Orientation and Dem Extraction from Alos-prism Images Using the Srtm-dem as Ground Control

This paper presents a methodology of image orientation and DEM extraction from ALOS-PRISM stereopairs without the use of GCPs. It uses the approximate geo-location information derived from the sensor navigation instruments and the SRTM-DEM. The method consists in the extraction of a relative DEM, which initially is not correctly geo-referenced. This relative DEM is then matched to the SRTM-DEM, allowing for the determination of positional correction to the image orientation. Although the SRTMDEM has a much coarser resolution, the positioning accuracy of this matching method was found, in previous work, to be of the order of 5 meters in plan coordinates, using DEM patches of 5 by 5 km. Tests were realized with PRISM images of a relatively mountainous region in Portugal, with heights ranging from sea level to 1400 meters. The horizontal accuracy, assessed with 56 independent check points identified on orthophotos of 0.5 meter resolution, gave an RMSE of 2.7 m and 3.9 m in x and y directions. In terms of vertical accuracy an RMSE of 2.8 meters was found for a total of 45 ICPs measured on a DEM, derived photogrammetrically for mapping scale 1:10,000. The overall accuracy is compatible with the standards for 3D data extraction for 1:25,000 scale mapping, and has the advantage of not requiring any field survey for ground control acquisition.