rem 
N 
F: 5,6 wide 
rcm 
7: 6,3 wide 
liamson Co, 
ion values 
eters). 
stereo-plotters a systematic height error was obtained amounting to 0.8 per mille 
of the altitude from the edge of the model to its centre instead of 0.2 per mille, 
which had been expected owing to the optical distortion. 
Since a large number of photographs had been taken with the cameras con- 
cerned before the disturbances were discovered, a correction plate was made by 
the camera makers for compensation of the total distortion in the stereo-plotter. 
The distortion curve obtained empirically from the ice model measurement was 
used as standard for the computation of this compensation plate. Check measure- 
ments of these compensation plates showed excellent results. In height measure- 
ments of ice models with compensation plates a mean height error amounting 
to + 0.05 per mille of the altitude was obtained. 
The method described above for determining empirically the total distortion 
of an aerial photograph has proved a rapid and sufficiently accurate method for 
the practical photogrammetrician. It is particularly suitable for Swedish con- 
ditions. Precisely at the time when the aerial cameras have to pass a final check 
before the beginning of the photographing season, there are plenty of suitable 
ices in Central Sweden. Checking is quick and requires no field work. It seems 
also to be an advantage that checking is done in the same manner and with the 
same methods as for the utilization of aerial photographs for practical stereo- 
plotting. It is frequently the stereoscopic height accuracy that determines the 
flying altitude and the methods to be used for a certain map-plotting task. The 
ice model measurement gives directly the height errors in different parts of the 
model with or without compensation devices. 
It can perhaps be objected that the layer of air above the ice surface may 
result in a refraction that deviates from normal. However, the ice surface can 
probably affect only the layer of air close to the ice, and its properties are probably 
of minor importance for the geometric properties of the bundle of rays. 
REFERENCES 
1. Leijonhufvud, A.: On astronomic, photogrammetric and trigonometric refraction. Rikets 
allmänna kartverk, Meddelande nr 13, 1950. 
2. Tham, P.: Lens Distortions in Photogrammetry. Ingeniórsvetenskapsakademiens handlingar 
1946. 
3. Hallert, B.: Über die Bestimmung der radialen Verzeichnung von Luftaufnahmen. »Zeitschrift 
für Vermessungswesen» 1956: 4. 
Sthlm 1956. Isaac Marcus Joktr.-Aktiebolag 
  
  
  
  
  
  
  
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