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Transactions of the Symposium on Photo Interpretation

the timber volume. Until now all efforts to get a direct estimate of the timber
volume by aerial photographs of tropical regions have been unsuccessful but
it is hoped that this Symposium will be able to make some contribution in
this respect. If so, this will be a new point in favour of aerial photography, but
even without a complete direct photo inventory technique, photo inter
pretation has already been proved to be of great value. Aerial photography is
particularly useful in those regions where no detailed topographic maps are
available. They provide the only solution to obtaining, by simple photo-
grammetric techniques, a good forest map of the region of sufficient accuracy.
This map can be used to locate the existing producing forests or to indicate
those areas where timber production looks favourable for the future.
Besides maintaining or opening up production forests in our tropical areas
there is still another aspect of the large value of our tropical forests which must
not be underestimated. Everywhere in the world, and particularly in parts of
the tropics having a high annual rainfall, forests act as a protection against
erosion. In the upper reaches of river systems particularly, this forest cover
secures a maximum absorbtion of rainfall which leads to a better regulation
of streamflow and a prevention of flooding and silting. This automatically
implies that existing forests in such areas have to be preserved for the future.
When the original forest cover has been removed by overcutting, burning or
grazing, steps have to be taken to establish new afforestation. In the prepara
tion of adequate working plans for the improvement of watershed conditions,
aerial photographs are an important tool since they provide the requisite in
formation in the shortest possible time. The determination of the area to be
preserved for this purpose must be made in accordance with other living re
quirements of the local population. This entails the study of the agricultural
systems applied, grazing possibilities for cattle, etc. The result will be a sound
system of land classification for the whole watershed region, and will provide
the basis for the cooperation between agriculture, animal husbandry, for
estry, etc.
Aerial photo interpretation also has become an important tool in the hands
of experienced engineers for all phases of engineering construction, dealing
with soil and rocks as surface materials. In a word, it may be said that vertical
aerial photographs can be used to indentify soil and rock textures, to outline soil
and rock areas having similar characteristics, to evaluate drainage conditions,
etc. Their application to specific project developments makes it possible to
appraise the suitability of site locations for dams, canals, highways, airports and
railroads; to conduct construction materials surveys; to develop sampling and
drilling programmes for detailed investigation of soil and rock materials; and
to prepare land use, drainage, and engineering soil maps.
Admittedly, this same work can be accomplished by conventional field
methods. However, the ease with which detailed information can be obtained
from aerial photographs by well-trained and experienced engineering geol
ogists permits engineering planning to proceed with a wider perspective and