Application of High-Order 

Stereoscopic Plotting Instruments 

to Photogeologic Studies

By C. L. PILLMORE

PROCEDURES AND STUDIES IN PHOTOGEOLOGY

GEOLOGICAL SURVEY B U L I/E TI N 1 0 4 3-B

A discussion of accuracy of 

measurement and advantages \ > 

of using stereoscopic plotters in 

geologic interpretation and mapping

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957
UNITED STATES DEPARTMENT OF THE INTERIOR

! i -.* >. .

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolari, Director

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

' ' ' Washington 25,' D. C.' - ' Price 15 cents (paper co'ver)
CONTENTS

Page

Abstract__ .............................i....................... 23

Introduction.._--___--___-__-_---___---_____--___-____________-__- 23

Instruments __-___-__-_-_-_--__-__-__---_-___-______-__________-__ 24

Kelsh plotter_______________________________________________ 25

Multiplex plotter._____________________________________________ 25

ER-55 plotter.._______________________________________________ 26

Comparison__ _______________________________________________ 26

Accuracy of measure ments__________________________________________ 27

Inherent accuracy__----_--_____---__-_- J -____-__-____--____-__ 27

Focal length of lens and scale and quality of photography __________ 27

Nature of geologic features--______-----___-___--______________- 28

Experience of operator.________________________________________ 28

Importance of scale---___-_-_---__------_--_____________-__________ 29

High-altitude photography______________________________________ 29

Importance of base map______._________________________________ 30

Economic aspects__----_-_-__-_--_-___--___-____-___-_-_______--_ 30

Advantages in geologic studies._______----_-__________-____________I 31

Application to specific geologic studies.__-_-_----___--__-____________ 33

Isopach maps_________________________________________________ 33

Structure-contour studies.______________________________________ 34

Special compilation projects____.________________________________ 34

Literature cited___________________________________________________ 34

in

426806--51

APPLICATION OF HIGH-ORDER STEREOSCOPIC 

PLOTTING INSTRUMENTS TO PHOTOGEOLOGIC STUDIES

By C. L. PILLMORE

ABSTRACT

The Kelsh, multiplex, and BR-55 projector-type plotters, all high-order precision stereoscopic plotting instruments, are applicable to many photogeologic 

studies. The use of these instruments for most geologic mapping projects is 

justified by their ease of operation and the high quality of results.

The large area per stereoscopic model covered by so-called high-altitude photography (at a scale of approximately 1:60,000 or smaller) in a high-enlargement instrument such as a Kelsh plotter enables geologists to survey large areas 

rapidly and to map many geologic features reliably. Areas in which detailed 

geologic mapping is desired may be appropriately studied by use of low-altitude 

photography (at a scale of approximately 1:20,000 or larger) in one of these instruments. Special geologic studies where numerous stratigraphic thicknesses or 

altitude measurements are essential, as in isopach or structure-contour studies, 

may be readily carried out for many areas by using stereoscopic plotting instruments. The use of stereoscopic plotting instruments, particularly when combined 

with field observations, may result in a more accurate and reliable geologic product than is obtained by use of simple stereoscopic "paper print" plotters, and, under some circumstances, than is obtained from field compilations. The reason for 

this high accuracy is that the anaglyph (stereoscopic model) may be reoriented, 

both horizontally and vertically, to a base map. However, reliability of the 

final product depends upon the base map and the geologist's interpretation of 

contacts, faults, and other geological features.

INTRODUCTION

Geologic interpretation of aerial photography by means of highorder stereoplotting instruments is a relatively new technique. The 

two instruments most commonly used to date (1956) are the Kelsh 

plotter and the multiplex plotter, although the ER-55, a projector 

 developed in recent years by the U. S. Geological Survey, will probably be much used for photogeologic interpretation in the future. 

These instruments are available for use by Geological Survey geologists; some commercial organizations are beginning to use them for

 'lil'-." I v ...!.' :' ' . . ' . ' . - . : ,:'.' 

23
24 PROCEDURES AND STUDIES IN PHOTOGEOLOGY

geologic purposes, but the so-called heavy plotters, such as the stereoplanigraph or the autograph, are rarely available to the geologist 

because of their high cost.

