ily 
such as slopes, ditches, dikes and protruding rocks. In 
a vertical aerial photograph it is not always easy to 
recognize slope changes in difficult terrain. Its definition 
and interpretation by a continuous line is very often 
difficult, in case floating mark is not guided along a 
contour line. In such a region a greater number of points 
should be measured to give a higher density of points on 
a wider strip of terrain on both sides of the presumable 
break line. But even distinctly visible break lines in 
a terrain cannot be determined mathematically in such a 
simple way as a contour line. In contrast to the contour 
lines, which are two-dimensional curves, the break lines 
usually are spatial curves, the mathematical definition 
and digital adaption of which is difficult. Such lines 
have to be measured and recorded by carefully selected 
single points. 
Even if the larger part of an area can be recorded by contour 
lines or by a height grid, a few break lines like the top 
and toe of slopes on ditches and dikes will always be 
present. 
Generally for each photogrammetric restitution of a digital 
terrain model, an investigation under a mirror stereoscope 
will show, how to measure in any range, what details have 
to be considered for the engineering purpose at hand and 
what point density is required. 
The result of the measurements in form of punch tape and 
coordinate list goes to the computing centre. In order to 
reduce measuring time in the restitution instrument, the 
photographic material should be prepared in a pre-inter- 
pretation stage. It will be up to the engineer in charge 
of that task how good and complete, how suitable and 
conscientious, but also how economically the terrain will 
be recorded for further processing and how close the new 
digital terrain model fits to reality. To assure satis- 
factory results, a thorough training of the operator is 
required to make him familiar with the purpose of the