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The terrain corridors generally have a long and narrow form 
and planimetric control points can thus be arranged in the 
form of a traverse through the corridor. The traverse points 
are signalized before the photography. The advantage of terres- 
trially measured control points compared to photogrammetri- 
cally triangulated control points consists of the possibilities 
Of using the control points as both reference and control points 
for the staking out of the centerline in the terrain. 
The development of electronic devices for length measurements 
has facilitated the use of terrestrial technique for the mea- 
surement of photogrammetric control and improved its efficiency. 
As an example of these devices may be mentioned Distomat DI 10 T, 
Geodimeter 6 B and 700, and Tellurometer. See fid 3, 4.and 5. 
The required vertical control is procured in a conventional way 
using levelling, trigonometric or phototrigonometric height 
measurement, depending on topography and vegetation. 
The control can be achieved or extended by photogrammetric trian- 
gulation methods. A very interesting system for analytical trian- 
gulation has been developed at the Institute of Technology in 
Helsinki by Prof S Halonen (14) and Dr E Kilpelà (18). The prac- 
tical experiences of his method show that the best result is 
achieved when the strip triangulation is made over four base- 
lines, according to fig 6. 
The theoretical accuracy, accounted for in the form of standard 
errors of planimetry and elevation is also shown in rig 6. This 
theoretical values have been verified in practical works. 
Another interesting similar system has been developed at the 
Institute of Technology by Prof J Talts (24). This method of 
calculation is used by Mr BR Adolfsson in (4). The result of his 
practical application is also encouraging. 
Similar results using aerial triangulation have been reported 
by Dr F Ackermann in (1) and (2). 
It can thus be stated that the recent development in the field 
of aerial triangulation means a good step forward in improving