The Kelsh and multiplex plotters, and the type using EK-55 projectors, utilize the principle of projection of light of complementary 

colors through glass diapositives of overlapping aerial photographs. 

The anaglyph, or stereoscopic model, is viewed on a white surface, 

called a platen, mounted on a tracing table containing a floating light 

source or pinpoint of light vertically above a plotting pencil. Because the platen can be moved vertically on the tracing table and 

because the tracing table can be moved horizontally, any point on the 

stereoscopic model can be located or measured with respect to any other 

point. Inasmuch as the stereoscopic model is orthographically true, 

setting the floating dot on the apparent ground surface at various 

places permits accurate horizontal positioning, and lines or geologic 

contacts can be plotted orthographically onto a base map. The use 

of high-order stereoscopic plotting instruments assures accurate compilation of geologic data, provided that interpretation is correct and 

provided there is sufficient control to orient the model.

Although high-order stereoscopic plotting instruments have been 

used for many years for topographic mapping purposes, these instruments were first used extensively for geologic interpretation in 1954 

by the U. S. Geological Survey. The instrument most widely used 

thus far for photogeologic purposes is the Klesh plotter because of its 

high enlargement factor and the high resolution of the stereoscopic 

model. The enlargement factor is herein defined as the relation between the scale of the stereoscopic model and the scale of the original 

photography. An instrument using EK-55 projectors also projects a 

model of high resolution and may become widely used for geologic 

interpretation and mapping. The multiplex plotter has been used for 

compilation of geologic field data, but it has not proved to be entirely

adequate for detailed photogeologic interpretation.

INSTRUMENTS

One of the main advantages in using high-order stereoscopic plotting 

instruments for photogeologic study is in working with an oriented 

three-dimensional true-scale model of tlie terrain upon which accurate 

measurements both vertically and horizontally can be made. The 

detail of the model, as observed on the platen, is related to the scale, 

focal length, and quality of the original photography, the processing

Of the diapOSitives, and the type of instrument being used.

Smaller stereoscopic plotting instruments, utilizing paper prints 

of aerial photographs, provide an orthographic solution through a 

series of metric compromises that is less accurate than that provided by
HIGH-ORDER STEREOSCOPIC PLOTTING INSTRUMENTS 25

anaglyphic type instruments, such as the Kelsh plotter. In general, 

the "paper print" plotters are more difficult to operate than the anaglyphic instruments.

KELSH PLOTTER,

The Kelsh plotter, developed by the U. S. Geological Survey, utilizes 

9%- by 9%-inch glass diapositives of aerial photographs commonly 

termed Kelsh plates. Because Kelsh plates are the same scale as the 

original photography the resolution of the image observed on the 

platen is excellent. Two projectors are linked to the tracing table so 

that light rays passing from the projectors through the glass diapositives are restricted to the platen; in the other plotters the entire stereoscopic model is illuminated. The enlargement factor of the Kelsh 

plotter is about 5 times, making the scale of the stereoscopic model 

(model scale) 5 times that of the original photography from which the 

diapositives were made. For example, Kelsh plates made from 

1:20,000-scale photographs taken through a 6-inch camera lens would 

result in a model scale of approximately 1:4,000. The Kelsh plotter 

can be adjusted to accommodate both 6-inch- and 8%-inch-lens photography. A description of the Kelsh plotter, of the correct method of 

placing the glass diapositives in the instrument, and of the orientation 

procedure for clearing parallax from the steroscopic model is given 

by Scher and Thompson (1954).

The high enlargement factor of the Kelsh plotter permits the use 

of small-scale aerial photography to a great advantage, especially when 

making reconnaissance studies of large areas. The scale of the photography used in the Kelsh plotter is ideally determined by the geologic problems involved in the area to be studied; that is, in areas 

that are to be mapped in considerable detail, low-altitude or largescale photography should be used when available; in areas in which 

detail is not so important, so-called high-altitude or small-scale photography, if available, should suffice. Recent tests indicate that lowoblique photography can be accommodated in the Kelsh plotter.

