Then the camera was translated horizontally until the center of the 
upper lens surface coincided with the plumb line through the crossing 
point of the files. The levelling of the focal plane frame of the camera 
and the adjustment of the cross spirit levels were carefully checked. 
When the camera was ready for exposure the correct levelling of the 
focal frame plane was checked with the cross spirit levels. The foot 
screws of the triangular frame were used for adjustment, if necessary. 
For final checks of the correct localization of the camera above the 
center control point theodolites on the ground were used, The elevation 
of the camera was determined from the control point in the tower and 
was checked with the theodolites on the ground. 
2.3 The photography 
For the photography glass plates and film were used. The glass plates 
were primarily Gevaert Diapositive 19 X 19 cm, Aviphot Pan 30 
23 X 23 cm, Kodak P 200 24 X 30 cm and Ilford Special Lantern 
Plate. The film was Kodak, Aerographic Super XX. 
For the handling of the glass plates a special dark sack was const- 
ructed and placed over the camera after the levelling of the focal plane 
frame. In the sack the glass plates were simply placed on the frame. 
The exposure was made with a simple lid shutter and the exposure 
time was chosen according to the exposure meter. The sensitivity of the 
negative material was chosen rather low. 
The plates were developed in the dark room in the tower. For the 
exposure of the film the ordinary film magazine was used and the 
camera was operated as during real aerial photography. The develop- 
ment of the film was made in the laboratory of the Geographical Sur- 
vey Office in Stockholm. 
The exposures were made under different conditions. Various filters 
were for instance used. As will be demonstrated below one camera was 
heated and cooled in order to investigate the influence upon the geome- 
try of the photographs from varying temperature conditions. 
2.4 The measurements of the image coordinates 
The measurements of image coordinates were made in three different 
instruments viz, a precision single image comparator SIP Mu 214 B, 
Nr. 277 at the Division of Mechanical Engineering at the R. Institute 
of Technology, a Zeiss stereocomparator and an autograph A 7 no. 310 
both at the Division of Photogrammetry. 
Most of the photographs were measured in the autograph A 7, since 
the measurements in this instrument are most convenient. 
The instrument was first carefully adjusted after test measurements 
according to the grid method with an especially constructed and meas- 
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