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13 
Working in the way we have already described, we obtained 
the following: 
a) the model to be surveyed was encased in a special frame de 
fined by 12 control-points placed in the best positions; 
b) the precísion with which it was possible to find out the 
co-ordinates of these points was probably in the order of 
a tenth of a millimetre; 
c) in each photogram the main point almost co-incided with one 
of the control-points on the edge of the piece; 
d) given the rectangular form of the piece itself and the 
presence of the wires, the relative orientation could also 
be made easily and accurately. 
Particular care was taken in resolving the problems of 
light, which however did not present any great difficulties, 
since the sheet metal piece had been painted grey as a precau 
tion. 
It is important to note that all the exposures taken with 
the three cameras and used in this research were done in the 
space of a few hours, to make sure that the framework and the 
model remained stable. 
Given the varied principal distances of the individual 
cameras, the average scale of the photograms was: 
1. TMK Zeiss = 1/30 
2. Off. Gal. = 1/12 
3. P/31 Wild = 1/18 
In Fig. 4, as an example, we have shown the left photo- 
gram taken with the TMK - Zeiss camera. 
Photogram stereo-pairs have been used to carry out exte- 
rnal orientation on both analogue and analytical instruments. 
It was not possible to complete all the tests which were desi 
rable because of a series of delays in organizing the experi- 
ment.