26 
  
  
Table 1. 
; Cross-section | Number of points 
Type or terrain interval per section 
Flat 20 5 
Norma | 20 10 
Rough 10 20 
  
  
  
Photography scale 1:5 000 
  
  
  
4.1.2 Digital Terrain Model - DTM 
The first DTM at. the Road Administration was introduced in 1967. 
It was originally based on photogrammetric measurements but was 
developed to include also terrestrial measurements. 
The DTM was supposed to give improved possibilities of investi- 
gating a number of horizontal alignments in the preliminary de- 
sign without any additional collection of data concerning the 
terrain topography. 
However, the experiences gained through the years have shown that 
DTM-technique is much more used in planning and design of dwel- 
lings, industries, airports etc than in highway design. 
Some of the reasons are the following. Road investments are to 
a great extent concentrated upon by-pass-roads and upon improve- 
ments of the existing road network. In these cases the horizontal J 
alignment is either given or influenced by a number of non-topo- U 
graphic factors. Only minor changes in the alignment remains in 
the final design. In these cases there is little sense in using 
a DTM. 
Using a DTM in the final design may, however, offer a more flex- 
ible way of organizing terrestrial measurements than the conven- 
tional way of levelling longitudinal and transverse profiles. 
A DTM is normally integrated in a series of computer programs to 
be used in designing. A decision to use a certain program suite 
does often mean at the same time the use of a certain DTM with 
all its specific qualifications. 
A more flexible way to use different computer programs for design 
could be obtained if the DTM's had a partly standardized out-put. 
Such a proposal has been given by Mr Bo G Hallmén, Markdata Con- 
sulting. Stockholm, in a research report on the use of coordinates 
in the building process. The research was financed by the Swedish 0 
Council for Building Research. 
It is proposed that DTMs should supply - as an out-put on magne- 
tic tape or disc - height matrices describing as well the ground 
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