Stereo Evaluation of Cartosat-1 Data for French and Catalonian Test Sites

DLR's Remote Sensing Technology Institute has more than 20 years of experience in developing spaceborne stereo scanners (MEOSS, MOMS) and the corresponding stereo evaluation software systems. It takes part in the CARTOSAT-1 Scientific Assessment Program (C-SAP) as a principal investigator for a Catalonian test site (TS-10) and as Co-Investigator for a French test site (TS-5, Mausanne-les-Alpilles). Rational polynomial coefficients (RPC) are provided by the distributing Indian agency as a universal sensor model for each scene of the CARTOSAT-1 stereo pairs. These RPC have to be corrected via ground control points (GCP) as they are derived from orbit and attitude information with an accuracy far worse than the pixel size of 2.5 m (normally, post facto pointing accuracy is in the range of a few hundred meters). GCP of sufficient (sub-pixel) accuracy are derived from high-resolution orthoimages of the Catalonian Survey Institute (ICC) and from JRC DGPS campaigns, respectively. The GCP are identified in the aft images of the stereo pairs because of their superior radiometric quality and then transferred to the fore images via local least squares matching. From these GCP an affine correction to the delivered RPC is estimated. The residual vectors at the GCP are reduced to sub-pixel values. Because of zero denominator problems with the original RPC in small regions of the stereo pairs new RPC are estimated Using the corrected RPC and mass tie points from automatic image matching the digital surface models (DSM) are derived from the CARTOSAT-1 stereo pairs via forward intersection and subsequent interpolation. These DSM are compared to the available sufficiently accurate DEM/DSM provided by ICC and JRC. For the comparison a 3D shift between the individual DSM is estimated via least squares adjustment. Height accuracies (in terms of 1 σ) of 2.5-4 m are achieved. The shifts between the orthoimages of the two looking directions computed on the basis of the reference DEM and the computed DSM are assessed via image matching and are found to be in sub-pixel range.