UTILIZATION OF SATELLITE SPOT DATA 
This satellite devoted to remote sensing is described else 
where and will not be presented here. It has been already 
seen that SPOT images rectified with D.T.M. could be used 
for revision of topographic maps in areas which are already 
mapped. In new countries SPOT images through their stereos 
copic possibilities will provide a new way of obtaining 
digital relief data and of establishing a general cartogra 
phy at 1:100 000 with an altimetric accuracy of about 10 to 
20m. Let us see now the process which could be used : 
1. Triangulation 
It will be possible to use aerial triangulation methods 
for determining unknown points with stereopairs equipped 
with some control points. But in certain cases SPOT will 
give the possibility to survey an area without knowing 
any control point : by using at the same time two ins 
truments HRV (High Resolution Visible), the first one 
being oriented towards the unknown area, the second one 
being oriented towards an area where control points are 
available, it will be possible to locate new control points 
in the unknown area. 
2. Calculation of D.T.M. 
When a sufficient number of control points are available, 
then it is possible to determine the geometrical model of 
every image. Two methods can be used : 
- analytical stereo-plotting of a stereo-pair : the opera 
tor follows and records contour lines or profiles of the 
D.T.M. 
- direct processing of the information in digital form 
through a computer. This kind of processor often uses 
epipolar lines for aerial photographs to find homologous 
points ; in the case of SPOT images where these lines do 
not exist, it is possible to find locally "quasi- 
epipolar" lines. 
In both cases an accuracy of about 10 to 20 m could be 
obtained, which is compatible with the scale of 1:100 000, 
D.T.M. making it possible to rectify oblique images from 
parallaxes due to the relief, and to obtain orthophotos 
providing a cartographic ground for a map at 1:100 000. 
CONCLUSION 
When the digitization of the whole relief data of France 
will be finished, through the methods described above, IGN 
and other interested organizations will have a complete data 
base, able to be constantly up-dated. 
In countries where accurate maps are not yet available, SPOT 
images will make it possible to calculate D.T.M.'s.