TheShuttle Radar Topography Mission (SRTM) is an international research effort that obtaineddigital elevation models on a near-global scale from56°S to60°N,[2]: 4820 to generate the most complete high-resolution digital topographic database of Earth prior to the release of theASTER GDEM in 2009. The technique employed for generatingtopographic data by radar is known asinterferometric synthetic aperture radar. It flew onboard the 11-daySTS-99 mission in February 2000.
Intermap Technologies was the prime contractor for processing the interferometric synthetic aperture radar data.[citation needed] The elevation models derived from the SRTM data are used ingeographic information systems. They can be downloaded freely over the Internet, and their file format (.hgt) is widely supported.
The Shuttle Radar Topography Mission is an international project spearheaded by the U.S. National Geospatial-Intelligence Agency (NGA), an agency of theU.S. Department of Defense, and the U.S. National Aeronautics and Space Administration (NASA).
The mission consists of aninterferometric synthetic aperture radar system based on the olderSpaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR), previously used on the Shuttle in 1994. It features two antennas, a critical change from SIR-C/X-SAR, allowing single-pass interferometry. One antenna was located in the Shuttle's payload bay, like in SIR-C/X-SAR. The other was located on the end of a 60-meter (200-foot) mast that extended from the payload bay once the Shuttle was in space.[2]
Like in SIR-C/X-SAR, the SRTM radar antennas work in both X-band and C-band. C-band provides wideraperature and hence wider coverage under the tracks, whereas the X-band has a narrower aperature but higher resolution.[3] The SRTM mission orbit was designed for the coverage of the American C-band mission, not the German-Italian X-band mission, hence the many gaps in X-band coverage.[4]
The American data releases are based on the C-band data whereas the German data releases are based on the X-band data. No merging of the two bands have been done.[6] All C-band processing was done on the 1-arcsecond (1″) resolution level.[7]
SRTM void filling with spline interpolation inGRASS GIS.
The C-band elevation datasets are affected by mountain and desert no-data areas. These amount to no more than 0.2% of the total area surveyed,[8] 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 onlyinterpolation 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.
Relief map ofSierra Nevada, SpainExample of relief map from SRTM1 (central Nevada)
The elevation models are arranged into tiles, each covering onedegree of latitude and one degree of longitude, named according to their south western corners. For example, "n45e006" stretches from45°N6°E to46°N7°E and "s45w006" from45°S6°W to44°S5°W. The resolution of the raw data is onearcsecond (1″, 30 m along the equator) and coverage includes Africa, Europe, North America, South America, Asia, and Australia.[9] For the rest of the world, only three arcsecond (3″, 90 m along the equator) data are available.[2]: 4821
Each 1″ tile has 3,601 rows, each consisting of 3,60116 bitbigendian cells. The dimensions of the 3″ tiles are 1201 1201. The original SRTM elevations were calculated relative to theWGS84 ellipsoid and then theEGM96 geoid separation values were added to convert to heights relative to the geoid for all the released products.[10]
Version 2.1 (~2005) is an edited version of v1. Artifacts are removed, but large voids are not yet filled. There are 1-arcsecond (1″) data over the US.[6]
Version 3 (2013), also known as SRTM Plus, is void-filled. It features global 3″ data and US 1″ data.[12] It was released by NASA LP DAAC in November 2013.[13] Voids were filled primarily fromASTER GDEM2, and secondarily from USGS GMTED2010 – or USGS National Elevation Dataset (NED) for the United States (except Alaska) and northernmost Mexico according to the announcement.
SRTM-GL1 (2014), global 1-arcsecond (30 meter) release sharing the "version 3" mark.[14][7]
The terminology regarding versions and resolutions can be confusing. "SRTM1" and "SRTM3" refers to the resolutions in 1 and 3 arc-seconds, not the versions of the format. On the other hand, "SRTM4.1" refers to a specific filled version by CGIAR-CSI. It is recommended to add a "v" in front of the version number to disambiguate.
