Dem Generation by Spot Hrs

The HRS (high resolution stereo) is an autonomous imaging system on SPOT 5. It has been designed especially for the generation of digital elevation models (DEMs). The combination of a forward and a backward view do allow a stereoscopic imaging with a height to base relation of 1.2 and just 90 seconds time interval. Some images, which are usually not accessible and used only by SPOT Image for the generation of DEMs, have been made available for the SPOT HRS study team. The test area Bavaria has been analysed. Just based on the known view direction, the orientation of a HRS stereo pair has been adjusted with the Hannover program BLASPO. The mean square Z-discrepancies at the control points of 3.9m corresponds to an x-parallax of 0.6 pixels, a realistic value for points located in areas with sufficient contrast. Depending upon the area, between 80% and 90% of the points could be matched successful by a least squares method. The determined ground points do describe a digital surface model (DSM) and not the requested DEM. Points located not on the solid ground have to be removed. For the 17 million points of one HRS stereo pair this can be made only by an automatic process. The Hannover program RASCOR removed approximately 35% of the DSM points. Nevertheless in dense forest areas several points still may be located not on the ground. Corresponding to this, in the forest areas a height shift against the reference data could be seen. But also the relative accuracy in the forest is not so good like in the open areas caused by limited contrast and the not visible ground. In the open areas a vertical accuracy in the range of 5m and a little better has been reached with a slight dependence upon the tangent of the slope.