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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