INTRODUCTION 
Most of the papers of the Symposium 1974 of Comm. VII have dealt with the 
identification of features and pattern in air- or spaceborne imagery. The 
interpretation of such imagery isn’t limited to identification problems. 
Identification is only one step within the interpretation process. This pro 
cess rather may be directed to certain specific purposes as for instance to 
inventory conditions of a forest land, an urban environment, a water body 
a.s.o., to monitoring changes in those conditions, to research implications 
of identified objects, quantify detected objects in number, volume, height or 
area extent. 
This paper discusses the question of how one can characterize and define the 
structure and detailed roughness of natural surfaces like the surfaces of 
forest canopies, vegetation types, certain ecotypes, micro-geomorphological 
phenomena a.s.o. 
Everyone who knows something about the implications of the surface structure 
and surface roughness for topoclimatic and energetic processes and the energy 
balance in the earth’s near atmosphere can imagine how important and desirable 
it is to have some useful, detailed and reliable but also simple surface charac 
teristics . 
This paper cannot describe those implications in detail. Therefore only a few 
brief remarks: The energy supply, the absorption of incident sun energy, is 
modified by the optical and morphological properties of the surfaces. The 
more energy a surface can absorb, the greater its energy efficiency and there 
fore the greater its role in the heat balance, in evaporation and, in general, 
in topo and macro-climate. Surface characteristics have still greater influ 
ence on the transfer of heat from the surface to surrounding air. The so 
called aerodynamic roughness increases with the roughness of the surface. The 
rougher the surface, the greater the turbulence and therefore the more effi 
cient the heat exchange. The important hygienic role of the surface structure 
for cleaning polluted air by vertical mixing is obvious. (For more details 
see for instance: Baumgartner 1971). 
A DIGITAL SURFACE MODEL 
A digital surface model for the characterization of the detailed effective 
surface’s roughness has been developed ("effective” relative to energetic and 
topoclimatic processes). The model is based on the concept of the DTM; it is 
really a special case of a DTM: Using large scale aerial photographs, a very 
dense network of measuring points within the relative and absolute orientated 
stereo model is led over the area which it is to characterize. For each point 
one has to register the x, y and z coordinates, x and y are given by the lay 
out of the network but one has to measure z of each point by the floating mark 
of the stereoplotter. 
There are various possibilities for the lay-out of the network. A regular