TheNational Geospatial-Intelligence Agency is responsible for most of the data cleanup work seen in version 2.1. It maintains its own high-resolution version and a number of undisclosed void-filled versions containing data from additional sources. Such an undisclosed version was used to fill the voids in ASTER GDEM2, which was in turn used to fill the voids in SRTM version 3.[7]
SRTM-GL1 is a void-filled digital elevation model with 1-arcsecond (30 meter) resolution, or alternatively a high-resolution version of "SRTM version 3". It was released in 2014. It is available from theUnited States Geological Survey web site[15] and the NASA data catalog.[14]
The United States Government announced on September 23, 2014 over a United Nations Climate Summit that the highest possible resolution of global topographic data derived from the SRTM mission will be released to public.[16] Before the end of the same year, a 1-arc second global digital elevation model (30 meters) was released. Most parts of the world have been covered by this dataset ranging from 54°S to 60°N latitude except for the Middle East and North Africa area.[15] Missing coverage of the Middle East was completed in August 2015.[17]
Jonathan de Ferranti published a short review of the new SRTM-GL1 data product in 2015. The effective resolution is about 50 metres, compared with 100 meters for versions 1 and 2 of ASTER GDEM. Voids remain around Mount Everest and the Swiss/Italian Matterhorn. There are some artificial details (bumps and pits), but at a lower amplitude than ASTER GDEM.[18]
Groups of scientists have worked on algorithms to fill the voids of the original SRTM (v2.1) data. Three datasets offer global coverage void-filled SRTM data at full (3-arcsecond) resolution:
The CGIAR-CSI version 4 provides global coverage using interpolation. The latest version is 4.1 of 2007. The resolution is 3″ or 90 m. Data sources include SRTM version 2 (3″) and a number of auxillary DEMs of comparable resolution.[19]
The USGS HydroSHEDS 3″ (90 m) dataset was generated for hydrological applications and is suitable for consistent drainage and water flow information. References are provided on the algorithms used and quality assessment.[20] HydroSHEDS has since been spun off into its own website with many derived products.[21] As of December 2025, a 12 m HydroSHEDS v2 based onTanDEM-X data is being worked on.
The void-filled SRTM data from Viewfinder Panoramas by Jonathan de Ferranti[22] are high quality at full SRTM resolution. The data is filled using local survey maps and photographs. The OpenTopoMap website uses this fill. 3″ and 15″ resolution globally, with 1″ resolution for: USA, Canada, Europe, Antarctica, New Zealand, Greenland, Scandinavia (last updated 2022).[23] Future 1″ data will be based primarily on SRTM-GL1.[18]
Due to how radar works, the SRTM data is contaminated by non-terrain features such as trees and buildings.
Geoscience Australia released a derived 1″ dataset with trees and other vegetation features removed covering Australia in November 2011 under the CC-BY 4.0 license. There are three versions: one deriving from direct removal of vegetation using vegetation maps, one derived from smoothing of the former, and one derived by hydrological enforcement (i.e. adjusting the elevation to match known water flow paths) of the smoothed version.[24]
The SRTM also carries the X-SAR instrument operated by theGerman Aerospace Center (DLR) andItalian Space Agency (ASI). The resulting dataset is usually called SRTM/X-SAR, or SRTMX for short. The grid resolution is high at 25 meters, but it has many gaps due to the narrower instrument aperture (only capturing 50 km wide areas). The data was made public in May 2011.[26][27] A visualization of SRTM/X-SAR coverage is available from the EOC Geoservice of the Earth Observation Center (EOC) of the German Aerospace Center (DLR), which also offers downloads.[4]
^Reuter H.I, A. Nelson, A. Jarvis, 2007,An evaluation of void filling interpolation methods for SRTM data, International Journal of Geographical Information Science, 21:9, 983–1008 – 'the ‘finished’ grade version of the data (also referred to as Version 2) still contains data voids (some 836,000 km^2)'; 836,000 is 0.164% of the Earth's 5.1×10^8 km^2 surface
Hennig, T., Kretsch, J, Salamonowicz, P, Pessagno, C, and Stein, W., The Shuttle Radar Topography Mission, Proceedings of the First International Symposium on Digital Earth Moving 2001, Springer Verlag, London, UK.
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