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4.2 Geocoding using simulated data 
With steep looking angles like those of ENVISAT, relief displacements are a common distortion in the SAR i imagery. 
Because of these distortions, points in the imaged scene are displaced towards the SAR platform (foreshortening) so 
that the images cannot be used directly for cartographic purposes, e.g. updating of geographic information systems. The 
procedure to correct for relief displacements, which is referred to as geocoding is basically a first-order transformation 
between the SAR image co-ordinate system and a selected geographic co-ordinate system, usually UTM. Implicitly 
contained in the transformation is that distorted ground range co-ordinates are restored to their actual positions by 
means of a digital terrain elevation model. The unknown parameters of the co-ordinate transformation are estimated 
from a number of ground control points, which are located at easy to recognize points such as crossroads or buildings in 
both the SAR image and an UTM map and have a known elevation. With known transformation parameters, a grid in 
UTM co-ordinates can be set up and for each point in the grid the corresponding point in the SAR image can be 
identified. 
  
Figure 8. Geocoded SAR image of simulated ASAR data of the Black Forest in Germany. 
The above procedure has been applied to a SAR image of the Black Forest, near the city of Freiburg in Germany, Figure 
8. The image has the characteristics of ENVISAT's ASAR operating in image mode, it has been simulated with 
PHARUS. The accuracy of the geocoded image is estimated at 100 m, which demonstrates the ability of ASAR for 
geocoding purposes. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 345