PHOTOGRAMMETRY APPLIED TO EARTH SCIENCE IN THE 

U.S. GEOLOGICAL SURVEY 

G EOLOGISTS AND HYDROGEOLOGISTS have 

traditionally prepared their maps by fieldsurveying techniques, either by sketching pertinent information directly on topographic or planimetric base 

maps or by annotating data directly on aerial photographs or on transparent overlays. Geologic mapping 

with only a topographic map as a base is still common, but when aerial photographs became widely 

available in the 1940's their use superseded much 

direct plotting on base maps. Nowadays in the Geological Survey, photogrammetric techniques are commonly used both for geologic mapping and for 

compiling data from aerial photographs. 

Geologists who map on aerial photographs take 

advantage of the three-dimensional quality of the 

stereoscopic model and the wealth of its inherent 

image detail for interpreting and accurately delineating geologic features. Details that can be seen and 

mapped on the photographs, however, must be transferred to a base map for compilation and publication, 

and much of the accuracy gained by careful annotation can be lost by inaccurate transfer. Excessive 

radial displacements caused by high topographic 

relief, and distortions caused by tip or tilt of the 

aerial camera, must be corrected in the transfer 

process. These corrections can be made visually by 

careful and skillful terrain interpretation and freehand sketching, or instrumentally by the use of photogrammetric techniques. For areas of low relief, where 

radial distortions are small, details mapped on aerial 

photographs can be adequately transferred to a base 

map by tracing or by using a simple optical projector 

to make adjustments for scale differences; if tip or tilt 

cause distortions, a tilt-correcting device, such as a 

sketchmaster, can be used to correct them. Many 

mapping projects, however, are in rugged areas of 

high relief where radial distortions must be corrected 

photogrammetrically for accurate compilation. 

HISTORY AND DEVELOPMENT 

During the 1940's and early 1950's, the use of 

aerial photographs for geologic interpretation and 

mapping increased rapidly, and new instruments and 

techniques were devised. Though limited in capability, 

radial planimetric plotters were used for constructing 

planimetric base maps and transferring geologic 

annotations from the paper prints onto the base map. 

Higher order projection-type photogrammetric plotters 

such as the ER-55 and Kelsh were used routinely for 

topographic contouring and base-map control. These 

plotters were first used in 1954 by Survey geologists 

who soon afterward began teaching other geologists 

to set up and orient the stereomodels. The Survey's 

Kelsh and ER-55 plotters were widely used for geologic mapping during the 1960's and the early 1970's. 

The PG-2 plotter replaced the projection plotters, 

because it combines high plotting accuracy with an 

excellent viewing system for stereoscopic inspection. 

It also is the only instrument that (1) with equal ease 

uses black-and-white or colored paper prints and film 

or glass transparencies, (2) has a high-quality viewing 

system incorporating a readily changed, variable magnification, (3) has an illuminated floating mark that 

can be easily seen in dark parts of the image, and 

(4) has an ease of operation that enables geologists 

with little use experience to operate the machine as 

soon as the stereomodel has been oriented. Though 

designed for topographic mapping, the PG-2 plotter is 

particularly well suited for geologic studies because 

the paper prints used by the geologist in the field can 

be placed directly in the plotter so that field annotations, together with additional data observed in the 

stereoscopic model, can be plotted directly and 

accurately onto a topographic base map. This plotter 

also enables the operator to sit comfortably in a welllit room while viewing the model and at the same 

time referring back to field notes and photographs. 

The projection-type plotters required subdued room 

lighting and, therefore, were inconvenient for studying map annotations and field notes simultaneously. 

Once the photographs are properly placed in the 

machine and parallax is cleared from the stereoscopic 

model, the PG-2 plotter is simple to operate. However, certain procedures must be followed to clear the 

parallax and level and scale the model to a base map. 

At the Geologic Division Plotter Laboratory in 

Denver, a photogrammetric technician orients the 

model for geologists who prefer not to do so themselves. Features seen in the model can then be 

plotted by tracing their images, just as with other 

stereoplotters. Many geologists use the machines only 

earnmetry applied to earth science in the U.S. Geological Survey 
to trace lines already mapped on the photographs, 

simply compiling previously mapped geology (thus the 

needed capacity of the plotter to accommodate paper 

prints). Other geologists, using field stations or measurements mainly as guides to mapping, prefer to 

map directly from the stereomodel, followed by field 

checking-a combined procedure that is highly effective for mapping areas of good exposure and little 

vegetation, such as deserts, high mountains, and 

polar regions. 

