An improved technique for generation of Digital Elevation Models (DEM), capable of dealing with full scene images (100x100 km) coming from an interferogram obtained with ERS satellite data is presented. Starting from an interferometric processor aimed to the geocoding of smaller areas, now we show the new improvements based on the use of a direct geocoding algorithm and Ground Control Points (GCP), in order to calibrate some imprecisions which appear in the case of very wide swaths. The direct geocoding algorithm is based on the transformation of phases directly into heights over a UTM grid, without an intermediate transformation of phases into heights in slant range. This technique provides more accurate results. A generated DEM of the test zone of Tarragona (Spain) and its error assessment are presented. Introduction A critical step in the interferometric process [3] is the transformation of unwrapped phases into geocoded heights. Several imprecisions in orbit generation, timing knowledge and geocoding can lead to significant height errors when we are working with wide areas. It is easy to observe false height slopes along the image during the process due to this imprecisions [1]. On the other hand, if we work with small areas these errors are more difficult to observe because their propagation is not so critical. Our goal in this paper is to present some techniques developed by our research group to obtain high quality DEM's (Digital Elevation Map) of small or wide images. Direct geocoding for generation of precise wide-area elevation models with ERS SAR data.

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