MULTIPLEX PLOTTER

The multiplex plotter uses glass diapositives on which the original 

9- by 9-inch negative is reduced about 4 times to a size about 1% 

inches square. The enlargement factor of the multiplex is about 2y2 

times the original photography scale. Therefore, multiplex diapositives made from 1:20,000-scale photography, taken through a 6-inch 

camera lens, would project to a scale of about 1:8,000. Owing to the 

great reduction of the image during printing of the diapositives, and 

owing to the enlargement (approximately 12 times) of this image in 

projection, the stereoscopic model obserjved on the platen is usually
26 'PR<5eEI)UBES AND STUDIES IN FHOTOGEOLOGY

slightly fuzzy, and the fine details of the terrain that are important1 

in photogeologic interpretation may be difficult to distinguish. In 

the Geological Survey the multiplex is being used to advantage especially in compilation of geologic field data onto a base map. A description of the multiplex and the procedure for orienting the diapositives 

in the instrument is discussed fully by Loud ( 1952) .

ER-55 PLOTTER

The term "ER-55 plotter" is used herein as any stereoscopic plotter 

using ER-55 projectors. The ER-55 projection assembly consists 

of ellipsoidal reflector- type projectors which utilize aerial photography 

reduced to a 4- by 4-inch glass diapositive. This projection assembly 

produces sufficient light to obtain an excellent image ; the enlargement 

factor of the instrument is 3.4 times, about midway between that 

of the Kelsh and multiplex. The projectors are also adaptable to 

low-oblique photography taken along or normal to the flight line 

(inclination of camera lens axis 20° from vertical). The photogeological aspects of low-oblique photography are at present being 

evaluated. The use of convergent low-oblique photography in a 

stereoscopic plotting instrument permits greater accuracy of vertical 

measurements at any given scale than standard vertical photography. 

Also, at a given scale a convergent model covers greater area than a 

vertical model. As the projectors are adaptable to both vertical and 

low-oblique photography the ER-55 plotter should prove to be very 

useful to the geologist. ER-55 projectors can be attached to the 

multiplex or similar-type frame with the use of supporting brackets 

and a few minor adjustments, depending upon the particular type of 

frame available.

A discussion of low-oblique photography and of the Twinplex plotter and a short description of the ER-55 projector have been given 

by Hopkins, Radlinski, and Thompson (1952).

One of the major differences between the KelSu plotter and the 

multiplex and ER-55 plotters is tke freedom of movement of the 

ER-55 and multiplex projectors. Because of the linkage of the Kelsh 

projectors to the tracing table and because of the construction of the 

superstructure that supports the projectors in the plotter, movement 

of the individual projectors is restricted to the x direction, which parallels tK© projector line of the instrument. HoweYCr, In the multiplex 

and the ER-55 plotters, each individual projector may be moved not

only in the a? direction but also in the y direction and the » direction. 

Because of this latitude of movement of projectors, particularly to
HIGH-ORDER STEREOSCOPIC PLOTTING INSTRUMENTS 27

compensate for greater scale variation due to high relief, the EK-55 

and multiplex plotters may be advantageous for study of high-relief 

areas. As more than two projectors can be used on a single plotter, 

depending upon the length of the supporting bar, multiplex and 

EK-55 plotters are also especially adapted to obtaining control between widely separated control points by bridging (see Loud, 1952).

ACCURACY OF MEASUREMENTS

The accuracy of measurements and spot elevation readings made 

with high-order stereoscopic plotting instruments in photogeologic 

studies depends upon the following factors: (1) The inherent accuracy 

of the instrument; (2) the scale, quality, and focal length of the original photography from which the diapositives were made; (3) the 

nature of geologic features, such as contacts; and (4) the experience of 

the operator.

INHERENT ACCURACY

The inherent accuracy of the instruments is an estimation of the 

minimum error expected under optimum conditions of the other three 

above-mentioned factors. Under these optimum conditions the Kelsh 

plotter is estimated to be accurate to within 0.1 millimeter of parallax. 

The estimated error of the EK-55 plotter is slightly greater than that 

of the Kelsh plotter. For the multiplex plotter the estimated error is 

higher mainly because of the lower enlargement factor. Because the 

stereoscopic model in the multiplex plotter is usually not as sharp as 

that in the Kelsh plotter or the ER-55, owing to the loss of resolution 

during reduction in printing and owing to enlargement in projection, 

the optimum reading is seldom realized. However, the Kelsh and 

ER-55 plotters usually display excellent images in the stereoscopic 

model, and optimum readings are commonly obtained when low-altitude photography is used.

