View source for SRTM

[[Image:Srtm 1.jpg|thumb|250px|The SRTM was flown on an 11-day mission of the [[Space Shuttle Endeavour]] in February of 2000.]]
The '''Shuttle Radar Topography Mission''' ('''SRTM''') is an international research effort that obtained [[digital elevation model]]s on a near-global scale from 56°&nbsp;S to 60°&nbsp;N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the [[ASTER GDEM]] in 2009. SRTM consisted of a specially modified [[radar]] system that flew on board the [[Space Shuttle]] [[Space Shuttle Endeavour|Endeavour]] during the 11-day [[STS-99]] mission in February 2000, based on the older ''Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar'' (SIR-C/X-SAR), previously used on the Shuttle in 1994. To acquire [[Topography|topographic]] (elevation) data, the SRTM payload was outfitted with two radar antennas. One antenna was located in the Shuttle's payload bay, the other &ndash; a critical change from the SIR-C/X-SAR, allowing single-pass interferometry &ndash; on the end of a 60-meter (200-foot) mast that extended from the payload bay once the Shuttle was in space. The technique employed is known as [[Interferometric Synthetic Aperture Radar]]. 

The elevation models are arranged into tiles, each covering one [[degree (angle)|degree]] of latitude and one degree of longitude, named according to their south western corners. It follows that "n45e006" stretches from [[45th parallel north|45°N]] [[6th meridian east|6°E]] to [[46th parallel north|46°N]] [[7th meridian east|7°E]] and "s45w006" from [[45th parallel south|45°S]] [[6th meridian west|6°W]] to [[44th parallel south|44°S]] [[5th meridian west|5°W]]. The resolution of the cells of the source data is one [[arc second]], but 1" (approx. 30 meter) data have only been released over United States territory; for the rest of the world, only three-arc-second (approx. 90-meter) data are available. Each one arc second tile has 3,601 rows, each consisting of 3,601 [[16 bit]] [[bigendian]] cells. The dimensions of the three-arc-second tiles are 1201 x 1201.

The elevation models derived from the SRTM data are used in [[Geographic Information Systems]]. They can be downloaded freely over the Internet, and their file format (.hgt) is supported by several software developments.

The Shuttle Radar Topography Mission is an international project spearheaded by the U.S. National Geospatial-Intelligence Agency ([[National Geospatial-Intelligence Agency|NGA]]) and the U.S. National Aeronautics and Space Administration ([[NASA]]).

== No-data areas ==
[[File:Srtm voidfilling grass gis.png|right|thumb|350px|SRTM void filling with spline interpolation in [[GRASS GIS]].]]
The elevation datasets are affected by mountain and desert no-data areas. These amount to no more than 0.2% of the total area surveyed, but can be a problem in areas of very high relief. They affect all summits over 8,000 meters, most summits over 7,000 meters, many Alpine and similar summits and ridges, and many gorges and canyons. There are some SRTM data sources which have filled these data voids, but some of these have used only [[interpolation]] from surrounding data, and may therefore be very inaccurate. If the voids are large, or completely cover summit or ridge areas, no interpolation algorithms will give satisfactory results. Other developers, including [[NASA World Wind]] and [[Google Earth]], have improved their results by using 30-[[arc-second]] data in the interpolation process, but, due to the poor resolution of these data, and very poor quality of some of them, they have further improved their earth viewing services by adding data from other sources. Readers with Google Earth software can examine an example of the most recent results by clicking on ([[Mount Everest]]) and tilting the image.

== Void-filled SRTM datasets ==
[[Image:Maps-for-free Sierra Nevada.png|thumb|250px|Relief map of [[Sierra_Nevada_(Spain)|Sierra Nevada]]]]
Groups of scientists have worked on algorithms to fill the voids of the original SRTM data. Two datasets offer global coverage void-filled SRTM data at full resolution: the [http://srtm.csi.cgiar.org/ CGIAR-CSI versions] and the [http://hydrosheds.cr.usgs.gov/ USGS HydroSHEDS dataset]. The CGIAR-CSI version 4 provides the best global coverage full resolution SRTM dataset. The HydroSHEDS dataset was generated for hydrological applications and is suitable for consistent drainage and water flow information. [http://hydrosheds.cr.usgs.gov/references.php References are provided] on the algorithms used and quality assessment. The void-filled SRTM data from [http://www.viewfinderpanoramas.org/dem3.html Viewfinder Panoramas] are high quality at full SRTM resolution, but [http://www.viewfinderpanoramas.org/coverage%20map%20viewfinderpanoramas_org3.htm coverage] is limited to areas of high mountain void incidence, and some areas north of 60 degrees of latitude.

== See also ==
* [[Advanced Spaceborne Thermal Emission and Reflection Radiometer]]
* [[Interferometric Synthetic Aperture Radar]]
* [[Digital elevation model]]
* [[National Geospatial-Intelligence Agency]]
* [[TerraSAR-X]] is a newer satellite with higher resolution
* [[SRTM Water Body Data]]

== External links ==
* [http://www2.jpl.nasa.gov/srtm/ Official NASA SRTM site]
* [http://dds.cr.usgs.gov/srtm/ NASA's server with SRTM data tiles] - Please read the accompanying documentation
* [http://www.maps-for-free.com Maps-For-Free.com] Free global relief maps
* [http://www.geosar.com GeoSAR, the Testbed for SRTM] - Currently owned and operated by Fugro EarthData
* [http://glcfapp.umiacs.umd.edu:8080/esdi/index.jsp 1-Degree SRTM data tiles in GeoTIFF format] - UMD's Global Land Cover Facility
* Void filled SRTM data at [http://srtm.csi.cgiar.org/ CGIAR-CSI] and [http://droppr.org/data/map/srtm/ Droppr]
* [http://hydrosheds.cr.usgs.gov/ USGS HydroSHEDS] - Full resolution SRTM-based DEM for hydrological applications 
* Software that can read and process SRTM data: [http://www.visualizationsoftware.com/3dem.html 3dem], [[GRASS GIS]], [[SAGA GIS]], [http://www.mapwindow.com/ MapWindow GIS], [http://www.dgadv.com/dgtv DG Terrain Viewer/Void Killer], [http://vterrain.org/ Virtual Terrain Project]
* [http://www.viewfinderpanoramas.org/dem3.html Viewfinder Panoramas] - Unofficial SRTM data with voids corrected using topographic maps
* [http://pub7.bravenet.com/forum/537683448/ Discussion forum] for SRTM data users 
* [http://www.atlogis.com/metamaps.html?lat=13.35938&lon=0.35156&zoom=3&layers=000000000B0 Atlogis Meta-Maps]: Online-Viewer for relief maps generated from SRTM-Data.
* [http://www.latlontoelevation.com/ LatLonToElevation.com] Free web application to extract SRTM elevation data given an input file of lat/lon coordinate pairs.
* [http://sourceforge.net/projects/srtm-matlab http://sourceforge.net/projects/srtm-matlab] MATLAB-based SRTM reader

[[Category:Geographic information systems]]
[[Category:Space radars]]
[[Category:Digital elevation models]]