A CONTRIBUTION TO THE REPORT OF THE WORKING GROUP IV/1 HIGHWAY 
AND RAILWAY ENGINEERING, OF THE ISP TO THE HELSINKI CONGRESS 
IN 1376 
by Richard O Mahan, 
U.S. Forest Service 
THE GENERATION AND APPLICATION OF THE DIGITAL TERRAIN MODEL 
Abstract 
A recent trend in the use of photogrammetry has been the develop- 
ment of digital photogrammetry, the result of the increasing use 
of the electronic computer for the data reduction. A further pro- 
duct of digital photogrammetry is the Digital Terrain Model (DTM). 
The terrain is defined by many spatial coordinates observed as 
discrete points on the surface of the earth, using stereomodel of 
aerial photography and a precision digitized stereoplotter. A 
technique has been developed to determine the authenticity of the 
DTM in the form of a RMSE resulting in some guidelines in the po- 
sitioning of the spatial coordinates, the number of points requi- 
red, and the compatibility of the computer algorithms that will 
perform the calculations. Authenticity of the DTM is essential to 
the applications involving engineering projects where the cost of 
moving earth is an important consideration. Significant advance- 
ments will be made in the application of digital terrain modeling 
as computer algorithms are developed that will generate specific 
answers from the manipulation of the spatial coordinates that accu- 
rately simulate the surface of the earth. 
What is a Digital Terrain Model? 
  
The electronic computer has made a significant impact on the way 
that we conduct ourselves in our daily work. Many problems can be 
solved in a timely manner with the computer now, that to do the 
same calculation a few years back would be just too tedious and 
time consuming to even think about. The preparation of maps as 
well, has not escaped this influence. 
However, there is difficulty in getting the electronic computer to 
read maps the way that we are accustomed. The computer just cannot 
look at a map, see a contour, and relate that to a level of the 
terrain. The computer recognizes number - not contours, streams, 
ridges and other features that are normally symbolized and shown 
on maps. Let one reduce the map symbols to numbers that the compu- 
ter can recognize, then wonders can be performed. 
Now consider topography, normally symbolized in the form of con- 
tour lines. Topography is generally the first requirement for an 
engineering project. Since many engineering firms now have access 
to the electronic computer, it is to their advantage to have topo- 
graphy expressed in such a way that the computer can understand it 
- hence the need for the Digital Terrain Model (DTM) is clearly 
identified. 
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