In the 1980's the Survey devised a computerassisted geologic mapping system that uses the PG-2 

plotter to (1) enhance the capabilities of the plotter 

for geologic mapping, (2) provide a means for geologists to make geologic maps more efficiently, and (3) 

develop new ways to apply computer technology to 

geologic studies. The system is capable of producing 

geologic maps and geologic-map products such as 

structure-contour and isopach maps, overburden and 

interburden maps, and surface profiles for geologic 

cross sections-all from stored digital information 

generated by the plotter, supplemented by subsurface 

data entered from a two-axis digitizing table. In some 

projects, it will be possible to go directly from stored 

data from photographs to published maps without the 

necessity for drafting or scribing. In addition, a set of 

computer routines designed specifically to aid in geologic studies has been developed jointly by the U.S. 

Geological Survey and the Geological Survey of 

Greenland in Copenhagen, Denmark. This routine, 

called Geoprogram, allows the geologist to integrate 

the visual stereoscopic model with the threedimensional mathematical model of the geology. A 

rapid interchange of information between the computer and the operator is one of the most useful 

aspects of the system. As the tracing carriage is 

moved about the stereomodel, the Geoprogram provides the kinds of information the operator needs for 

making structural interpretations by the near-instant 

generation of mathematical planes representing 

geologic attributes. For example, dips and strikes of 

any planar surface can be computed automatically by 

simply tracing the floating mark along the outcrop of 

the surface or by occupying three or more points on 

the surface. Once a plane has been computed, the 

floating dot can be guided by the computer so that it 

remains on the plane throughout the model and thus 

allows comparisons of geologic surfaces, mapping of 

covered contacts, and precise measurements of inclined strata. The dot can also be controlled by a 

surface-model grid, which allows projection of curved 

geologic surfaces, rather than flat planes, throughout 

the model. 

Analytical plotters, which are fully computer controlled, are not restricted by the limitations of the 

mechanical analog plotters. Computer-controlled systems offer the additional advantages of working with 

a wide array of photographic materials ranging from 

hand-held, 35-rnm stereophotographs of any orientation to satellite imagery. All that is required to utilize 

the stereoimagery is control in the form of x-y-z coordinates for at least three points that appear on both 

images of the stereopair. Mappers should be familiar 

with these new instruments and techniques. 

How TO GET AERIAL PHOTOGRAPHS 

The great increase in use of aerial photography in 

the United States in the past few decades has resulted in millions of photographs. These range from 

low-altitude photographs of small areas with great 

detail to high-altitude photographs that cover more 

than 100 square miles in one exposure and to photographs from space that cover vast regions of the 

planet. 

The U.S. Geological Survey's Earth Science Information Center (ESIC) maintains records of aerial 

photographic coverage of the United States and its 

Territories, based on records from other Federal 

agencies, State governmental agencies, and commercial companies. From these records, called the Aerial 

Photography Summary Record System, ESIC can 

assist you in finding the photography to meet your 

needs. (Address is at the end of this section.) Records 

go back as early as the late 1930's for certain areas. 

Most early photographs were taken on black-andwhite film. Color and color-infrared films were introduced later. Contact paper prints, film positives, or 

enlargements of these photographs can be ordered. 

224 The computer-assisted geologic mapping system 
Contact paper prints 9 x 9 inches presently cost about 

two-thirds as much as film positives; film positives, 

however, produce the highest quality images for 

photointerpretation and mapping on the PG-2 plotters. For official use, the Geologic Division Plotter 

Laboratory in Denver can borrow available compilation photography or "quad-centered" high-altitude 

(1:80,000-scale) film positives and paper prints from 

National Mapping Division's photographic libraries. 

Quad-centered photographs are those taken at the 

centers and north-south borders of 7112'-quadrangleseach photograph portrays the area of more than one 

7112'-quadrangle. 

Before ordering aerial photographs, you must identify the area of needed coverage. Send ESIC the geographic coordinates (longitude and latitude) of your 

area of interest, and ESIC will promptly research 

your request. If the geographic coordinates are 

unknown, ESIC strongly recommends circling, pinpointing, or outlining the area on a USGS topographic 

map or a State or local highway map to indicate the 

desired area-the more specific, the better. Also state 

if stereoscopic coverage is required. 

ESIC cooperates with the Earth Resources Observation System's (EROS) Data Center in Sioux Falls, 

South Dakota. Both ESIC and the EROS Data Center 

take orders for aerial and space photographs and 

space images. If the exact roll and frame numbers are 

known, ESIC and the EROS Data Center can provide 

addresses, prices, and order forms for ordering from 

many other Federal and State agencies and commercial companies. For information write or contact: 

Earth Science Information Center 

U.S. Geological Survey 

507 National Center 

Reston, VA 22092 

User Services Section 

EROS Data Center 

U.S. Geological Survey 

Sioux Falls, SD 57198 

All ESIC and EROS photograph and image orders 

must include project accounting or prepayment by 

check or money order. Each print of a photograph is 

custom made, so all orders must contain the correct 

information. 

How to get aerial photographs 225