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are almost always using the kind of interpretation of aerial photographs as taught at the 
I. T.C. Special mention should be made ofthe work in Australia, Sudan, Ethiopia, Pakistan, 
Congo, Ruanda Urundi and Rhodesia. 
Recently, research projects were started by I.T.C. staff members in Switzerland and 
Luxemburg. The object of these is to improve the interpretation techniques for soil surveys 
in different landtypes and to enlarge on the general experience in the use of air-photos 
for soil survey. 
Special attention during the last four years was given to the difference in cost, both as 
to time and money for the carrying out of soil surveys with and without the use of aerial 
photographic interpretation. It was found that for soil surveys on a reporting scale of 
approximately 1 : 300.000, the cost, using a.p.i., was only one tenth of what without a.p.i. 
For soil surveys to be published on a scale of 1 : 50.000, the cost is about 25 to 35%. The 
gain in time, which for development projects is often still more important, is of the same 
order. 
The vertical exaggeration to be seen in the stereoscopic image, has once more proved 
to be extremely important for soil survey purposes. Therefore, the use of wide angle 
photography is always preferred, especially in the normal areas for which soil surveys 
are made. 
Apart from other subjects, the methods for producing reporting maps, and for the 
publishing of those maps, receive special attention both at I.T.C. and at other institutes 
in the Netherlands. This is necessary, because often the time necessary to produce the 
final map is longer than the time for interpretation and field work combined. 
VIII. 3. Photo-interpretation for geology 
The geological survey is using air-photos to an increasing extent for geological mapping 
in the Netherlands, while at several universities as well as at the Mining Department of 
the Technical University at Delft geological air-photo interpretation is becoming more 
and more part of normal working routine in connection with geological field work abroad 
(Southern France, Spain, Austria, etc.). 
At the I. T.C. in Delft some photogeological research work was done on different scales 
on photos of overseas territories (New Guinea, Surinam, Egypt, Spain, Ceylon, U.S.A.) 
of which geological field maps were partly available. These were indispensable for the 
determination of certain lithological features which could be delineated, but not actually 
determined in the photos. They served as a valuable substitute for field-keys. 
Some interesting experience was gained by working on 1 : 120.000 scale photos (taken 
with a Wild RC 9, Super Aviogon, F 88,45 mm at 10.000 m altitude). As expected, they 
proved most valuable for geological interpretation and equally suitable for topographic 
mapping on a scale 1 : 50.000. Since a single photo covers about 760 square km they 
can be most strongly recommended for geological surveys over widespread areas. As the 
resolution is excellent, no detail is lost and certain geological features stand out far more 
clearly by the greater compactness of the morphological and vegetational pattern than 
in larger scale photos. 
Such small scale photography is recommended for example for photogeological recon- 
naissance mapping in New Guinea and Surinam (especially in areas of high relief, where 
a 20.000 scale is unsuitable). Moreover such photos stand an enlargement to a 40.000 scale 
without losing any of their detail. They could then also be used for other purposes than 
geological work, e.g. forestry. 
In view of the strong exaggeration of dip angles by the wide angle lens, stripping for 
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