METHODOLOGICAL DIFFERENCES AS AGAINST THE CONVENTIONAL DTM 
The most remarkable difference in the methodology as against the conventional 
DTM is that we are going to measure the really detailed roughness of the 
natural surfaces, particularly of the vegetation. This roughness appears as 
a trouble-factor in the conventional DTM. Photogrammetrists and engineers 
are mainly interested in the ground surface (solid line in fig. 2b). There 
fore special methods are necessary to eliminate the surface of the vegetation 
cover (i.e. Kraus 1974). From the topoclimatic or biophysical or ecological 
point of view one is particularly interested in this detailed surface structure 
of the vegetation cover (dashed line in fig. 2b). 
A second important difference between the conventional DTM and the here dis 
cussed detailed surface model is the required density of the measuring grid. 
With some reservation it is true that the denser the grid the better the 
approximation to the real surface and its details. Restrictions of this 
general rule depend on the properties of the objects which are to be measured 
or on the purpose of the investigation. 
Distances of 15,20 or more meters from point to point and row to row, which 
are usual and can be useful in the DTM — for instance for the automatic con 
struction of counterlines — don't meet the requirements of a model which 
should characterize the detailed roughness of natural surfaces like forest 
canopies. 
In the first investigation, during the hand made approach we used 2 mm-distances 
within the model (= 10 m in the nature). Then in the second, half-automatic 
approach with the Zeiss-set we have used 0,3 mm distances (= 1,5 m in the 
nature). The last distance seems to be sufficient for the digital character 
ization of canopies of nature forest stands and of the vegetation surface of 
parts of certain ecotypes. The Zeiss-set allows measurements with distances 
between 0,1 mm and 50 mm. The optimal distance in a special case depends in 
connection with the scale of the photographs from the surface properties of 
the objects and the purpose of the research. But also economic considerations 
may affect the definitive choice of the distance between measuring points 
within the model. The required or at least desirable high point density leads 
to huge numbers of z — measurement per area unit. Therefore the size of the 
measuring area must be limited or the z's must be measured within sample plots. 
In the last case standard plot sizes and lay outs of points should be used. 
This makes all following operations of data processing and evaluation easier 
and the results more comparable. 
It should be mentioned at this point that an experienced operator needs 5-10 
seconds for one z-measurement. 
EVALUATION OF THE MODEL 
The digital model of the surface's roughness consists of the total number of 
x, y, and z coordinate-sets registered for the area which was under investiga 
tion. The evaluation of the model can be given in graphs (= analog) or