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The geological-geotechnical photo interpretation is facilitated 
by a simultaneous study of the photographs with the aid of a 
mirror stereoscope in connection with the Balplex evaluation. 
The choice between the different evaluated centerlines within 
the terrain corridor is preceeded by an analysis of the conse- 
quences and cost of each alternative from the point of view of 
construction, vehicle operation, maintenance etc. The detail 
mapping that is required for the second phase of preliminary 
design can in this way be limited to a narrow area around the 
centerline which is finally chosen. 
There exist of course cases, e.g. in urban areas, when the ac- 
tual terrain corridor from the beginning from different rea- 
sons is so limited that the entire corridor ought to be mapped 
in detail. Then the planning and analysis of alternative cen- 
terlines are made by aid of the map. Completing topographic 
and geological information can be received by a simultaneous 
study of the corresponding aerial photographs in a mirror ste- 
reoscope. 
The detail maps are generally made at the scale of 1:2000 and 
the contour interval is 1l m or 2 m depending on the topography. 
The maps are made in a conventional photogrammetric way but as 
the flying height for the photographs mainly is estimated by a 
compromise between the requirements on good topographic infor- 
mation over the entire terrain corridor for the evaluation of 
alternatives and the requirements on geometrical accuracy for 
the detail mapping, great attention must be paid to the geo- 
metrical quality of the photographs and of the evaluation equip- 
ments. Efficient methods for the calibration of aerial cameras 
and for the adjustment and calibration of evaluation equipments 
are available today. Reference is made to Hallert (13). and 
Kaasila (16). 
The chosen centerline is fitted, in detail, to the terrain, 
according to the intentions from the previous phase, by aid 
of the detail map. Thereby consideration is given more in de- 
tail to the requirements from the points of view of aesthetics 
and visibility or sight distance, which is important for traffic 
safety. In the design work the information from the map can be 
completed by trial staking in the field in complex situations.