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COMMISSION VII Ol 
| Appendix to the INT. ARCHIVES O | 
| PHOTOGRAMMETRY, Vol Xil, 19 
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NEW TECHNIQUE IN PHOTOGEOLOGIC INTERPRETATION AND MAPPING 
USING VERTICAL AERIAL PHOTOGRAPHY IN CONJUNCTION WITH THE 
KELSH PLOTTER 
By Paul P. Orkild, Geologist 
U. S. Geological Survey / 
The use of the Kelsh plotter in photogeologic inter- 
pretation and mapping is one of the more significant ad- 
vances in photogeologic techniques in the past few years. 
Particularly promising is the use of the Kelsh plotter in 
conjunction with so-called high-altitude photography 
(1:60,000 to 1:70,000 scale). Because photography at this 
small scale enables large areas to be viewed at one time, 
the geologist is better able to understand and interpret 
broad geologic relationships. Although map accuracy is 
retained when this high-altitude photography is used in 
routine photogeologic compllation at scales as large as 
1:20,000, loss of some geologic detail is noticeable in 
the stereoscopic model. A significant factor in the cur- 
rent use of high-altitude photography 1s the reduction by 
as much as 90 percent in the number of photographs nor- 
mally required for interpretation and plotting, when 
1:20,000-scale photographs and simpler photogrammetric 
instruments are used. 
The Kelsh plotter is a projection-type stereoscopic 
plotting device with which an optical model is created 
beneath the projectors by means of the anaglyphic princi- 
ple, Such models have proved to be valuable in the study 
and mapping of geology. The models are measurable in x, 
y, and z, directlons. 
The Kelsh plotter plots orthographically by means of 
a tracing table with a platen having an illuminated dot 
in its center. Thus measurements in the x and y dir- 
ections may be scaled directly from plotted positions on 
the manuscript base. The geologist is able to measure 
vertical intervals (the z direction), such as strati- 
graphie intervals and amount of throw on faults, by 
raising or lowering the platen in the anaglyphic model 
until the dot of the platen rests on the feature that 1s 
to be measured. Readings of altitude are taken either in 
millimeters of parallax, which can be converted readily 
to feet or meters, or made directly in feet or meters, 
depending on the type of tracing table used. Slopes can 
be measured directly within the model by the three-point 
if Publication authorized by the Director, U. S. 
Geological Survey