Derivation of DEM and Orthoimages Using HRS Stereo Data and HRG data from SPOT 5 Satellite

Kurzfassung

The HRS stereo camera system on SPOT 5 produces image stereo pairs with two optics looking forward and backward with „b20 degrees with respect to the nadir direction. The data have a spatial resolution of 10 meter across track and a 5m ground sampling distance along track for obtaining higher accuracy of the parallaxes for the DEM generation. The swath of the HRS is 120 km (12000 CCD elements) and one acquisition sequence can be as long as 600 km along track. After the ISPRS Commission I Symposium in Denver in November 2002, the HRS Scientific Assessment Program has been established. This program gives the user community the opportunity to test HRS data, which are usually not available, for generating DEM and for comparison with other DEM generation methods. Further it should provide CNES an international scientific performance assessment of the HRS which will be taken into account for future programs. Two test sites are analyzed: south-eastern Bavaria in Germany and the city of Barcelona with surroundings in Spain. In the case of Barcelona, also the nadir looking high resolution channels HMA and HMB have been acquired additionally and can be used in the processing. For the derivation of the DEM, the DLR own stereo processing software, developed for the MOMS-2P three line stereo camera is used. As a first step, the interior and exterior orientation of the camera, delivered as meta data (DORIS and ULS), which should lead to an absolute pointing accuracy of better than 15 meters, are extracted. A dense image matching, using nearly every pixel as a kernel center (region-growing), provides parallaxes for most pixels. The quality of the matching is controlled by forward- and backward matching of the two stereo partners using the local least squares matching method. In the case of Barcelona also the nadir looking channels are used for blunder reduction in the matching process. Having the mass points of correlation from the matching process as well as the exterior and interior orientation of the camera system, the surface heights can be calculated using forward intersection. This is done by least squares adjustment for the intersection of the image rays. The irregular distribution of points in object space after the forward intersection has to be regularized into a grid of about 20 x 20 meter pixel size and filtered by a DEM filtering process. The interpolation is performed by a moving plane algorithm. The derived surface models are compared with the high precision reference DEMs by several methods regarding height accuracy and error budget depending on surface properties etc. As a second step, orthoimages are generated from the images of the two looking directions, using the derived DEM and accuracy analysis is carried out regarding the absolute accuracy in comparison to orthoimages from airborne data and other ground control points.