilitY------------·----------- 72-74 

See specific instruments. 

Interpretation, defined .. _--------- _________ .. 

Interval me!ISurlng device. _______________ 48; fig. 21 

Intrusive rocks. See Igneous rocks. 

Isopach mapping _____________________________ 58-59 

J 

Joints .•.•...... -------------------- 16, 18, 19, 23-24 

K 

KEK plotter .. ·-------------------~---------- 45 Kelsh plotter _____________________________ 47: fig. 18 

Kfactor ________________________ .54-55, 72-74; fig. 37 

L 

Landform .. ----------------- ____ ----- 6, 12-13, 17,34 

Landslides .. --------------------------------_ 39 Land usc •.. __ ._. ______ .. __ .. ______ .. __ ._._... 37 

Layout._ See Control net. 

Lens angle, relation to parallax measurement .. 

relation to radial displacement ___________ _ 

Lineations ... ·.----------------------- ___ 21, 29,30-31 

Lithologic guides. _________________ "---------- 31 

Page 

Lithology. See Igneous rocks; Metamorphic 

rocks; Sedimentary rocks. 

M 

Mahan plotter .... ---------------------------- 45 

Measurement, vertical. See Parallax. 

Metamorphic rocks ________________________ 16, 18-19 

Multiplex plotter _________________________ 47; fig. 16 

Multiscope. _. ____ . ____ . ---------------.... --- 42, 45 

0 

Ore deposits, interpretation of aerial photographs in search for _______________ 30-33 

Orientation of photographs _______________ 52; fig. 25 

need for correct orientation_______________ 54 

Orthophotographs .• _____ . ----.. ---------..... 69 

Overhead projectors .. ------------------------ 67, 68 

Overlay, drainage .... ------------------------ 19 

in correcting horizontal position.. . 56 

p 

Parallax, formulas ••.• ------------------------ 53 geologic uses or. __________________________ 56-63 

measure of.. ______________________________ 50-55 

Parallax bar. See Stereometer. 

Parallax ladder.·---------------------- 41-42; fig. 12 Pattern, drainage __________________________ 11, 26,27 

lines ____________________________________ 9, 11,29 

soils.------------------------------------- 11,29 

vegetation .•.• ______ .• -------------------- 11 

See also Elements of soli pattern. 

Permafrost •.. ____ •• -----.----·-----.•. --.• ---- 37-39 

Permeability .. ---------------------------- 16,34-35 

Petroleum geology, interpretation of aerial photographs in ___________________ 24-30 

Photogrammetric systems •. ------------------ 69-72 

Photographic tone ..• ------------------ 6-8, 16,35-36 

Photographs, geometry or. ___________________ fig. 1 

printing of. ... _. __ ------•• ----.• ----.--... 7 

scale .... ---------------------------------- 2, 4 

twin low-obllque.------------------------ 2, 4 

Physiographic guides------------------------- 31-32 

Physiography. See Landform. 

Pingos ....•• _____ . _. ___ ••• ----.. -----••••..•• - 38 

Platen .•.. ___ ••..•. ___ -----------.---.--.. -.-. 47 

Plotting, in cross section______________________ 69 

orthographic ....• ---------------------- 67-68,69 

Plutonic rocks. See Igneous rocks. 

Polygonal microrelief. ____ •.• ---_.--.----.. --. 37-38 Pseudoscoplc view ________________________ 5; fig. 95 

R 

Radial displacement, correction for .. --•... -•• 56, 68 

measure oL.-------------------------- 56,74-75 

use In determining heights .... ------------ 5fi 

Radial drainage. See Drainage, pattern. 

Radial planimetric plotter ________________ 42; fig. 14 

Recognition elements------------------------- 6-13 

Reflectance. See Photographic tone; Spectral 

refl.ectance. 

Relief displacement. See H.adial displacement. 

s 

Scale of photographs, relation to focal length 

and flying height. __ ------------- 2, 4 

relation to parallax measurements ________ 4, 74 

significance in Interpretation ....•... 11-12, 14,21 

Sedimentary rocks _________________________ 15,16-17 

Shadows. _____________ • ___ • ___ ----------_---- 13 

229 
230 

Page 

Shape __ -------------------------------- 12-13,34,35 Size ______________________ -- __ -- ________ -_____ 13 

Sketchmaster---------------------------- 42; fig. 13 

Slope, direct determination oL --------------- 63-64 

measurement by parallax method _____ 54, 56-57 

Soils, permeability_-------------------------- 34-35 

transported, landform or__________________ 34 

Spectral reflectance _____________ ------ ____ ---- 7-8 

Stereometer------------------------------ 41; fig. 10 Stereo slope comparator __ . ________________ 63; fig. 33 

Stereo slope meter ______________________ 42; fig. 11 

Stereotemplets ______________________ ------- __ 67 

Stereotope ________________________________ 45; fig. 15 

Stratigraphic thickness, determination oL __ 57-58 

Stream gradient, determination oL___________ 63 

use in detecting tilt_______________________ 55 

Structure _________________ --------- ___________ 19-24 

Structure contouring __ ----------------------- 6Q-62 

Structural guides _____________________________ 3Q-31 

INDEX 

Page 

Surficial materials __ -------------------------- 17, 33 

Syncline. See Folds. 

'I' 

'I'emplets___ _ _ __ __ _ _ _ ___ _ _ __ _ ___ _ __ ___ __ _ _ _ __ _ 67 

Texture, drainage ________________ 9, 17, 26-27, 34-35 

photographic ___ -------------------------- 9 soiL __________________________ --------____ 9 

topographic _____________________________ _ 

'I' haw lakes ______________ -------______________ 38 

TilL ___________ ---- __ ----____________________ 55 

'fone. See Photographic tone. 

Topography, significance in delineating structtue_ -------------------- 19, 21, 24, 27, 29 

See also Landform; s·hape. 

Trellis drainage. See Drainage, pattern. 

Twin low-oblique photography. See Photographs. 

u 

Unconformity-------------------------------- 24 UniversaJ tracing table _____ . ___________ 48, 55; fig. ~2 

v Page 

Vegetation, as indicator of structure 11 21 23 

in interpretation of permafrost---~~~~~~---· '37 

in interpretation of soils ___ --------------- 36 

significance in search for ground water____ 40 

use in differentiating rock types ________ 15, 17, 18 

See also Elements of soil pattern. 

Vertical exaggeration, cause of ________________ 13-14 

correction for _____________________________ 63,64 

relation to true distances and slopes ______ 13-14 

Vertical positioning___________________________ 69 

Viewing, methods oL________________________ 4-5 

Volcanic rocks. See Igneous rocks. V factor ______________________________________ 73-74 

w 

Water supply. See Ground water. 

Wave length_-------------------------------- 4, 7-8 

U.S. GOVERNMENT PRINTING OFFICE: 1960 0-553966