Method of measuring 
Considering the extent of the area, photogrammetric measuring was 
the only realistic solution when choosing the method of measuring. 
Flying altitude and the position of the photostrips 
The choice of flying altitude was made taking into consideration partly 
the quality tolerances mentioned above, partly a number of other fac- 
tors. 
As already mentioned, there is a common map sheet system for the 
Stockholm region. Every map sheet covers an area 200 X 320 metres. 
The aerial photography should be carried out in such a way that practi- 
cal agreement is achieved between map sheets and stereo models in order 
to diminish the control point measuring as well as the stereo plotting. 
When mapping in the scale 1: 400, a number of small details have to 
be plotted (kerbstones, poles, wells etc.). In order to see these clearly 
when stereo plotting, a relatively large negative scale is required. 
Further, the photographing should be planned so that a clear view is 
obtained between buildings and into woods. Thereby the need of expen- 
sive additional field work is reduced. 
Considering the stereo instrument to be used, the flying altitude should 
be chosen so that the best use is gained of the transmission ratio of the 
instrument. 
In Sweden all aerial photography for mapping is carried out with 
cameras with the image size 9" X 9" and the camera constant 6". For 
mapping in the scale 1: 1000—1 m, which is the most usual map type 
for planning purposes, the photographing is mostly done from an alti 
tude of 900 metres (scale of the negative 1:6000). In some cases such 
photo material has been used for mapping in the scale 1: 400—0.5 m. 
Experience has shown that the scale 1: 6000 of the negative in several 
cases is too small, i. a. difficulties arise in the interpretation of the pho- 
tographs. In many cases it may also be difficult to achieve the geometric 
accuracy desired. 
The main part of all large scale mapping in this country is done in 
stereoautographs of the type Wild A8. In order to make the best use of 
the instrument, one of the negative scales of approximately 1:6 000, 
1:5000, 1:4000 or 1:3000 should be used for mapping in the scale 
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