Shuttle Radar Topography Mission

The Shuttle Radar Topography Mission (SRTM) is an international research effort that obtained digital elevation models on a near-global scale from 56° S to 60° 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 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 topographic data, the SRTM payload was outfitted with two radar antennas. One antenna was located in the Shuttle's payload bay, the other – a critical change from the SIR-C/X-SAR, allowing single-pass interferometry – 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 of latitude and one degree of longitude, named according to their south western corners. For example, "n45e006" stretches from 45°N 6°E to 46°N 7°E and "s45w006" from 45°S 6°W to 44°S 5°W. The resolution of the raw data is one arcsecond (30 m), but this has only been released over United States territory. A derived one arcsecond dataset with trees and other non-terrain features removed covering Australia was made available in November 2011; the raw data are restricted for government use. For the rest of the world, only three arcsecond (90 m) data are available. Each one arcsecond tile has 3,601 rows, each consisting of 3,601 16 bit bigendian cells. The dimensions of the three arcsecond tiles are 1201 x 1201. The original SRTM elevations were calculated relative to the WGS84 ellipsoid and then the EGM96 geoid separation values were added to convert to heights relative to the geoid for all the released products.

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 widely supported.

The Shuttle Radar Topography Mission is an international project spearheaded by the U.S. National Geospatial-Intelligence Agency (NGA) and the U.S. National Aeronautics and Space Administration (NASA). NASA transferred the SRTM payload to the Smithsonian National Air and Space Museum in 2003; the canister, mast, and antenna are now on display at the Steven F. Udvar-Hazy Center in Chantilly, Virginia.

No-data areas
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 1-arc-second for the United States and 3-arc-second for the rest of the world, 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.

Void-filled SRTM datasets


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 CGIAR-CSI versions and the 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. References are provided on the algorithms used and quality assessment. The void-filled SRTM data from Viewfinder Panoramas are high quality at full SRTM resolution. Since November 2012 there is free and global coverage at 3 arc seconds available.

In November 2013, LP DAAC released the NASA Shuttle Radar Topography Mission (SRTM) Version 3.0 (SRTM Plus) Product collection with all voids eliminated. Voids were filled primarily from ASTER 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.

Highest Resolution Global Release
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. And before the end of the same year, a 1-arc second global digital elevation model (30 meters) has been released. Most part of the world has been covered by this dataset ranging from 54 degrees south to 60 degrees north latitude except for the Middle East and North Africa area. MIssing coverage of the Middle East was completed in August 2015.

Users
In early June 2011, there were 750,000 confirmed users of SRTM topography dataset. Users in 221 countries have accessed the site.