3 
work in details is required, in principle, however, the use for oil exploration is a solved 
problem. The photography used so far was 1 : 40,000 and smaller, but normally with 
1:60,000 as a limit. The results of the photo-interpretation are plotted in the photoscale 
and transferred to a base map by means of especially determined passpoints, mostly taken 
from a slotted templet layout. In rather flat terrain this is an easy operation and the lack 
of a base map needs not to be considered as a disaster. The big oil companies have good 
topographic services to fill the gap of the missing base map. 
Much more difficult and less developed is the photo-interpretation of cristalline rocks, 
the older formations, metamorphies and intrusives. We believe that much work should 
be done on the study of igneous and metamorphic rocks and on sedimentaries of the older 
formations, mainly of Cambrian and Precambrian age. It is very likely that for this kind 
of interpretation a wider variety of scales of photography will be required, even mixed 
with colour photography on a larger scale. Also from the cartographic point of view this 
work will be more difficult since it takes place in mountainous terrain. Transfer of details 
is then difficult and it will be a condition for efficiency that a good topographic base 
map, printed in grey, will be available. 
c. Soil maps for agriculture. 
In number B2 of the LT.C.-publications Dr. P. Buringh gives a description of the 
development of the photo-interpretation for soil survey as this took place in the I.T.C., 
which to a certain extent may be considered as the cradle of this technique. In number 
AB1 of the same series I indicated a few subjects which have to be investigated, such 
as the photo-interpretation for soil survey of areas in which lime stone is a dominating 
parent material, for soil erosion surveys and for soil and water conservation surveys. 
Since a soil survey is hardly ever carried out in mountainous areas, the cartographie 
problem is easy. We see, however, very few soil services equipped with a topographic 
service like the oil companies and some geological and forest services. Therefore a plan- 
ning organisation should take such measures with the topographic service of the country 
that the necessary base maps are provided in time. 
Important for the planning is also to know that a good semi-detailed soil survey 
requires in general photographs in the scale 1 : 20,000. The smaller scales result either in 
more fieldwork or in less detailed soil maps. 
d. Air-photography for forestry. 
The photo-interpretation for forestry is even older than that for geology. I need for 
this only to mention the name of Hugershoff. In older days, however, it was mainly the 
geometrical element, in particular the height of trees, which interested the forester. At 
present the forest inventory of the tropical jungle by recognition of tree species is very 
important. The determination of the correlation between timber volume and factors as 
average tree height, density of the crown canopy and crown diameter is important for the 
inventory of the forests also in moderate climates. 
The scale of photography in regular use for this work is 1: 20,000 and even larger. 
It may be however that the use of the highest quality of camera and emulsions could 
eliminate these larger scales. 
In countries in which forests are in full exploitation also maps are needed. They are 
in a large scale to show the ways along which the logs are transported. The scale is 
sometimes even 1:10,000, only with a few contours and in general with a limited 
precision. 
Since many forests are in mountainous terrain and even in very high mountains, the 
transfer of the results of forest-photointerpretation is rather difficult because of the 
large reliefdisplacement. The planning should have as result that the forester obtains in