20 
With these aspects as a background the National Road Administra- 
tion and the National Land Survey are in cooperation with four 
communities in the western part of Sweden concerning production 
of a new basic map in the scale of 1:5 000. The map consists of 
the following editions on transparent material 
- orthophoto, 
- planimetry, 
- contour lines, 
- property boundaries. 
In the route location stage geological and geotechnical informa- 
tion is also important. This information is available through 
geological maps and photo interpretation. The use of geotechni- 
cal photo interpretation has increased quite a lot during the 
last years. In the route location stage it is carried out on the 
same aerial photos (1:30 000) that are used for the topographic 
studies. The photos give a good general view of the terrain and 0 
it is possible to study essential geological features and the \ 
connection between different types of terrain and soil-forming 
processes. Areas with bad geotechnical conditions can thus be 
identified. 
The result from the topographic and geotechnical studies is the 
delimitation of terrain corridors. The final choice between these 
corridors is based on a traffic-technical and traffic-economical 
analysis. This analysis takes into account the cost of road 
construction and vehicle operation for the proposed routes but 
also the fitting of the highway network into the development of 
the community. 
3. PRELIMINARY DESIGN 
3.1 Aerial Photography 
  
A deeper examination of the terrain corridor delimited in the E 
route location stage is now performed. Aerial photography of the 
corridor is generally required for this examination. The width 
of the corridor is usually ] to 2 kilometers. 
The aerial photography is normally performed from 1 500 or 2 000 
metres flying height corresponding to the photography scales 
1:10 000 and 1:13 000. The choice of photography scale is in- 
fluenced by requirements on an overall view of the terrain cor- 
ridor which is best achieved if one flight strip covers the who- 
le corridor. The choice of photography scale is also influenced 
by requirements on geometrical accuracy. Thus the final choice 
of photography scale will be a compromise between these two re- 
quirements. 
For economical reasons, the highest possible flying height is ad- 
vantageous. It is therefore important that the aerial cameras and 
the evaluation instruments are regularly calibrated and adjusted 
in order to keep the systematic photogrammetric errors within 
control. v 
The aerial photography of corridors is generally carried out in 
single strips in sequence one after the other. Blocks of strips 
are not very common. 
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