SRTM – Shuttle Radar Topography Mission
The Shuttle Radar Topography Mission (SRTM) is an international project spearheaded by the National Geospatial-Intelligence Agency (NGA), NASA, NIMA, and the German and Italian space agencies.
SRTM obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth. SRTM consisted of a specially modified radar system that flew onboard the Space Shuttle Endeavour during an 11-day mission in February of 2000. SRTM collected an unprecedented 8.6 Terabytes of interferometric C-band Synthetic Aperture Radar (SAR) data (equivalent to about 14,317 CDs). This data will be processed to produce a rectified terrain-corrected mosaic of approximately 80% of the Earth's land surface topography (between 60 degrees North and 56 degrees South latitude) at 30-meter resolution. This would be the most accurate and complete topographic map of Earth's surface that has ever been assembled.
The processed SRTM radar data can be tailored to meet the needs of the military, civil, and scientific user communities. But other uses of this data include improved water drainage modeling, more realistic flight simulators, navigation safety, better locations for cell phone towers, and even improved maps for backpackers. Just about any project that requires accurate knowledge of the shape and height of the land can benefit from this data. Some examples are flood control, soil conservation, reforestation, volcano monitoring, earthquake research, and glacier movement monitoring.
To acquire topographic (elevation) data, the SRTM payload was outfitted with two radar antennas, and a technique called radar interferometry was used. In radar interferometry, two radar images are taken from slightly different locations. Differences between these images allow for the calculation of surface elevation, or change. One antenna was located in the shuttle's payload bay, the other on the end of a 60-meter (200-foot) mast that extended from the payload pay once the Shuttle was in space. Virtually all of the land surface between +/- 60 degrees latitude was mapped by SRTM. Processing of the C-band data took two years. During this time, JPL generated and released "showpiece" derived products.
The original data came with data voids, where insufficient contrast was available in the radar data to extract the elevation. These data voids tend to occur over water bodies (lakes and rivers), areas with snow cover and in mountainous regions (for example, the Himalayas has the greatest concentration of no data voids in the original data). The CGIAR-CSI SRTM dataset has undergone post-processing of the NASA data to “fill in” the no data voids through interpolation techniques (see the Data Processing and Methodology page for detailed description). The result is seamless, complete coverage of elevation for the globe.
With the release of SRTM V2 by NASA, all of the spikes and wells have reported been removed from the 'Finished' data (although the Australian data has not yet been processed) and the coastlines have been masked for unambiguous definition of coastal areas. However, many void areas still exist. These have commonly been interpolated out. The BLACKART program available from this website was one of the first utilities available for such interpolations. Several other GIS applications have added this ability, including 3DEM and MicroDem.
Interpolation provides an OK solution in areas of low relief. In mountainous areas it can produce very poor results, for example turning mountain tops into plateaus and filling in valleys. BLACKART attempted to remedy this by merging alternative DEM data sets into the SRTM data and then patching the holes with the merged data. This was sometimes better, but still suffered from problems. Standard error between the DEM datasets produced uneven transitions in the patched area. For practical purposes, the DEM data available for merging was of much lower resolution than the SRTM target.
The Consultative Group for International Agriculture Research - Consortium for Spatial Information (CGIAR-CSI) has confronted the task of improving the situation by processing the native SRTM data using the best available patching solutions. For example one of the data sets they use is data from Jonathan de Ferranti's Pathfinder Panorama web site. Mr. De Ferranti uses an innovative (if not laborious) technique of contour line extension to provide much better patch data. In this process, the contour lines are first generated from the SRTM. Then the missing or corrupted contour lines are fixed by extending adjacent contour lines, using (usually Russian topo) contour map data. This is probably done by a manual or semi-manual method. Once the contour lines are prepared, the contour map is interpolated to produce a DEM. The DEM is then merged with the source SRTM, preserving the valid data and applying the patch. As a result new data is added to the map and transitions are much more continuous.
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