70
course, they cannot be used for conventionnal cartographic
output at the 1:25 000 scale.
When a stereo plotting of 1:30 000 photographs is under
taken for the revision of the 1:25 000 map, then we use
it directly and we obtain so the same data as we would by
digitization of the contour lines sheet.
VARIOUS USES FOR DIGITIZED RELIEF
There we shall review all the applications that are either
operational or under development at IGN-F.
1. Cartographic output of con tours
Experiments have been made in order to use digitized data
for drawings boths at 1:25 000 and at derived scales. To
achieve this, IGN-F owns a plotter which may be used for
drawing on a scribecoat. The drawing quality which is re
quired for cartographic outputs results in major cons
traints, particularly for compilation work. Three main
types of problems usually arise :
(a) Which contours should be retained when changing scale.
Generaly, every other contour is kept, but it is ne
cessary to choose a number of intermediate contours
in some areas ;
(b) Smoothing of contours may prove extremely difficult
in valley bottoms ;
(c) It is absolutely necessary to have a logical link bet
ween contours of different origins (either stereopairs
to 1:25 000 sheet, or from 1:25 000 to 1:50 000), in
order to plot top quality lines.
All these problems can be solved by automatic means most
of the time, but cases remain where manual work cannot be
done without ; then the above described interactive system
can be used.
2. D.T.M. Computation
D.T.M. 's of various grid sizes are computed at IGN-F,
using the contour files. These D.T.M.'s are used in two
main ways :
- direct uses, which we are going to describe in this
paragraph ;
- derived cartographic outputs.
(a) Automatic orthophotography :
Photographic coordinates of points, the ground projec
tion of which are on parallel straight profiles, are
computed from a D.T.M. and fed into the device which
produces the orthophotographies.