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-— Bins 
In the case of rolling terrain, such as mountains of 
medium altitude, the determination by way of contour 
lines will be in most cases the most advantageous method. 
Tests showed that volumes of artificial and natural earth 
bodies of round shapes can be better determined by photo- 
grammetrically measured contour lines than by measuring 
point by point cross sections or by grid measuring. However, 
for use in the digital terrain model, the contour lines 
have to be recorded point by point in a digital way. That 
is much easier done with contour lines of constant z-value. 
For instance, it would be possible to record the xy- 
coordinates automatically at constant time intervals when 
tracing the contour lines. Since tracing of contour lines 
would be slower in more difficult terrain, a denser net 
of points would be obtained automatically in those places 
wherever needed to assure a good fit to the shape of the 
terrain. It would also be possible to record those points 
at a greater density and to reduce afterwards the number 
of points to a suitable and necessary quantity with the 
help of the computer. For the selection of an economical 
density of points along the contour lines the criterion 
could be the curvature = which is the variation of the 
angle of the tangent T over the length of arc L, thus 
A dt 
RL 
In this we can use as sufficient accurate approximation 
the angle of the chord instead of the angle of the tangent 
and the length of the chord instead of the length of the 
arc between neighbouring points on a contour line - both 
are values which are computed directly from xy-coordinates. 
With the elimination of redundant points a reduction in 
storage space and computer time is obtained as well as 
eliminating the task of point selection for the stereo- 
operator. Measuring and recording point by point along 
contour lines is probably the simplest method for obtaining 
digital terrain data, and furthermore it should not cause 
too many difficulties for the instrument manufacturer. 
The third typical terrain incorporates sudden changes,