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.