GVII-24 
PHOTOGRAMMETRIC ENGINEERING 
In most cases the mapping of structures has been facilitated by the absence 
of a dense vegetation cover, but in one case in Southeast Asia an investigation 
has been made into the possibility of tracing structures in densely forested areas, 
the work being carried out on mosaics at a scale of 1:31,680. 
Whereas the main function of photographic interpretation in British geo 
logical work is still concerned with the tracing of structures, a certain amount 
of direct lithological interpretation has been done. But it is emphasized by the 
interpreters that in general this is possible only where some detailed knowledge 
of the area in question is already possessed from ground investigations, as in the 
case of work in Jordan referred to below. 
Many of the British lithological studies have been carried out in areas of hot 
arid climates and it is emphasized that climatic circumstances are important 
in interpreting rock types. In particular it is pointed out that, in the Saharan 
and Middle East arid regions, outcrops of sandstones, grits and quartzites which 
contain iron always show up as dark or black zones on the photographs. Again, 
photogeological methods have been applied to specific detailed problems by the 
commercial companies, one example being the plotting of the outcrop of a com 
paratively narrow band of manganese-bearing and characteristically stained 
flags and shales in the Duna-Feinan district of Jordan. 
The interpretation of the geophysical results of airborne magnetometer and 
scintillation counter surveys is regarded as being outside the scope of the present 
report. It should be noted, however, that the interpretation of surface features 
which may be related to the anomalies revealed by this type of survey is a spe 
cialized branch of geological and morphological interpretation which is rapidly 
increasing in importance. 
As a state office for geological broad mapping the Geological Survey of 
Sweden has had a very great use of air photos and photographical maps for ore 
prospecting and geological works, especially in northern Sweden, where the 
roads are sparse, the country is difficult to traverse, and the topographical maps 
leave much to desire. For other state offices the Geological Survey investigates 
such problems as the river erosion at the water plants. At these works air photos 
have been very valuable. Repeated photographing after some years in the same 
locality will give an exact and objective picture of the progress of the erosion. 
Because of the land upheaval in Central Scandinavia and the erosion of the rivers, 
landslides often have happened in the steep clay slopes of the rivers. A region 
of current interest in this respect is the valley of the Gota River between Lake 
Yanern and the sea at Gothenburg. Here, old slide scars have been mapped by 
the aid of air-photos. 
In mapping and examining peat bogs in south and middle Sweden air pictures 
and photo maps have saved much work of manual survey. 
In the United States, since 1952, numerous geologic mapping projects involv 
ing primarily interpretation of geology from aerial photographs have been under 
taken and completed; certain projects are continuing. Of particular significance 
is the variety of terranes that have been studied. These include well exposed, 
gently folded rocks of the Colorado Plateau area, in western United States; 
poorly exposed, tundra-covered sedimentary rocks of northern and northwest 
ern Alaska; heavily forested igneous and metamorphic rocks in southern and 
southeastern Alaska; and sparsely forested areas in Alaska underlain in large 
part by superficial deposits. A variety of projects of different mapping scales, 
objectives, and techniques used have been undertaken within these terranes. In 
northern and northwestern Alaska general mapping of stratigraphy and struc 
ture has been completed at scales of 1:96,000 and 1:250,000. Numerous poten