2 PHOTOGRAMMETRIC ENGINEERING 
pense, with a minimum of field work. Maximum accuracy and completeness may 
be additional objectives. The map may be reconnaissance or detailed, partial or 
complete, preliminary or final, according to circumstances. It may be intended 
primarily to show regional stratigraphy and structure, or soil distribution, in 
a systematic way. Or, in petroleum geology, it may be intended only to delimit 
localities for further ground study, for geophysical testing, or for drilling. In 
mining geology, it may be restricted to features or localities with which minerali- 
zation is believed to be associated. In engineering geology, it may be concerned 
with data on construction materials, foundation conditions, drainage, classifica- 
tion of materials for excavation, location of such features as landslides which 
present engineering difficulties, and erosional and depositional phenomena of 
streams and shores as related to engineering operations. In brief, the distribu- 
tion and disposition of rock and surficial deposits are delineated with reference 
to particular purposes. 
The materials, equipment, and procedures used by the photo interpreter 
in working toward the above objectives may be considered next. The photos 
most commonly used are contact prints of single-lens, black and-white verticals 
made with cameras having focal lengths of from 6 to 12 inches, and using pan- 
chromatic film. Enlargements are sometimes used for detailed work, and 
mosaics are frequently employed to supplement the individual prints. Tri- 
camera photos are sometimes used for reconnaissance. Oblique photos of 
selected localities are considered valuable by some workers as an auxiliary to 
the standard verticals. Sharpness of detail together with a proper degree of 
contrast are essential specifications of the photos, and the time and season 
of photography are best adapted to insure these characteristics under the 
existing conditions of vegetation, drainage, and relief. 
Continuous strip photography seems to have found little use outside of 
military operations, having been tried only to a limited extent in such work as 
highway engineering. ; 
Some interest has been expressed in the use of infra-red photography on an 
experimental basis, but little seems to have been done in this field except by 
foresters. Much greater interest has been shown in the possibilities of using 
color photography, but systematic utilization has been hindered by compara- 
tively high costs, the need for special equipment in handling transparencies, and 
lack of standardization in the photographic product. One worker (Laylander, 
1952), however, has been conducting intensive experiments with color film in 
connection with mining geology, and considers the results very promising. 
Scale of the photos used ranges from 1:10,000 or larger to 1:40,000 or 
smaller. A scale of approximately 1:20,000 is perhaps most widely used in 
America for general purposes, owing partly to its availability. The use of larger 
scales is generally limited to detailed study of restricted areas, for engineering 
or mining operations. The smaller scales are used mainly in reconnaissance 
work. In general, the smallest scale suited to a particular project affords maxi- 
mum economy of effort. 
The equipment most commonly used in photo interpretation is compara- 
tively simple. Most important is some type of stereoscope. Both reflecting and 
refracting types are widely used, the latter seemingly being preferred by a small 
majority. Some type of stereometer, such as the parallax bar, height finder, or 
stereocomparagraph, is widely used also. Other equipment used by some inter- 
preters, mainly for transferring the results of interpretation to base maps, 
include the Sketchmaster, Kail radial-line plotter, K. E. K. plotter, M ultiscope, 
and Kelsh plotter. Other equipment occasionally used comprises a light air-