| Geometry to the rescue!
The images acquired by Earth observation systems cannot be transferred
to maps as is, because they are geometrically distorted. These distortions
are due to errors in the satellites positioning on its orbit,
the fact that the Earth is turning on its axis as the image is being
recorded, the effects of relief, etc. They are amplified even more
by the fact that some satellites take oblique images.
Some distortions, such as the effects of the Earths rotation
and camera angles, are predictable. They thus can be calculated
and correction values applied systematically. Satellites also have
sophisticated on-board systems to record very slight movements affecting
the satellite. This information is used mainly to correct the satellites
position (when this is necessary), but can also be used to correct
the images geometrically.
The producers of satellite images generally propose applying the
most elementary corrections based on the satellites known
information. So, in the case of SPOT images it is possible to buy
geometrically uncorrected images (level 1A images).
However, a large number of users work with images in which the distortions
have been corrected systematically (level 1B images).
These corrections can be made without any special knowledge of the
terrain. The accuracy of locations in these SPOT images is of the
order of 500m.
To improve the precision of the corrections, reference points or
Ground Controle Points, GCP (identified on a topographical map or
in the field by GPS) must be available. The SPOT images that have
been corrected in this way are accurate to approximately 50m and
the data that they contain may be presented on a given map projection,
meaning that the images are superimposable on a map.