10 
lines or edges and also natural edges on the ground have been 
photographed from the air using varying flying altitudes, air- 
craft speeds, exposure data etc. The microdensitometer measure- 
ments on the aerial negatives and the computations have been per- 
formed by the Institute of Optical Research at the Royal Insti- 
tute of Technology in Stockholm. 
In order to study how the spectral variation in objects in aerial 
photographs influences height measurements in stereo models RAK 
has performed some experiments. Aerial photography with differing 
film types was performed in 1971 over Ská-Edeby airfield, where 
a large number of signals in five different colours were placed 
on the ground before the aerial photography and spirit levelled. 
This experiment is à continuation of an experiment which started 
in 1969 and also dealt with investigation of the visibility of 
the coloured signals in different types of photographic materials. 
In eonnection with the aerial photography with the Lear Jet 24 
from an altitude of 9,200 metres which covered an area of 
125,000 km^, an experiment of photographing from the maximum 
operational ceiling of the aircraft, 15,700 metres was carried 
out covering an area of 5,400 u^. The experiment was successful 
and the photographs have been interpreted regarding the visibility 
of different objects. 
Photogrammetric Instruments 
During the latest four year period the number of photogrammetric 
instruments has been fairly constant. An increase in the number 
of instruments can be recorded only for automatic co-ordinato- 
graphs, automatic co-ordinate printers and comparators. Table 3 
shows the number of instruments of different types used in pro- 
duction. Instruments owned by educational institutions are thus 
not included in the figures. 
e