It is not necessary to deeply study these figures to note that there is no resemblance 
among the behaviours of the mean curves. The mean errors of each coordinate, in 
each test of the two groups, have a behaviour almost completely different. Also, the 
good resemblance between the mean curves in coordinate X of the three tests 13, 14 
and 15 is probably due to chance and therefore is not sufficient to modify the first 
impression. 
In both groups of all three tests, two tests have been obtained by analytical triang 
ulation, while the third has been obtained by the instrumental triangulation. No 
particular resemblance is evident between the results of the two analytical triangula 
tions. 
Also, the expectation of a greater regularity of the results of the analytical triangu 
lation, with respect to the ones obtained in an analogical way, does not seem to be 
satisfied by the data contained in the mentioned figures. 
However, these statements can have no general character since they are based on 
too scanty a number of proofs obtained under particular conditions, thus not permitting 
a general conclusion. 
2 - COMPARISON BETWEEN THE INTRINSIC ACCURACY OF A BLOCK AND ITS ABSO 
LUTE PRECISION 
The investigation into accuracy one can reach in the triangulation and 
adjustment of a block obtained by means of proofs executed on an experimental pol 
ygon, runs the risk of losing a part of its practical value if one cannot deduce from 
these data relating to isolated cases a principle of evaluation of the accuracy of any 
other block carried out according to analogous principles. 
In our opinion, it is not sufficient to increase the number of the typical cases of 
reference and to assume as accuracy of the blocks of that type, the experimental accur 
acy deduced from the block of reference. 
Several circumstances, dependent on ground, flight, camera or plotting apparatus, 
operator, chosen points and so on, can markedly alter the results and lead to erroneous 
judgements. 
There are elements intrinsic in the blocks, on whose basis one can give a judgement 
on the accuracy. In practice one can refer to them to judge the quality of the results. 
These elements are: 
— discrepancies between points plotted in successive models; 
— discrepancies between points common to adjacent strips; 
— discrepancies of the known points used for adjustment of the block. 
In this paragraph we shall compare these three intrinsic elements with the indi 
cation of the absolute precision of the blocks obtained by the above described methods. 
Table 8 contains all the data on the absolute and intrinsic accuracy relative to 
the 15 independent tests carried out by the centres. Some data have been obtained 
from the reports of the centres themselves; some others have been directly calculated; 
some others, finally, have been taken from the preceding tables of this chapter. 
The mean square values of the errors on the points used to adjust each block 
have been calculated by the Secretary’s Office by means of the data contained in the 
general tables of the differences or have been given by the centres themselves in their 
respective reports. 
178