U.S À —4 PHOTOGRAMMETRIC ENGINEERING 
American and European make, instruments that are normally employed in the 
production of standard accuracy maps. Discussions are informal and the student 
is encouraged to present his own problems so that his training might be guided 
to fit his particular needs. 
GEOLOGIC ORIGINS OF PHOTOGRAMMETRY 
À review of the important photogrammetric developments in the United 
States is given. Here one learns that photogrammetry in the United States 
began with the work of two brother geologists, Clarence W. Wright and Fred 
E. Wright, who first employed a panoramic camera for recording geologic 
details in Alaska in 1904. This review serves to tie together the present with the 
not too distant past when photogrammetry and geology first came together. 
Brief information is given on methods and instruments used by different 
organizations in order to suggest possible application to present geologic map- 
ping problems. The new trends and developments in photogrammetry are dis- 
cussed; these are expected to have a considerable effect upon geologic mapping. 
STEREOSCOPY 
Since stereoscopy is the foundation for many of the photogeologic studies, 
considerable attention is given to the underlying principles, namely how stereo- 
scopic vision is formed and the factors affecting visual acuitv, and how the 
visual model is formed under the stereoscope and under the double-projection 
stereoplotting instrument. Some of the factors producing vertical exaggeration 
in viewing pairs of photographs under the stereoscope are discussed ; this is an 
important factor in photogeologic interpretation. The principle of anaglyphic 
projection is demonstrated as a prelude to description of the modern stereoscopic 
plotting machines, such as the Multiplex, Kelsh, and ER-55 which employ this 
principle in constructing a geometrically-correct stereoscopic model of the ter- 
rain. Arrangements are made whenever possible to give each student an ortho- 
rater test to measure his visual acuity and ability to see stereoscopically. 
STEREOPLOTTING 
The student is given a prescribed course of exercise in handling the Multiplex 
and Kelsh plotters, the two instruments most widely used in the United States. 
Experience tells us that there is a good reason for an early acquaintanceship 
with these instruments; it is best to permit the student to learn the physical 
characteristics of each machine first; then the theory on which the instrument is 
based; finally back to the instrument for the practice stage, with a better knowl- 
edge of each mechanical adjustment. 
In addition to practice with the double-projection instruments employing 
the principles of anaglyphic projection such as the Multiplex, Kelsh, and ER-55 
plotters, the student obtains a brief introduction to the so-called heavy plotters 
of European make employing the principles of mechanical projection: the Zeiss 
Stereoplanigraph (usually included in this group although based on the principle 
of optical projection), the Wild A-5 and A-6 Autograph and the Wild stereo- 
plotting machine. A step-by-step explanation of the instruments is given de- 
scribing as closely as possible the procedures followed in a practical mapping 
operation. 
MAP PLANNING 
Here the student learns that to obtain maximum benefits from photogram- 
metric techniques, careful planning is required at the beginning of every project. 
It is often cheaper to fly new photography correctly designed than to attempt 
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