USA —2 PHOTOGRAMMETRIC ENGINEERING 
UNDERLYING FACTORS 
Many geologists already were using aerial photographs as contact prints or 
enlargements for plotting field geology, and were combining field work with 
photographic information to obtain quantitative data such as dips, strikes, 
thickness of beds, profile measurements, and the like to a high degree of ac- 
curacy. À few geologists were combining aerial geologic mapping with contour- 
ing by photogrammetric methods to develop structure contours. 
For maximum results in the use of aerial photographs for geologic mapping a 
greater familiarity with photogrammetric instruments is required and especially 
a knowledge of their limitations. In addition it was believed that, as much of 
the national economy was dependent on the search for minerals and fuels, and 
we had on hand many new instruments and techniques brought about by re- 
search in recent years, these instruments could be applied to the task. 
NEED FOR TRAINING 
Since 1945 many problems have come up which primarily concerned the 
application of instruments that were developed for topographic mapping to the 
problems of geologic mapping. As aerial photographs were being used as contact 
prints or enlargements on which to plot detailed geology, one of the recurring 
problems was that of relating, or transferring, geologic data from aerial photo- 
graphs to base maps. Double-projection instruments, because of their simplicity, 
are able to supply the link between the perspective of the aerial photograph and 
the orthographic projection of the map, and so are immediately applicable to 
this type of problem. To effectively use instruments, however, requires consider- 
able training. 
From the viewpoint of the photogeologists, who had already made consider- 
able use of the stereoscope in the field, photogeologic mapping under double- 
projection instruments might prove even more attractive, since in many cases 
these instruments could combine stereoscopy with the ability to make many 
measurements similar to those he was now making on the ground. Only a few 
geologists, however, had training in these instruments, which heretofore have 
not been available for their use. 
ORGANIZATIONAL ADVANTAGES IN THE GEOLOGICAL SURVEY 
Our organization is unique in that it has always been one of the largest 
operators in the United States engaged in both topographic mapping employing 
photogrammetric means, and geologic mapping. Actually, this close association 
of topographic and geologic mapping goes back more than 75 years, to the for- 
mation of the U. S. Geological Survey, for the authorized purpose of classifying 
public lands, and examining the geology and mineral resources of the United 
States. 
During the past fifteen years, there has been a very great increase in the 
application of photogrammetric methods to topographic mapping. The Geologi- 
cal Survey also developed during this time an integrated program for research 
and development in the three major elements of photogrammetric mapping: 
aerial photography, plotting equipment, and technical procedures. The results 
of this research program led to the development of new and more efficient pho- 
togrammetric instruments. Among these was the ER-55 Projector designed to 
be used with either vertical or twin-low oblique photography and the Twinplex 
plotter; these instruments replaced, to some extent, the use of standard Multi- 
plex equipment for bridging and for some stereoplotting procedures. During this 
time, also, the Kelsh plotter went through a rapid development which led in 
1951 and 1953 to two standardized Geological Survey models. The latter models 
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