Analysis of Digital Elevation Models Determined by High Resolution Space Images

Digital Elevation Models (DEM) generated by automatic image matching of high resolution space images are used often for very high resolution orthoimage production. The published accuracies of such DEMs are not in any case realistic and do represent often only the quality which can be reached in flat areas with good contrast and without influence of buildings and vegetation. In the mountainous area of Zonguldak, Turkey, Digital Surface Models (DSM), representing the height of the visible surface have been generated by means of stereo models taken by TK350, ASTER, SPOT 5 and IKONOS. As reference, GPS control points and a DEM of the topographic maps 1:25 000 are available like also the DSM of the SRTM X-band and C-band. The DSMs have been reduced by an automatic process to DEMs and are analysed separately for forest and open areas and as a function of the terrain inclination. The image matching of TK350 photos did require a scratch removal and a low pass filter because of several scratches and the film grain. Nevertheless, the results could be accepted only in the open areas. In the forest the image contrast was not sufficient and this was leading to large blunders and not accepted point matching. ASTER is using the near infrared band (NIR) for the stereo coverage and here we do have a good contrast also in the forest. Caused by this, the image matching was without any problems also in the forest. In the available SPOT 5 level 1A and level 1B images, the grey value variation in the forest is limited. With both products approximately the same accuracy has been achieved. Two IKONOS scenes taken with a time interval of 2 month have been used also for the DEM generation. The difference of 20° in the sun elevation caused quite different shadows, so a matching was not possible in the forest areas, but also in the build up parts it was difficult. Quite different results have been reached in another area where the IKONOS images have been taken from the same orbit; here the results have been excellent like also with QuickBird images taken with a time interval of just 10 days. The height models of the Shuttle Radar Topographic Mission (SRTM) are also referring to the visible surface, so in the forest areas a height shift can be seen. In addition there is a strong dependency upon the terrain inclination. The SRTM elevation models are better like the DEMs based on medium resolution space images.