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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.