1.2 THE ORIGIN OF PROGRESSIVE SAMPLING 
For more than 10 years an efficient alternative photogrammetric 
procedure adopted to the terrain curvature has been well known. 
Makarovic (/2/,/3/,/4/,/5/) has proposed Progressive Sampling, 
which has now become available as software on analytic restitution 
systems. The basic idea is the densification of a regular grid only 
in those regions where the topography requires it. The terrain cur- 
vature is analysed with several criteria such as the slope or the 
second height difference. If these criteria exceed a terrain-re- 
lated limit the rough grid interval is bisected. This procedure 
continues until the criteria fulfills the limit. Composite Sampling 
which is also suggested by Makarovic (/4/) extends Progressive 
Sampling taking into consideration additional morphologic infor- 
mation for the terrain analysis. Hence breaklines and structure- 
lines have to be interpolated in the regular grid. 
1.3 PROGRESSIVE SAMPLING WITH TRIANGULAR INTERMESHING 
Grid intermeshing of the terrain is a common but not a flexible 
method for data capture in regions with variable relief. The dis- 
advantages become evident when additional morphologic information 
is to be interpolated in the raster instead of their direct use 
for the intermeshing procedure. This lead WILD to the concept of 
triangular intermeshing where breaklines and structure-lines 
always form the edges of triangles. The original information as 
it vas measured is then used for the terrain analysis. Many other 
advantages: result from the use of triangles (/6/): 
triangular intermeshing is closer to the classical terrain inter- 
polation task, 
irregularly-distributed points approach the terrain in an optimal 
way, 
dead areas may be taken into account through use of its enclosing 
polygons, 
the number of points to be stored is reduced to a minimum. 
Another major advantage is the prediction of extrema in the terrain 
and the direct positioning in the model. 
For triangular intermeshing the second height differences as a cri- 
teria for the terrain analysis are substituted by taking directly 
the terrain interpolation model which is afterwards again used for 
the DTM generation. For WILD DTM processing packages this is the 
Zienkiewicz-function (/7/). This link between acquisition and gener- 
ation of DTM's becomes a further advantage of the triangular inter- 
meshing. 
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