+2 
E 
  
    
Such residual errors cause also some differences between the 
  
machine values and the ground values of the co-ordinates of the control 
points. The knowledge of these differences and a random check of the 
precision of identification, eventually done by repetition of plotting, 
would give a sufficient security for checking the plotting. For this stage 
of work too, all field measurement destined to determine the differences 
between real distances and plotted distances ought to be abolished and 
this method is still considered by many people as revolutionary, but in 
practice used by photogrammetric surveyors. Some tests give an 
increasing of the planimetric differences; the distance between two 
points, when this distance increases. 
If such a law has been verified by tests, this is due to the pro- 
gressive error made by the tester, not by the executor. The tester 
certainly will have made his measurements by using tacheometric 
methods or by surveyors staffs, where the probability and the magnitude 
of the error increases with the measured distance. On the contrary, in 
photogrammetry, when the points belong to the same stereogram, there 
may be a greater or lesser error with their position, but only in function 
of their less or greater distance in respect of the control points. 
If, on the contrary, the points belong to different stereograms, also 
if really their reciprocal distance is smaller than those we have consi- 
dered between points of the same stereogram, we may notice much 
greater errors due to a not sufficiently accurate connection of the overlap 
of the two stereograms. (Schermerhorn-Witt: Photogrammetry for 
cadastral survey - Ph. X/2). 
À rational method to test surveys made by means of photogram- 
metry for large scale maps, could include: test, done following the 
classie criteria, of the planning operations which follows the classic 
methods of triangulation and levelling; check of the preparation of the 
photograms and of their orientation, based on the measurement of the 
precision attained in the re-establishement of the control point co-ordina- 
tes; check of the connection between neighbour stereograms, based on 
the repetition of the determination of the machine co-ordinates of the 
common points between the two stereograms; check of the precision of 
identification, based on the repetition of the plotting in the most 
doubtful points of the stereogram; eventual check, by sight, of the 
accuracy of the reconnaissance; check of the preservation of the 
characteristics and of the adjustment of the taking cameras and of the 
plotting apparatus. The most part of these checks could be made 
without sending technicians to the fields.