Locally within the broad areas [ust mentioned are smaller areas where more 
detailed study is desired. In southeastern Utah, for example, detailed measurements 
of stratigraphic intervals are made from aerial photography so that isopachous maps 
of a particular formation may be made, lsopachous maps are those that show lines of 
equal thicknesses of rock formations. These equal=thickness lines, or isopachous lines, 
may reflect old swales and associated locally thickened parts of formations that are 
in places loci of radioactive deposits. In figure 6 the thickening of one formation is 
apparent; from appropriate measurements on aerial photographs it is commonly possible 
to delineate such locally thickened strata, which may represent old stream courses. 
Herein lies the Immediate value to the prospector and developer of radioactive de- 
posits, 
Research 
Operations in photogeologic work of course would not be possible without 
Instruments, and progress would probably not be as rapid without research in instruments, 
either in devising new instruments or in adapting existing ones to different jobs. Pro- 
gress would also suffer without research in photogeologic study per se, using existing 
instruments as Incidental to that study, and of course research in recording media, 
primarily film types, can be considered essential in the evolution of photogeology. 
Thus, photogeologic research may be grouped Into three broad categories, namely, 1) 
instrument design and adaptation to photogeologic problems, 2) study of selected 
geologic problems through the medium of aerial (and terrestrial) photography, and 3) 
development and adaptation of film types to geologic problems. At present no research 
on selected geologic problems is being carried on by the Geological Survey. 
Although many photogrammetric instruments have been adapted to geologic 
study of aerial photography, the relatively recent use of stereoplotting instruments, 
particularly the Kelsh plotter in conjunction with high-altitude photography, stands 
out. In areas tested, vertical photography of about 1:60,000 scale was enlarged to 
a working scale of approximately [:12,000 in the Kelsh plotter. For the areas selected, 
and for the results desired, this combination of instrument and photography proved 
highly successful. It was demonstrated not only that satisfactory Interpretive results 
can be obtained from 1:60,000-scale photography, but also that they can be obtained 
in less than half the time required when the normally employed 1:20,000-scale photog- 
raphy is used. Further, It was demonstrated that reliable positioning of geologic detail 
was obtained in this procedure,