(25) 
Zeiss RMK P10 are also calibrated by the manufacturer and checked in the 
Netherlands in a manner very similar to that for the RC-5. The Williamson 
OSC 6" is also calibrated by the manufacturer and is checked by the user for 
location of the principal point using both an auto-collimation procedure with a 
mercury mirror and the Williamson auto-collimator. In general, in the Nether 
lands, the expression “principal distance” is used in preference to the calibrated 
focal length as it is considered to be an adjusted numerical value and not actually 
a focal length. The value is adjusted so as to distribute the distortion in such a 
way that is suits best the plotting conditions of the user. KLM Aerial Surveys, 
the Ministry of Public Works, and Topographic Service require that the principal 
distance be such that the absolute values of positive and negative distortion are 
equal. Royal Dutch Schell use for the Williamson 6" Ordnance Survey Camera 
the condition that at the 35° zone the distortion be 0-16 mm. 
New Zealand 
In the procedure used for camera calibration in New Zealand targets are 
photographed by the contractor and measurements are made on the photographs 
by a government laboratory 10 . 
Sweden 
In Sweden the “camera constant” or principal distance and the principal 
point are defined in accordance with German terminology and camera calibration 
is carried out by a visual method using a goniometer. A special photogrammetric 
office of the Geographical Survey Office of Sweden has made a considerable 
investigation of distortion. Samsioe and Tham 7 have reported on a number of 
their studies. They have developed a particularly effective type of vacuum 
back for holding the film in contact with its supporting plate. They have 
prepared nomograms for the correction of height measurements due to distortion 
of the wide angle Topogon objective, and have also devised and developed a 
procedure for shaping the film supporting plate in a camera in order t6 compensate 
for the lens distortion. Their tests have shown this procedure to be very 
successful and have established that it introduces no imperfections due to strain 
in the film or to loss of image definition which might arise from the displacement 
of the film from its designed film plane. 
Switzerland 
The Wild RC-5 and RC-7 cameras are calibrated by the manufacturer. 
Principal distance or calibrated focal length is defined as that which makes the 
maximum positive and negative distortions equal in magnitude. The principal 
point is defined to be the “symmetrical centre of distortion”. Calibration is 
by visual means using a goniometer developed from the Wild T-3 transit. Per 
missible limits are as follows: 
Calibration tolerances ¿2 microns 
Asymmetry for RC-7 ±0-01 mm 
Asymmetry for RC-5 ±0-02 mm 
Flatness for focal plane ±0-02 mm 
Distortion curves for three Wild lenses are given in Hgs. 6, 7, and 8.