PHOTOGRAMMETRIC TRAINING FOR GEOLOGISTS U.S.A.-7 
perspective field of the photograph and the map field by a precisely plotted 
grid. À one-centimeter grid is photographically transferred to one of a pair of 
diapositives and placed in a Kelsh model or other appropriate double-projection 
instrument, where a stereoscopic model is formed. The grid lines, which appear 
monoscopically in the model, can, however, be plotted precisely upon a map. 
Both the gridded photograph and its counterpart, the stereoplotted grid, are 
now available to the field geologist as a means of communication between 
photograph and map. 
Another photographic material, also developed as a result of the exchange of 
ideas of the geologist and photogrammetrist, is the orthographically-restituted 
photograph. This is a photograph in which distortions due to tilt and relief are 
methodically removed by mechanically scanning a stereoscopic model of the 
terrain, and re-photographing the images, corrected for relief and tilt, upon a 
fresh piece of film. A new stereoscopic photo-scanning instrument, which recti- 
fies the relief perspective of a photograph to that approaching orthographic 
presentation, is called an “Orthophotoscope.” The photographic product of the 
instrument, still in the development stage, is called an ‘“Orthophotograph.” 
Many geologists believe that the Orthophotograph will have an important ap- 
plication in geologic mapping. 
A number of new photogrammetric instruments that have been designed for 
geologic use are demonstrated; among these are a photogrammetric profile 
plotter for double-projection instruments, and other devices for making dip 
and strike measurements. 
Related cartographic techniques employing plastic-coated materials for 
photographs and maps are shown; these materials, which can be scribed by a 
pointed graver, are rapidly replacing pen-and-ink drafting, in many cases. 
Other important developments and their possible effect on geologic mapping 
are discussed. Among these are the new ER-55 photogrammetric projector, 
which is replacing the Multiplex, and the Twinplex, a new device for plotting 
with twin low-oblique photography. These technical advances are, in a practical 
sense, indirectly hastening the adoption of photogrammetric instrumentation 
by geologists by making available older but by no means outmoded equipment. 
Geologists of the U. S. Geological Survey have already acquired Multiplex and 
Kelsh plotting equipment and are employing these instruments in their work: 
more recently the ER-55 projector has been ordered. 
SUMMARY AND DISCUSSION FEATURE 
One of the unusual features of the course is a general summary and discussion 
at the end in which both the students and instructors participate. The graduates 
are invited to present their criticisms and suggestions; the instructors are there- 
by kept alert to provide first-class instruction. Some of the most attractive 
features of the course have resulted from this open discussion. A better under- 
standing of mutual problems of the two largest Divisions of the Geological 
Survey has been made possible through the informal contacts the course has 
provided. 
OUTLINE OF THE COURSE 
It will be interesting to many of our colleagues to have an outline of a typical 
ten-day course in familiarization with photogrammetric principles, instruments 
and methods, as follows: 
First Week 
Ist Day Aims of the course 
Background of photogrammetry 
Principles of stereoscopy