ITALY 
  
General Direction of Cadastral Survey and of the Technical Services 
of Finance Department 
On the final testing of contours in air 
surveyed maps on large scale. 
Communication of Prof. Eng. ALFREDO PAROLI 
to the 7th International Congress of Photogrammetry at Washington, 1952. 
The accuracy of contour lines in air suerveyed maps on large scale (from 
1:5000 to 1:500) can be verified by directly measuring intersections of the ground 
(using the tacheometer and the stadia) and comparing the profiles thus obtained 
with the corresponding ones derived from the map. The differences of height of 
corresponding spots in the two profiles must not exceed the limits of error which 
are preliminarly established and vary according to the slope of the ground and to 
the map scale. 
From a technical point of view, no objection can be made practically on 
regard of the above-said testing system, notwithstanding it is resulted too slow 
and too laborious and permits to ascertain the accuracy of contours of a small 
zone only, i. e. in proximity of the points, where the above mentioned intersec- 
tions cut the contour lines. 
The search after other testing systems allowing a quicklier, less laborious 
and more extended verification of mapped contours appears therefore obvious 
as well as opportune. 
A testing system we have experimented since a long time and which re- 
sulted simple, quick and very efficient, is the «method of the second plotting». 
It requires office work only. The method means — as the denomination says — 
that during the plotting of contours or during the operations for final testing of 
the map, tracing of an adequate number of contour lines must be repeated. 
The second plotting carried out by means of the same or by an analogous 
apparatus like that used for the first plotting, permits, first of all, an imme- 
diate demonstrative verification of the degree of accuracy and can further be 
used for establishing the acceptability of the map. 
In fact, if the case may be that both the tracings would be without errors, 
the two runs obtained for a same contour line by the first and the second plot- 
ting will be affected by inevitable errors and will show, therefore, reciprocal. 
planimetric divergencies. The magnitude of these divergencies permits to dedu- 
ce the total mean error of the operation consisting of the two plottings consi- 
dered together. 
If the second plotting (for verification) is carried out with such care that 
it may be considered free from errors or at least affected by very small and the- 
refore neglectable errors, the magnitude of the above mentioned divergencies 
will only depend upon the error due to the original plotting. If, therefore, such 
divergencies remain between the fixed limits of error, the map may be considered 
acceptable; if the case is on the contrary, the map cannot be accepted and must 
be refused or corrected.