linear measurement, on a large scale. About 3.000 such measurements 
have been made since 1957. 
Aerial photography 
Most of the aerial photography has been done by the Geographical 
Survey Office of Sweden. For preliminary work, the Power Board has 
used photographs taken at high altitude. The regulation photographs 
taken by the Geographical Survey Office at 4.000 to 4.500 metres have 
been of great help. However, in many cases these photographs were 
unsuitable because the strips run north to south, whereas the streams 
in the North of Sweden usually flow northwest to southeast. The Pow- 
er Board often ordered strips taken along the streams from heights 
approaching 7.000 meters, which is the limit for the aircraft of the 
survey Office. During 1958 and 1959, photographs taken at a height of 
10.000 metres (in Operation RAK) were also used, with excellent re- 
sults in many cases. 
For detail projection, strips were taken at heights of 600 to 3.000 
metres. Here, too, there is a tendency to work from increasing altitudes, 
the reasons being that the quality of the photographic copy has been 
improving — for instance, the introduction of corrected transparencies 
and that the large volume of orders has made a reduction in the cost 
of the photogrammetric procedures necessary. 
  
Mapping 
A. Base mapping (1:8000 to 1:20000) 
For study in connection with the planning of hydroelectric power 
stations and impounding areas, the regions to be affected by a contemp- 
lated project are represented on plotted maps to a scale of 1:10000 with 
a contour interval of 5 metres. In certain cases it is not necessary to 
prepare maps. This will be explained later. 
Taking photographs 
— ; 
Base maps are made only by photogrammetry in the work of the 
Power Board; aerial photographs taken at 4.000 to 10.000 metres are 
used. The economic importance of working from high altitudes is great, 
as the location of markers and control points in the highlands of north- 
ern Sweden is expensive and time-consuming. 
Horizontal control 
The horizontal control is based on polygons measured with the geo- 
dimeter and tellurometer. The elongated form of the drainage area 
makes this type of network appropriate. Fig. 1 shows such a polygonal 
network covering the headwaters of the Luleä Rivers. The network 1s 
based on the first-order stations of the Geographical Survey Office. 
3