FOCAL LENGTH OF LENS AND SCALE AND QUALITY OF 

PHOTOGRAPHY

The altitude of the airplane and focal length of the lens used in the 

photography from which diapositives are made are very important 

factors in estimating the probable error in vertical measurements by 

use of the different instruments. The scale of photography is directly 

related to the focal length of the camera and to the height of the camera 

(altitude of airplane above mean terrain) at the time of exposure; 

that is, scale=focal length (in feet)/height of camera (in feet).

Most photography for topographic mapping purposes at present is 

taken with a wide-angle camera lens having a focal length of 6 inches. 

Some organizations use a lens with an 8^4-inch focal length, but Geo-
28 PROCEDURES AND STUDIES IN PHOTOGEOLOGY

logical Survey specifications usually require the use of the 6-inch lens. 

When the 6-inch lens is used the scale of photography taken from 

10,000 feet above mean terrain is 1:20,000; photography taken at 

30,000 feet above mean terrain is at a scale of 1:60,000. When 

1:20,000-scale photography is used in the Kelsh plotter the predicted 

error is about 2 feet for vertical readings, under optimum conditions. 

This error is proportionately larger when smaller scale photography 

is used. The predicted error for the EK-55 and multiplex plotters 

will be slightly greater primarily because their models are smaller in 

scale than the Kelsh model.

The quality of each individual negative is one of the factors that 

determine how close to the predicted accuracy the operator of the 

instrument can measure. If the film has been underexposed or overexposed, or if the pictures were taken under cloudy or hazy weather 

conditions, the stereoscopic model observed on the platen may not be 

sharply defined, and the probable error will increase.

NATUBE OF GEOLOGIC FEATURES

The accuracy of readings made on such geologic features as contacts depends upon how well the operator can see these features in 

the stereoscopic model. The quantitative and qualitative aspects of 

the stereoscopic model, of course, depend not only on the type of instrument being used but also on the lens focal length and the scale 

and quality of the photography. If other conditions are optimum 

and a geologic contact is a sharp tonal or topographic break, then a 

minimum error should be realized in making vertical measurements 

with stereoscopic plotting instruments. However, if talus or rock 

debris, a thin veneer of soil cover, or vegetation cover the contact, 

then the probable error will increase.

EXPERIENCE OF OPERATOR

The experience of the operator concerns not only his experience 

with photogrammetry and practice with stereoplotting instruments, 

but also his geologic knowledge of the area with which he is working. 

The geologist witK no experience in photogrammetry and no practice 

in using stereoscopic plotters may accurately position a geologic contact horizontally, but he may incorrectly position this contact vertically at a point 10 or 15 feet above or below the ground surface. An 

operator with photogrammetric experience may make accurate readings On the ground surface, but he may not be able to identify the 

geologic unit that this ground surface represents.

Experience has proved that an individual with normal or corrected 

vision can readily learn to clear parallax from Stereoscopic models,
HIGH-ORDER STEREOSCOPIC PLOTTING INSTRUMENTS 29

level them, and scale them to a base map using a pantograph. With 

little practice he can make reliable measurements with use of the floating dot and can plot geologic features expressed in the stereoscopic 

model. . . 

IMPORTANCE OP SCALE

The scale of the original photography determines the scale of the 

stereoscopic model, depending on which stereoscopic plotter is used. 

As stated previously the model scale and the instrument affect both 

the clarity of the image observed on the platen and the reliability 

of readings made with the tracing table. < Commonly the geologist 

must use photography that is already available. Many times this 

photography was not designed for geologic interpretation; however, 

the enlarged scale of such photography in the Kelsh or ER-55 plotters is usually acceptable for most geologic-mapping studies. In 

areas that are to be mapped in great detail, the geologist may find 

it desirable to obtain new photography at a scale large enough to 

permit him to distinguish very minor features in the stereoscopic 

model. In areas of reconnaissance study it may be desirable to obtain 

new photography of small scale. The use of a variable-ratio pantograph linked mechanically to the tracing table of any of the instruments described enables the geologist to plot the data observed on the 

tracing table directly and accurately on base maps of different scales.

