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17. 
In the publication, consisting of several reports, technical 
as well as economical problems are discussed. The process of 
design involves a large use of photogrammetric techniques, 
computers and plotters. Interesting results from analytical 
aerial triangulation for the planimetric control are reported 
as a complementary addition to the conventional terrestrial 
surveying of photogrammetric control. 
3. Digital Terrain Model.“ DTM 
The main principles of the Digital Terrain Model are based 
upon a registration and digitizing of the terrain topography, 
represented by a number of single points, terrain elements, 
contour lines etc, depending on the system of the individual 
DTM. The data, mainly in the form of X, Y and Z-coordinates, 
are stored in the computer on magnetic tapes or discs etc. A 
surface is then fitted numerically to this information by some 
form of interpolation. The interpolation is made linearly or 
according to a mathematical function of higher degree, depen- 
ding on the system of the individual DTM. The result of the 
DTM-interpolation can be obtained from the computer in the form 
of longitudinal and transverse profiles or in the form of a sur- 
face or single spots. 
Many different DTM:s have been designed during the recent years 
and in some countries they have also been introduced in the prac- 
tical work of Highway Design. They have however generally found 
the best market concerning the computation of masses in connec- 
tion with the design of industrial sites, large buildings, air- 
ports etc. In Europe the application of DTM in Highway Design 
has become most important in England and France, see pages 18-21. 
The main advantage of introducing a DTM in the Highway Design 
Technique is the improved possibility of investigating diffe- 
rent alternative horizontal and vertical alignments without any 
new data collection concerning the terrain topography. The DTM- 
data should give a better account concerning the calculation of 
earthwork quantities than data of measured transverse profiles, 
as the whole surrounding terrain surface is taken into account. 
On the contrary the DTM-data, interpolated according to a mathe-