Fig. 3. 
areas which are not of interest for the route, and to concentrate work 
in the sequel on the real problems. 
The preliminary planning 
The next stage in the planning, i. e. the preliminary planning requires 
photography from an altitude of approx. 2.000—2.300 m for a complet- 
ely satisfactory result. This cruising altitude has been chosen so that at 
the same time as the pictures give a sufficient survey they also ensure 
such accuracy in horizontal and vertical determinations that the work 
of staking out the route in the terrain can be done on the basis of the 
material obtained. 
The aerial photography is carried out in the spring before the appe- 
arance of the leaves, and it is planned in such a way as to ensure ade- 
5