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Aerial survey had a second and even morę important impact on geo-

morphology: it helped to form a link with neighbouring disciplines, such as geology, soil science, physical planning, etc. and thus contributed considerably to the transformation of geomorphology from a subject of purely academic interest into a modern science with many applications. Geological aerial photograph interpretation, for instance, is actually applied geomorphology as the geological features, such as rock types and structure, are normally not directly visible from the air, but their presence can only be deduced from related geomorphological phenomena. It is true that grey tones and vegetation elements sometimes are an in-dication for certain geological facts, but the most useful and freąuent indications for the photo geologist are relief and drainage pattern. The landforms occurring in a certain area depend on the rock types on the one hand and on the erosional processes which prevail under the given circumstances of relief and climate on the other. For a correct interpretation, the geologist should have a thorough knowledge of geomorphological types and processes. It sometimes occurs that different rock types show rather similar terrain forms in which case they cannot be properly distin-guished from the air. A minor change in lithology on the other hand, may in rare cases result in drastic changes in relief forms, which then are without or of little geological importance. It shoul be noted that the recognition of a geological phenomenon is only possible if its size is sufficient for the development of its characteristic topography. This fact, and not the photo scalę, sets the limit for the detailness of the photo geological map. It is therefore feasible to use smali scalę photography which gives the best generał view of the geological structure and which contributes to a higher production per man day.

The soil scientist, when engaged in photo interpretation, is also largely dependent on the geomorphologist as his object of study, the soil, is usually covered by vegetation and the soil profile cannot be seen at all. It is very fortunate that the boundaries of soil associations often coincide with geomorphological units, or at least have a simple relation to morphological boundaries. There are also other indications, like grey tonę, vegetation, land use, etc. but the landform study is basie to pedo-logical aerial photograph interpretation. The same holds true for land use mapping, in interpretation for engineering purposes, etc.

It is evident that, when compiling a geomorphological map with the aid of airphotos, or when applying geomorphological principles in the compilation of a geological or soil map, special stress is laid upon the ‘static’ aspects of geomorphology. The aim is to map the present terrain forms in their correct position and mutual relationship. Such landform mapping is nevertheless not merely descriptive. It is the insight in their genesis which allows for a classification and which clarifies their signi-ficance and their economic potentialities. Another type of geomorphological aerial photograph interpretation deals especially with the ‘dynamie’ aspects of this science. Emphasis in this case is not laid on the mapping of the existing situation, but on the active geomorphological processes and their effeets. The application of the studies is particularly, though not exclusively, in engineering.

The author carried out extensive aerial photograph interpretation — with associated field work — in Sumatra and in the Moluccas, Indonesia, where he became acąuainted with the latter two aspects of photogeo-

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