HIGH-ALTITUDE PHOTOGRAPHY

One of the most recent and promising developments in photogeology 

within the Geological Survey is the application of photography taken 

at altitudes as high as 35,000 feet, termed high-altitude photography 

(scale of 1:60,000 to 1:70,000, and commonly about 1:63,360 or 1 

inch to a mile), for use by the Army Map Service in the 1:250,000- 

scale topographic compilation of the United States. In the Kelsh 

plotter, this photography effects a model scale of about 1:12,000, and 

the estimated error in measuring vertical distances is 5 to 6 feet under 

optimum conditions. Because this photography is of small scale, 

some of the minor details of the terrain are difficult to distinguish in 

the stereoscopic model, but in areas suited to photogeologic study 

this loss of detail is not generally a deterrent in interpretation and 

mapping. High-altitude photography is especially useful for smallscale reconnaissance mapping because of the rapidity with which the 

operator can cover a large area. One stereoscopic model from highaltitude photography covers approximately 10 times the area covered 

by the stereoscopic model from low-altitude photography (1:20,000 

scale). Because the original photography scale is increased less with 

the multiplex than with the Kelsh plotter, high-altitude photography
AND STUDIES IN PHOTC-GEOLOGY

may not be as suited to geologic interpretation when used in the multiplex. Also, resolution in the multiplex stereoscopic model is less than 

that in the Kelsh stereoscopic model. In fact, geologic detail visible 

in a 1:12,000-scale Kelsh stereoscopic model from original 1:60,000- 

scale photography commonly may be more easily delineated than the 

same detail visible in a 1:12,000-scale multiplex stereoscopic model 

from original 1:30,000-scale photography. It may also be significant 

that this multiplex model covers only one-quarter the area represented 

by the Kelsh model.

High-altitude photography may be used in the ER-55 plotter, but 

the model will not be as large scale as the model obtained in the Kelsh 

plotter.

IMPORTANCE OF BASE MAP

The model scale of stereoplotting instruments is calculated from a 

base-map scale or from control points identified in the stereoscopic 

model. The accuracy of most measurements, both horizontal and 

vertical, depends on how accurately the scale of the model has been 

determined. Among the best available base maps for use with one of 

the high-order stereoscopic plotters are topographic maps compiled 

on stereoplotting instruments by the U. S. Geological Survey. The 

geologist should obtain an exact-scale copy of the base map printed 

on some stable material such as Mylar or metalmount paper, because 

the accuracy with which he can map geology by using stereoscopic 

plotting instruments is greater than the amount of error realized by 

the expansion or contraction of most less stable materials.

The quality of the base map is important because geologic features 

should be plotted as accurately as possible with relation to topographic 

and planimetric details. If a good base is not available, a base map 

can be constructed by photogrammetric methods by using ground control points within the map area. Many 1:250,000-scale topographic

maps compiled for the Army Map Service from high-altitude photography are available and can be used to advantage in areas that are not 

covered by recent large-scale topographic maps. Many topographic

details are lost in these maps because o± their small seal©, but thera

is generally sufficient control shown to approximately orient stereoscopic models from high-altitude photography in the Kelsh or EH 55 

plotters.

ECONOMIC ASPECTS

The most important advantage of photogeologic mapping by means

of high-order stereoscopic plotting instruments is the high order

of accuracy obtained at low cost, provided there is access to an acceptable base map or sufficient control available to construct a base

map. The main factor influencing the cost in a geologic mapping
HiGH;-pRpER STEREOscopiq, PLOTTING INSTRUMENTS 31

problem is the time in man-hours necessary to complete the job. .As 

mentioned before, 1 model obtained from high-altitude photography 

and used in either the Kelsh or ER-5,5.j plotters coyers an area approximately equivalent to about 10 models from low-altitude photography, or about % of a 7^-minute- quadrangle. Even with a minimum of control, data f or: special studies .such ,as; structure-contour 

maps, isopach maps, fence diagrams, control, for. isometric drawings, 

and detailed structural overlays, are readily available from spotelevation readings and horizontal and vertical measurements made 

