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11 
ng, as was the case, a stylus with a very fine conical point. 
Systematic inspections were done during the surveying to 
check the reliability of the mechanical and electronic parts 
of the apparatus. 
Note that the reading was made with a resolution of 0.05 
mm in the three co-ordinates, thereby giving an estimation 
which was exact to a tenth of a millimetre. 
It is useful to note briefly the fact that, apart from 
the various measuring systems related to the photogrammetric 
instrumentation that was used during the reading of the pro- 
grams, we used two measuring systems that could be called ab 
solute: 
a) a system that is related to the wire framework set up in 
the laboratory on the metal trestle with its origin in point 
502 and the direction of the axes X* Y* as indicated in 
Fig. 3; 
b) a system that is related to the model studied, with its ori 
gin in point 99 and the direction of the axes as indicated 
in the same figure. 
The system that is related to the framework has a degree 
of precision of 0.01 mm (the degree of precision of the mechani 
cal processes) as far as the co-ordinate X*and Z* are concer- 
ned; the degree of precision of the co-ordinate Y*, however, de 
creased to about Q1 mm because of the method used for measuring 
the wires. 
The system that is related to the model, obtanied with the 
afore-mentioned DEA apparatus, had a degree of precision of 
0.05 mm. 
We think, therefore, we can be sure that the spatial co-or 
dinates of the points in the two systems were determined with 
a degree of precision in the order of 0.1 mm. 
As we will see further on, through a comparison of co-or- 
dinates of one point in the two systems, useful information