Pu-Huai Chen 
  
SAR IMAGE GEOCODING USING A STEREO-SAR DEM AND AUTOMATICALLY 
GENERATED GCPs 
Pu-Huai CHEN, Ian DOWMAN, 
University College London, U.K. 
Department of Geomatic Engineering 
idowman @ ge.ucl.ac.uk 
KEY WORDS: Automation, Control strategies, DEM, Geo-referencing, RADARSAT, Simulation, Spatial data, Stereo 
SAR. 
ABSTRACT 
SAR image geocoding needs an accurate DEM, but such a DEM can be difficult to obtain. This paper shows that the 
needs of geocoding can be satisfied using the DEM generated by employing a rigorous geometric model and an 
optimized least squares correlation method with a region-growing approach. The geometric model used is independent 
of any manually selected GCPs and is robust to orbit errors. The required number of GCPs for correcting any systematic 
effects after space intersection is only two. When good quality orbit information is unavailable, GCPs or tie points are 
essential for SAR image geocoding, but selection and measurement of these points is often difficult. 
This paper proposes a SAR simulation technique to automatically provide control for geocoding without manually 
selected GCPs or tie points. The GCPs were generated using a small known DEM chip of size Ikm by Ikm with 
significant terrain relief. The RMS errors of the four automatically generated GCPs are less than 3.4m in range direction 
and 4.6m in azimuth direction. RADARSAT SAR images have been tested using a reference DEM and an automatically 
generated DEM to produce geocoded images. The RMS errors of the check points range from 13m to 21m in easting 
and 16m to 21 in northing. The results derived in this paper demonstrate that the stereo-SAR generated DEM can be 
used for geocoding in flat-moderate areas, and a higher level of automation can be achieved. 
1 INTRODUCTION 
Geocoding is to geometrically rectify a remotely sensed image, such as a Synthetic Aperture Radar (SAR) image, 
according to a specific map projection and a Digital Elevation Model (DEM) to eliminate terrain induced image 
distortions. The key factor to produce a geocoded radar image-map is the use of a correct DEM and reliable Ground 
Control Points (GCPs). However, DEMs are not always available and may be difficult to produce, e.g. in cloud-covered 
locations where optical data is not available or in underdeveloped areas where collecting GCPs is difficult. Two 
distinctive and practical methods have been developed for generating a DEM from SAR data, including interferometric 
SAR (IfSAR) and stereoscopic SAR, (Leberl, 1990). 
In terms of frequent mapping and global monitoring, however, experiments of the repeat-pass space-borne IfSAR 
system show that the method often gives poor results due to poor coherence and to different atmospheric and physical 
conditions. Compared with IfSAR approach, the stereoscopic SAR method, based on measuring the co-ordinate 
difference of a common ground point from an image pair and converting it to spatial data according to an appropriate 
geometric model, does not require such tight conditions. Another advantage of the stereoscopic SAR method is the 
ability of direct provision of the spatial information of any image point and of linear/area measures. It means that the 
stereo SAR method is relatively flexible. In theory, there also remain other conditions for stereo SAR, such as a 
reasonable intersection angle between each image for collecting distinctive parallaxes, allowing them to be transformed 
into height data. This condition is less significant, since multiple options of incidence angle of space-borne SAR data 
are available nowadays, such as RADARSAT data, (CSA, 1995). The Shuttle Radar Topography Mission (SRTM) was 
launched in early 2000, it uses a single-pass IfSAR system and is designed to carry out terrain elevation mapping of 
80% of the Earth’s land surface, (JPL, 2000). It will be shown later that the stereoscopic radargrammetric method can 
be used as an alternative tool for general mapping to the IfSAR approach, whenever the IfSAR method is not applicable 
or the IfSAR generated DEM is not available to common users. 
A rigorous geometric algorithm for generating spatial information with an error model to be validated is necessary to 
understand the full potential and limitations of extracting spatial information from stereoscopic SAR data. Also, the 
requirements of human operations and GCPs have to be reduced. Early work on ERS-1 data at UCL suggests that the 
stereoscopic radargrammetric approach is a promising tool for extracting elevation data from space-borne SAR data, 
  
38 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 
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