6 
  
39 
The Development of a DTM 
  
Most digital terrain models that are produced are byproducts of 
some other process. The digital data gathered in the process of 
scanning profiles in the preparation of orthophotos is a common 
way to collect data for the generation of a DIM. 
There are a number of problems in using data so gathered to de- 
velop an efficient DTM. Following are some of the compromises 
that are made: 
1. Prominent breaklines such as streams and ridges may be mis- 
sed if they run parallel to the scan line. 
2, [t is not practical to identify drain or ridge lines. 
3. When scanning a stereomodel along the profile line, one is 
watching a moving target and it is less likely that the 
"dot" will be right on the ground under these conditions. 
Better accuracy is achieved by bringing the "dot" to a stop 
and carefully setting it on the ground at a specific spot. 
4. There is a tendency to gather more data than is necessary 
since there is bound to be a considerable redundancy of data. 
How many times have we heard of the problem of thinning data 
gathered in the high-speed automated systems? This cry is 
because the computer, with all its power, cannot economical- 
ly handle so much data. 
5.. Accuracy of terrain data is always a problem but there is a 
tendency to obtain irregular data in proportion to the sophis- 
tication and speed of the scanning system. 
The following is a procedure that is being successfully used to 
define terrain by numbers. 
The slope of a line between any two adjacent terrain points of 
the DTM will conform to the slope of the ground between the same 
two points. Essentially, what this means, is that the position 
of the selected points will fall on the breaklines of the terrain. 
All of the computer algorithms hereafter referred to are designed 
to operate on terrain data which meets this criteria. 
How does one gather data that will meet this criteria? 
1. One way is to digitize existing topographic maps with an X Y 
digitizer that provides a means of entering the contour ele- 
vation from a keyboard. A topographic map can be digitized in 
such a manner that the data will meet the criteria. 
A line following technique is not generally recommended be- 
cause of the complications that seem to arise when one attempts 
to automate this. Chances are that an automated line following 
system will result in large amounts of redundant data and will 
increase the computer costs of manipulation with little gain 
in accuracy.