38 
It has been observed that there is considerable variation in the 
definition of a DTM, depending on whom one talks to. Planimetric 
features are often included within the DTM; others may consider a 
system of profiles along defined lines as a DTM and there are 
other variations of terrain modeling which mäy be valid for a 
particular algorithm. All digital terrain models, however, must 
have one feature in common. It must be structured so as to allow 
one to superimpose a horizontal coordinate on it and get back an 
elevation. This interpolated elevation must be equal to the ele- 
vation of the ground at that position and should be accurate to 
a specified limit. Fundamentally, this is the total requirement 
of a DTM, nothing more. One can enhance the model with some nice- 
ties such as identifying points that fall in the bottom of streams 
or lie on sharp ridges. One might also identify points which fall 
on the highest point of a hill. The use of such identification of 
terrain points can be useful when incorporated into the computer 
algorithms that one may use in developing products from the DTM. 
By far the most important points in a DTM are those which fall in 
the bottom of drains. The next most prominent breaklines are the 
ridges. Points along lines of a change in the slope of the terrain 
are next in importance. 
One must consider the number of points that are used to make up 
the DTM because computer costs generally are proportional to the 
number of points used in the calculations. The best DTM is the 
one that gives maximum definition while using the least number of 
points. 
The Purpose of a Digital Terrain Model 
  
The purpose of a DTM is to provide the map maker, engineer, geolo- 
gist, land manager or whomever, with a tool which will allow him 
to use the electronic computer to help solve his problems. Some 
of the products that are being produced from this tool are: 
1. Profiles along defined lines. 
2. Slope maps. 
3. Contours. 
4. Perspective views of terrain. 
5. Plots of strings of coordinates plotted on aerial perspectives. 
6. View maps showing hidden areas from designated viewpoints. 
/. Cross-sections for the design of roads. 
8. Profiles that guide certain types of projectors used in prepa- 
ring orthophotos. 
9. An accurate means of transferring data from aerial photos to 
maps without the use of a stereoplotter. 
  
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