method or by introducing a suitable surface into the model 
parallel to that slope. The angle of slope of the intro- 
duced surface can be measured with a clinometer. 
Because Kelsh diapositives are the same size and scale 
as the original photography, are on glass plates rather 
than paper, and because the Kelsh plotter has a moving 11- 
lumination system that gives a greater concentration of 
light and a strongly illuminated model, a high degree of 
resolution is preserved in the projected model. The model 
is about 5 times the original scale of the photography. 
Such enlargement contributes to the ease and accuracy of 
identifying and delineating geologic detail. This detail 
is compiled by guiding the tracing table so that the 
floating dot follows the feature on the stereoscopic model 
belng traced orthographically on the base map. The trac- 
lng table is mechanically linked to a variable-ratio panto- 
graph, which reduces the model to desirable compilation 
Scale. 
In the western United States several areas have been 
mapped using this high-altitude photography. The sparsely 
vegetated sedimentary terrane is ideally suited to photo- 
geologic studles. Continuous rock exposures over wide areas 
favor rapld correlation of stratigraphic units, particularly 
where there are different weathering and color characteris- 
tics of formations. A preliminary field check by the plane- 
table method showed that the positioning of geologic fea- 
tures using the Kelsh plotter met the acceptable standard 
of geologic mapping accuracy, that is, within 1/25 inch 
with respect to other planimetric or geologic features. 
For several special projects requiring more detailed 
study than is used in routine photogeologic mapping com- 
pilations have been made from medium-to low-altitude photo- 
graphy (1:40,000 scale or larger) in conjunction with the 
Kelsh plotter. The following are a few of the applications 
of the Kelsh plotter in such studies. 
Detalled studies of areas in the western United 
States were made using the Kelsh plotter and low-altitude 
photography (1:20,000 scale) to produce isopachous maps 
that might aid in the detection of uranium-bearing depo- 
sits. Geologlc detail was compiled at a model scale of 
about 1:4,000 and numerous measurements were made of 
formational units. The comparative ease of taking read- 
ings permitted a larger number of thickness determinations