Potential of Ers-1 Derived Orthometric Heights to Generate Ground Control Points for Absolute Orientation of Imagery and Dem Quality Evaluation

Though the ERS satellites are today out of service, the huge quantity of altimetric data collected during the so-called “geodetic missions” covers the globe with sufficient density for many mapping projects. This paper first describes the general principle of ERS altimetric measurement . Then, the paper shows the method adopted to take advantage of ERS high-energy measurements (specular), which come usually from water bodies (rivers, lakes,...). ERS heights are processed along with a middle-scale vector data base, through a software which attempts to associate specular measurements with a cartographic item. As a result, we get a list of exact altitudes, applying to water bodies easily visible on SPOT imagery. In the next section, the research and production works to extract very accurate altitude values over flat areas (10 km wide) are detailed. The method to select the relevant ERS measurements is explained. The validation stage, using test sites distributed all over the world, showed a 2 to 5m height accuracy, adequate enough to control a global height database, and as a valuable input into image block-adjustment process . Finally, this paper will focus on the quantitative evaluation of ERS altimeter accuracy relatively to terrain height and slope variations within the whole impact area of the altimeter radio pulse contributing to the return signal ; we show that, after correcting several systematic errors through an original simulation method, developed by GRGS, absolute vertical accuracy better than 10 meters is kept available with ERS altimeter data in moderately rough terrain areas without any ground geodetic infrastructure.