Looking Down on Heights

The two 30 m resolution images below show part of Germany's Iller Valley south of Ulm in Bavaria. The underlying data are from the synthetic aperture radar instrument on the ERS-1 and -2 satellites and were collected in December 1995. The digital terrain model on the left was generated by digitizing altitude isolines and is representative of the standard now available on the market, for a limited number of sites. On the right is a digital elevation model produced at DFD with its InSAR (Interferometric-SAR) processing chain. Digital elevation models additionally consider the height of any ground cover, such as constructions or vegetation. They are made by taking two images of the same point of the ground from slightly different locations and then combining them. Their phase differences can be used to produce an interferogram, whose fringes run along the lines of elevation and thus reflect the topgraphy.

The most obvious additional information in the InSAR DEM is the vegetation shown on the valley floor, including several densely forested patches at the top right, and the Iller River itself, snaking along the left hand side of the valley. Applications for high quality DEM include the modeling of surface drainage patterns or determining river catchment basins, essential information for identifying areas endangered by flooding or to support the planning and monitoring of road or dam construction. Another emerging application is in the telecommunications field, where DEM are being used to ascertain the optimal siting for antennas, or in the power industry, to optimize the routing of high tension transmission lines. (A power line is in fact visible in the high resolution version of the DEM - it is the straight dotted line running NW TO SE below the forested patches.)

left: DTM, level 2 processing, right: InSAR DEM (click on images for better resolution)

DFD is engaged in a project to generate digital elevation models of most of earth's land surfaces, based on interferometric data from ERS-1 and -2 tandem missions and from the Shuttle Radar Topography Mission (SRTM) scheduled for September 1999. In the tandem missions there is a 24 hour interval between the two recordings, and a spatial shift in the base line between 80 and 300 meters ("repeat pass interferometry"). In the SRTM mission the two images are recorded simultaneously using two antennas, one on a boom which extends 60 meters from the shuttle ("single pass interferometry"). Single pass interferometry avoids problems which can negatively affect the quality of the DEM such as changes in wind or other atmospheric conditions or the onset of precipitation between the two recordings.

For more information:

about the InSAR processing chain developed at DFD or about the the generation and processing of this particular image set contact Mr. Achim Roth in Oberpfaffenhofen at achim.roth@dlr.de

about the SRTM mission in general see http://www-radar.jpl.nasa.gov/srtm/index.html

about SRTM activities at DFD contact Mr. Hartmut Runge in Oberpfaffenhofen at hartmut.runge@dlr.de


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