Remote Sensing for Science, Education, Rainer Reuter (Editor) and Natural and Cultural Heritage EARSeL, 2010 Analysis of 3D Forest Canopy Height Models Resulting from Stereo-Radargrammetric Processing of TerraSAR-X Images Roland PERKO 1 , Hannes RAGGAM, Karlheinz GUTJAHR, and Mathias SCHARDT Institute of Digital Image Processing, Joanneum Research, Graz, Austria Abstract. The presented study focuses on the evaluation of 3D digital surface models and canopy height models resulting from stereo-radargrammetric processing of TerraSAR-X images. These models are compared to highly accurate LiDAR data. It is shown that the accuracy of these DSMs are very high over bare ground, however regions of forest are systematically underestimated, like known from InSAR processing. This bias of about 27% of the real canopy height is caused as the SAR signal in X-band penetrates into the forest canopy. Detailed analysis and evaluations are per- formed to understand and to correct the underestimation. Keywords: TerraSAR-X, stereo-radargrammetry, 3D mapping, canopy height model. Introduction Forest parameters are an important source of information for monitoring climate change issues, quantifying renewable resources, and in general to observe deforestation and forest degradation. These parameters can best be estimated when 3D information on forest, i.e., a canopy height model, is integrated into the classification process [1]. Forest is in general hard to map with optical sensors due to cloud coverage, which may be especially high over rain forests. So the question remains whether radar data, in particular high-resolution TerraSAR-X products [2], can be reasonably used for 3D forest mapping. For areas of forest, SAR interferometry using TerraSAR-X data cannot be applied practically due to the strong temporal phase decorrelation caused by the small wavelength of X-band [3]. Therefore, this study is based on stereo-radargrammetric processing which yields highly accurate results due to the exact pointing accuracy of TerraSAR-X [4,5]. The approach is based on the authors' previous works [6,7] where 3D surface reconstruction is performed by stereo- radargrammetry incorporating multiple images acquired at different look angles. It is known from studies using InSAR-based processing on airborne X-band data, that tree heights are systematically underestimated. The physical explanation for the observed underestima- tion is the downward penetration of X-band SAR signal into the canopy causing a shift of the In- SAR phase center (cf. [8,9]). The amount of this underestimation dependents on many factors, such as tree species, crown shape, forest density, tree height, terrain slope and look angle. A simulation revealed that this underestimation is in the range of 10 to 80% of the real canopy height [8] and that an improved retrieval is only possible through use of ancillary data on canopy parameters. In this paper we analyze if the same effects occur in stereo-radargrammetric processing of Ter- raSAR-X data, i.e., if the penetration into the canopy also changes the magnitude of the SAR signal. For doing so, the digital surface models (DSM) resulting from stereo-radargrammetric processing are compared to highly accurate LiDAR data at two test sites and for regions on bare ground and in forested areas. While over regions on bare ground the general quality of our 3D surface reconstruc- 1 Corresponding author; E-mail: roland.perko@joanneum.at