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