with the tracing table. .In addition simple.geology in well-exposed 

areas can be interpreted and mapped accurately with a minimum 

of field checking. The geologist can then spend his field season 

working out the more complicated areas, collecting samples, and 

measuring detailed sections necessary for stratigraphic studies. Because vertical control is readily available from the stereoscopic model 

valuable field time need not be used to obtain this control for special 

studies. Stereoscopic plotters are also useful in compiling geologic 

data that have been annotated on photographs as part of a field 

mapping project. ; ,

If diapositives of high-altitude photographs are used, it is suggested that low-altitude paper prints of the area also be purchased 

if available. These prints are an aid to field checking and are 

often very useful for detailed interpretatipn in areas which, for one 

reason or another, are not well defined in the stereoscopic model.

For areas in which very detailed interpretations are desired, diapositives made from low-altitude photography should be used. If 

rock exposures are numerous and,continuous and the geolpgy ,is not 

complex, a large part of the field work necessary to map distribution 

of formations can be eliminated.

Kelsh and multiplex diapositives can be purchased from many of 

the larger private flying companies, and ER-55 diapositives should 

become available in the future. Diapositives made from Geological 

Survey film and from Army Map Service so-called high-altitude film 

on file with the Geological Survey may also be purchased for any of 

the plotters from photograph sales offices within the Geological 

Survey. . 

ADVANTAGES ik QEQLOGIC STUDIES , '.- ' ; , .'i !'. ':' i ! . ;'.: i' > : '..->' ' ' ' I . I ,.,-.< 

The main advantage in the use of high-order stereoscopic plotting instruments such as the Kelsh or ER-55 plotter in the identification and interpretation of geologic features is that the geologist 

works with a large-scale optically corrected model of high resolution, and geologic features observed in this model may be plotted 

or measured rapidly and accurately, or both. Because more than
one geologist can view the stereoscopic model at the same time, consultation with experienced personnel may help to solve interpretation and identification problems.

If a geologist has access to literature or field data on a particular 

area, he may be able to identify the units of measured stratigraphic 

sections with a stereoscopic plotter by studying the relations of formations as to photographic tone, topographic expression, thickness, 

texture, and, sometimes, bedrock-controlled vegetation. Thicknesses 

of sedimentary units may be important in determining depositional environments and are therefore paid close attention by 

most geologists. If contacts are observable in the stereoscopic model, 

then numerous thickness measurements may readily be obtained by 

using a stereoscopic plotter. If thicknesses or attitudes are known 

to be constant, the geologist may be able to position covered contacts 

on the basis of thickness or attitude measurements.

The use 9f the floating dot as a spatial reference mark within the 

stereoscopic model is of particular significance. With proper use of 

the tracing table in uniformly dipping strata, the geologist can project contacts for hundreds of feet to intersect covered slopes.

Stereoscopic plotters may be particularly useful in obtaining structural data. Detailed structure studies with stereoscopic plotters 

usually have been confined to areas of relatively simple structure 

with numerous rock outcrops; however, in some areas structure studies 

have been made on the basis .of vegetation patterns observable in 

aerial photographs but indistinguishable or distinguished with difficulty on the ground. The success in obtaining reliable structural 

data is a direct result of the ability to make accurate vertical measurements by use of stereoscopic plotters. Areas of more complicated 

structure with good rock exposures are also suitable for study with 

stereoscopic plotters, but they'should be studied in conjunction with a 

correspondingly greater amount of field work.

The floating-dot principle of stereoscopic plotting instruments allows rapid, accurate measurement of altitudes. These measurements 

can be used to compute strike and dip by means of the standard threepoint method, and to construct structure contours, provided recognizable geologic horizons are persistent throughout the area. By use of

a tiltable surface with an attached clinometer on the tracing table,

reliable strike and.dip measurements can be made without mathematical calculations by aiming this surface to a dipping sedimentary

.Unit ,in the stereoscopic model.

Faults with as little-as 18 inches .of vertical .displacement may be

Identified in.some Stereoscopic models, depending on the nature of

tfie-faulted, strata, the kind of/fault, 'and the resolution and'scale of 

the model. Usually, faults are expressed as linear elements (linear
HIGH-ORDER STEREOSCOPIC PLOTTING INSTRUMENTS 33

features) which aid the geologist in interpretation in areas of vegetation or surficial cover, as well as in areas of exposed rock. The geologist mapping on stereoscopic, plotting iristruments may find it easier 

to visualize faulted areas and.to project stratigraphic units than is 

possible in the field. ,....

