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All of the products generated from the DTM use just two algorithms, 
one that determines the elevation of a unique coordinate and the 
one that determines the elevation of a point on a line which inter- 
sects a line connecting to two terrain data points. The real trick 
here is to get the appropriate terrain data points isolated from 
the tems of thousands of other data points in a manner that is 
economical or in orther words does not use up too much computer 
time. : 
One problem with the DTM is that one does not develop a topogra- 
phic map during the terrain gathering process. Contours can be de- 
veloped from the DTM but the planimetric detail is missing. How- 
ever, the DTM does provide the means for plotting this detail. 
A space resection can be performed on an aerial photograph to de- 
velop the rotational elements and the coordinate of the perspec - 
tive center. Using appropriate computer algorithms with the DTM 
and an X Y digitizer, planimetry can be plotted to scale on any 
map. 
This same technique allows plotting detail on an aerial perspec- 
tive. Think how this can be used to an advantage. Plotting finished 
plans on enlarged stereo pairs which allows a visual inspection of 
plans as related to the terrain. 
The Future of Digital Terrain Modeling 
  
It seems to me that the future of digital terrain modeling is tied 
to the electronic computer and the computer seems to be here to 
stay. Uses of the DTM are limited only by ones imagination and his 
ability to develop computer algorithms to manipulate it. 
It may turn out that the most important contribution that the photo- 
grammetrist can make to the topographic mapping process will be in 
providing digital terrain data rather than topographic maps as we 
know them now. The question of the future may well be "Does your 
DTM meet national map standards of accuracy".