Accuracy Analysis of Dems Derived from Aster Imagery

ASTER acquires along track stereoscopic imagery, with a spatial resolution of 15 meters. Automatic generation of Digital Elevation Models (DEMs) from these images is a well established process, implemented in many commercial software packages. It can provide relief information for areas with poor coverage of topographic mapping. This paper presents a study of the accuracy achieved in DEMs extracted from ASTER, for an area in Portugal, using the PCI OrthoEngine software. Images were orientated with ground control points (GCP) obtained from topographic maps. Experiments were carried out in reducing the number of GCPs. A number of 5 or 6 GCPs was always required to orient the images, in order to keep the accuracy achieved with larger numbers of points. It was possible to conclude that more use could have been done of the approximate orientation provided in the image header. A grid of points derived from the sensor position and attitude, estimated by onboard equipment, is given in the image header. The precision of these data was assessed as being better than 100 meters. A significant reduction in GCP requirements would be possible if more use of this information would be done. The vertical accuracy of the extracted DEMs, assessed with a 8 m DEM grid, produced by a local mapping agency, was of 9 meters in areas of mountains with low vegetation. The percentage of coverage of the DEM is very large, however the automatic filling of gaps, in areas where the stereo-matching failed, degraded the DEM detail and accuracy. A manual edition of gaps, according to user perception of relief forms is more appropriate when deriving DEMs for production and update of topographic map products.