APPLICATION TO SPECIFIC GEOLOGIC STUDIES

Stereoscopic plotting instruments can be used to produce isopach 

and structure-contour maps and other special maps entailing highorder accuracy. The Kelsh and ER-55 plotters are better suited to 

many special problems because of their high enlargement factors 

and the highly resolved models obtained from most of the common 

photography; however, the multiplex plotter is also suited to these 

studies if large-scale photography is available, or if the geologic requirements will permit use of small-scale photography.

ISOPACH MAPS

One of the most important advantages of isopach mapping by 

means of high-order stereoscopic plotters is that a very large number 

of readings can be taken, and many thicknesses computed, in a relatively short time. If the geologic contacts under consideration are 

sharp and distinct in the stereoscopic model, if dips are gentle and 

structure is not complicated, and if the operator has stereoscopicplotter experience, the majority of measurements will be as accurate 

as, or sometimes even more accurate than, those obtained in the field. 

However, easily recognizable rock units are not necessarily essential 

to successful isopach mapping by stereoscopic plotting instruments. 

Topographic, vegetational, or other criteria that will permit interpretation of geologic contacts may well be adequate to allow compilation of isopach maps.

Several factors may limit accuracy in isopach mapping by use of 

b,igh-order stereoscopic plotters Geologic contacts upon which readings are taken should preferably be sharp and distinct; talus, soil, 

and vegetational cover should be at a minimum. Ideally a tonal 

break between formations is preferred over contacts expressed by 

changes in topographic expression or changes in vegetation.

Good base-map control is important for detailed work but not absolutely necessary if accuracy requirements are not too high. Spot 

field checking during compilation of the project will enable the 

geologist to evaluate the accuracy of both thickness measurements and 

identification of geologic contacts. If optimum conditions prevail 

throughout the project area, and low-altitude photography is used in 

either the Kelsh or ER-55 plotters, the geologist can predict an error 

as low as 2 to 5 feet per 100 feet for most measurements.
34 :: ; PIM>CEpU$ES : JINP rSTTJDIES HJT, PHOTOGEOLOGT^,

STRUCTURE-CONTOUR STUDIES ..,..-

Similar principles and limitations apply, to structure-contour 

studies as to isOpach mapping by use of stereoscopic plotting instruments. Eesolution of detail in the stereoscopic model is : also important in structure-contour studies, and it may be advantageous to 

use the Kelsh plotter or EK-55 plotter. However, because the limiting error need not be too small in many studies of this kind, the 

multiplex plotter may be substituted when low-altitude photography 

is used. The surface to be contoured should preferably be clearly 

visible in the stereoscopic model and relatively free from cover, although vegetated areas of sparse rock outcrops have been successfully 

structure contoured from aerial photography.

SPECIAL COMPILATION PROJECTS

Stereoscopic plotting instruments offer the geologist many desirable 

features that are especially applicable to special compilation projects. 

Some lineations are mappable from the Kelsh plotter stereoscopic 

model that are indistinguishable or difficult to distinguish in paper 

prints. Example of these features are those expressed by alinement 

of tall trees in a dense forest, minute lines in darker grassland vegetation, and parallel elements thought to be foliation trends of metamorphic rocks. Most of these linear elements probably are controlled 

by bedrock. It is interesting to note that most of these features cannot be recognized easily in the field owing to a dense vegetational 

cover, and hence the accurate map positioning afforded by stereoplotting instruments facilitates the location of such features on the 

ground.

LITERATURE CITED

Hopkins, B. T., Radlinski, W. A., and Thompson, M. M., 1952, Twin low-oblique 

photography and the Twinplex plotter: U. S. Geol. Survey drc. 222.

Loud, T. I., Jr., 1952, Topographic instructions Chapter 307 a-e, Multiplex 

Procedure: U. S. Geol. Survey Clrc. 164.

Scher, M. B,, and Thompson, M. M., 1954, Kelsh plotter procedure: U. S. GeoL 

Survey Circ., 357.