4, THE FOCAL LENGTH TO BE ADOPTED FOR THE PLOTTER 
The question must be asked "What value for the focal length (fp) should be set on a plotter to 
complete the inner orientation process?" 
4.1 The value of c certainly should not be used simply because much may have happened to change the 
single perspective document (the photograph) since the instant of exposure. The happenings are 
impossible to define exactly and therefore one should resort to obtaining the best general value 
by taking measurements to available calibration data. The only available data are c and the 
coordinates of fiducial marks or distances between them. 
4.2 The operator should take measurements between the fiducial marks with a scale to an accuracy of 
0.02 mm to compare with the calibrated distances and then calculate a correction ratio to apply 
to e thus obtaining the setting value fp. The accuracy of 0.02 mm is stated as this is the 
normal limit of accuracy for adjustment provided. in analogue instruments. The difference between 
e and fp is often greater than 0.05 mm, especially when a projection printer has been used to 
produce film diapositives, and this is significant. 
4.3 What should not be forgotten is that the single photograph must be treated as a 3-dimensional unit 
and the three dimensions of the unit should be brought to the same scale, otherwise deformations 
will be present. 
4.4 The above arguments apply also to the mounting of photographs in analytical plotters and 
comparators regardless of computer program software. The latter should take account of residual 
errors. Definable errors in perspective should be corrected before the perspective is viewed. 
5, FREQUENCY OF RECALIBRATION 
Opinions and practices differ considerably on this subject. 
5,1 Some national departments and photogrammetric organisations and flying agencies are conscientious 
by recalibrating during the winter before the flying season; some are required, in an offer to 
tender for a task, to produce evidence of a calibration check within six months; some never have 
a recalibration done after a camera is purchased. 
5.2 Continuing to use a camera without recalibration must somehow be prevented. A firm that does 
this is not being honest with itself nor with the clients. The same remark applies to those who 
use projection printers for exposure of diapositives. For the printer, a colinearity check - - 
should be made using precise grid plates periodically. This should indicate axis deviation, 
tilts of the negative and diapositive planes and errors existing in compensation plates. 
CONCLUSION 
Mention has been made of other equipment apart from the camera. . This is because the camera is 
only the initial tool in the whole chain of operation to provide map plots or object coordinates 
regardless of whether analogue or analytical methods are employed. 
It is important always for those concerned with the Science of Measurement to endeavour to prevent 
deformations and errors from occurring at all by using good equipment which is well calibrated. 
The complete geometry of the single perspective is never known precisely - never as precisely as 
a camera is calibrated in the Laboratory - and often assumptions are made, with reference to stability 
and accuracy, which are different from the truth of the situations that exist outside the laboratory. 
The reader is referred to the paper of Commission 1 presented by Dr H. Ziemann (2) at Ottawa in 
1972. Image geometry, changes and effects are well discussed and, with the comprehensive reference 
list of 75 publications prior to that date, it is an excellent text. Much has been reported and 
published since and the reader is referred also to Photogrammetria (3) and also to the Bibliography 
of 102 references provided by Sabry F. El-Hakim (4) in a thesis dated 1979. 
1. Thompson, E.H. The Geometrical Theory of the Camera and its Application in Photogrammetry. 
The Photogrammetric Record, 2(10):241-263 (1957) 
2. Ziemann, H. Image Geometry - Factors Contributing to its Change, presented paper, 
Commission I, XII International Congress, Ottawa (1972) 
3. Patterson, J.B., Ziemann, H., Hakkarainen, J., Tayman, W.P., Carman, P.D. and H. Brown, 
all publications in Photogrammetria, 34 (1978) 
4. El-Hakim, S.F. Potentials and Limitations of Photogrammetry for Precision Surveying. 
Tech. Report No. 63, University of New Brunswick, Canada (1979) 
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