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PHOTOGRAMMETRIC TRAINING FOR GEOLOGISTS U.S.A.—5 
to use older photography that may be designed for some other purpose, even 
when such photography is provided gratis. The best opportunity to save time 
and money is at this point, by judicious planning and careful design of the 
photography to fit the scale, accuracy and use of the final product. The impor- 
tance of base-to-height ratio for spacing along the flight lines, and width-to- 
height ratio for spacing between the flight lines, is stressed. Sample problems in 
design of aerial photography are worked out. The importance of seasonal vari- 
ations, vegetative and forest cover, snow cover, and local conditions affecting 
photographic quality are brought out. 
During the first week much of the instruction is given to the orientation 
procedures and individual practice with the Multiplex and Kelsh, instruments 
which produce geometrically-true spatial models in a simple and easily-under- 
stood fashion. It should be remembered that many of the students have never 
handled instruments of this type before. Here the relationship of the photo- 
graphs to the taking lens-camera combination, the relative relation between 
successive photographs and the relation of the stereoscopic model to external 
conditions of control is explained. The student-operator having found that he, 
too, can level a model begins to draw his map; he finds that he too can make 
certain measurements of geologic features, an impressive discovery since he 
proves to himself that the floating mark—like his Brunton compass, hand level, 
aneroid barometer, planetable, and telescopic alidade—can become another 
useful tool for making measurements. 
STEREOSCOPES AND STEREOMETERS 
During the second week, photogeology, the interpretative aspect of geologic 
mapping with aerial photographs, is covered. Here the student learns to use 
correctly his most important tool, the stereoscope. Fortunately, the stereoscope 
is also the simplest of all the photogrammetric instruments. It is useful to learn 
that there are many different kinds of lens, mirror and prism stereoscopes avail- 
able, but that some stereoscopes have serious drawbacks for study of photo- 
geology; also, that there is a need for further improvement in stereoscopes for 
geologic use. In addition to good quality photography and a good background 
in geology, the techniques of photogeology should include: (1) integration of 
photo-interpretation with field work, (2)good stereoscopic vision, and (3) train- 
ing and experience in photo-interpretation and use of appropriate photogram- 
metric instruments. 
[n the laboratory exercises some of the simpler measuring devices, the stere- 
ometer (parallax-bar) and slope-reading instruments, are demonstrated. The 
student learns to make measurements for dip, strike, thickness of beds, and the 
like; measurements which he has often made in the past on the ground. 
RADIAL-LINE PLOTTING 
The principles of radial plotting, one of the fundamentals of photogram- 
metry, are demonstrated. The student is shown how to construct a graphic 
triangulation net using vertical photographs, and later he translates these 
principles into mechanical triangulation systems. A wide variety of graphic 
and mechanical radial methods is shown: the continuous radial plot; ''hand" 
transparent templets; slotted mechanical templets made of metal or cardboard; 
and stereotemplets, a new scheme for mechanical triangulation which incor- 
porates the geometrically-correct precise measurements of the stereoscopic 
plotting instruments into a convenient, mechanical templet system for extend- 
ing horizontal control. Stereotemplets are considered by the geologist to be cne 
of the outstanding recent developments in photogrammetry.