ITALY
General Direction of Cadastral Survey and of the Technical Services
of Finance Department :
On the most suitable equidistance of contour lines
in air surveyed maps.
Communication by Prof. Eng. ALFREDO PAROLI
to the Tth International Congress of Photogrammetry at Washington, 1952.
The usual equidistance of contour lines plotted from air photographs is
chosen with regard to the scale as well as to the aims of the map. In zones where
slopes of the ground are rather steep, it will, of course, be necessary to use lar-
ger contour intervals, in order to avoid that — becoming slopes steeper — the
contour lines will result planimetrically too near one another or even confound
themselves, rendering thus contours indistincted.
In the case of maps on large scale containing many planimetric details,
and particularly in the case of cadastral maps of zones, where landed property
is very small parcelled and requires drawing of many boundaries, an excessive
density of contour lines would render the picture of planimetry less intelligible;
and planimetry, of course, is in the cadastral map the essential element for the
civil and fiscal aims to which it is destined.
Thus, for the air surveyed maps of the Italian Cadastral Survey, which con-
tain also contour lines, it was decided to use a medium planimetric interval bet-
ween adiacent contour lines not inferior to 3 mm on the map.
If this medium interval results inferior to the fore-said limit, only a part
of contour lines is plotted. This means that — instead of the usual equidistance
— a larger contour interval (the double, quadruple, etc. of the usual one) is
used. For example, cadastral maps on scale 1:2000 have a contour interval
of 2 m, which has resulted suitable for a clear and evident altimetric represen-
tation. If, of course, slopes of the ground are rather steep, the contour interval
must be enlarged to 4 m; for steeper and steepest slopes, equidistances of 8 m,
16 m, etc. will be necessary.
This intuitive system for the choice of a suitable contour interval can be
applied in a more rational manner, avoiding any arbitrariness. For this purpose
