from one another (a method which is not used any more, 
except for intermediate contours). 
(c) Another programme, which only requires a video display 
terminal, enables to modify codes of contour lines 
that the operator chooses by indicating their number 
(we shall see later how this number is known to him). 
This programme is used when only a few errors or 
omissions are to be corrected, and do not require 
the use of the more complete programme described in 
(a ). 
(d) The current file status can be checked at any time 
by a plot which is produced by an on-line plotter 
almost immediately after the request is made, accor 
ding to the wishes of the operator, this plot may 
include all contours or only part of them, the coding 
of each contour and its characteristic number. 
(e) An automatic checking programme of elevations shows 
up errors which are very difficult to notice even 
when examining very closely the checkplot. 
(f) At last, we develop similar programmes for dealing 
with the spot heights. 
We could notice a big progress when this software came 
into operation, in comparison with the time the operators 
spent with off-line digitizers and batch processing ; 
and the advantages of the automatic digitization (raster 
scanning and raster to vector computing) which needs more 
correcting, have been considerably increased. 
2. Digitization by stereo plotting of aerial photographs 
In montainous areas, contour lines sheets are extremely 
crowded and very uneasy to digitize ; the manual digiti 
zation of such a sheet would require a very long opera 
tor time, due to the great amount of points to be digi 
tized and the difficulty of following the contours, 
because they are very close to one another and often 
interrupted in very steep areas ; so we would be lead 
to use the automatic digitization method ; unfortunately, 
the scanner cannot always keep apart two contours, even 
when using the 20 points to the millimeter resolution, 
so that the correction time would be almost as long as the 
time required by a manual digitization. In addition, when 
the slopes are very steep, a lot of contour lines are not 
necessary for understanding the terrain. So we decided to 
use directly the 1:60 000 scale aerial photographs ; for 
this purpose, IGN-F can use stereo plotters with a digital 
output on magnetic tapes : the operator plots the stereo 
pairs with a contour interval of 40 meters, plus interme 
diate contours in flat areas. The contours of the stereo 
pairs are then put together in 1:50 000 sheets, in order 
to obtain a similar result to that we woult by mean of a 
manual digitization. This method provides us with data 
precise enough for computing D.T.M., and is very faster 
and cheaper than the previously described ones ; but of 
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