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In this situation, the problem of the description of the points in order to permit 
a sure identification becomes difficult to be solved. In fact, different organizations 
follow different methods or principles to describe the points on the ground. In the 
organizations themselves, operators on the ground and operators of the instruments 
integrate the uncertainties in the identification of points by means of direct contacts 
which are extremely clarifying. When the research and determination of points on 
the ground and their description is done by one organization and use of those data 
for plotting is done by other organizations, difficulties of interpretative character that 
evidently have nothing to do with the photogrammetric proceedings are unavoidable. 
Notwithstanding the remarkable care taken by the I.G.N. in providing the Work 
ing Centres with a vast documentation on the position of the control points, the above 
shown situation is present also in these experimental investigations. Moreover, it 
is comprehensible that the users of the data lay an eccessive blame on this cause of 
errors and partially exagérate the difficulties of interpretation which they found. 
As one can note by reading the reports of the centres contained in chapter V, all 
the operators (exception is made for the operators of the I.G.N.) found some difficulties 
in the interpretation of the descriptions of control points and in their identification 
on the ground. 
This previous question, posed by all the centres, may invalidate or at least reduce 
part of the documentary and informative value of the results. In order to avoid this 
fact, the Presidency has tried to verify whether these situations of difficult interpreta 
tion were real and due to deficient documentation or wrong individuation of the con 
trol points, by means of an objective statistical analysis of the results of all the centres. 
2 - SELECTION OF THE ANOMALOUS POINTS 
The errors due to the above mentioned reasons are necessarily localized on 
the poorly identified control points; therefore, they are included in the values 
of the differences A obtained as shown in the preceding paragraph. 
A principle of probability has been chosen to separate the points with anomalous 
behaviour from the others. Namely, those points have been considered anomalous 
that in one of the three coordinates present a difference A higher or equal to 2 m A . 
The choice of this limit is evidently arbitrary; however, supposing a distribution of 
frequencies of the differences not very different from the Gauss distribution, the prob 
ability of these events is less than 5%. 
On the basis of this principle, we separated all the points with a value A higher 
than the limit we adopted. 
This method of analysis of the anomalous points is certainly independent on 
the proceeding by which the block has been adjusted. Therefore, from the same series 
of measures adjusted by means of two different proceedings, we shall obtain two blocks 
with the same anomalous points. 
This is the reason why the selection of the anomalous points has been executed 
only in 11 tests corresponding to 11 different and independent series of measures. 
The 11 tests taken into account are those that have been first chosen and precisely 
tests n° 1, 5, 8, 13, 14, 15, 16, 17, 18, 19, 20. 
Therefore, the data contained in tables 4, 5, 6 and 7 concern the anomalous 
points that are present in these tests. 
The total number of anomalous points for each test and each coordinate is shown 
in the last columns of table 3.