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 (elevation) 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. It follows that "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 supported by several software developments.
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).
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. 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
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, but coverage is limited to areas of high mountain void incidence, and some areas north of 60 degrees of latitude.
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.
- 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
- "Shuttle Radar Topography Mission: Mission to Map the World". Retrieved 2009-04-26.
- Nikolakopoulos 2006, p. 2
- "SRTM-derived 1 Second Digital Elevation Models Version 1.0".
- Nikolakopoulos 2006, p. 3
- Reuter H.I, A. Nelson, A. Jarvis, 2007, An evaluation of void filling interpolation methods for SRTM data, International Journal of Geographic 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
- Nikolakopoulos, K. G.; Kamaratakis, E. K; Chrysoulakis, N. (10 November 2006). "SRTM vs ASTER elevation products. Comparison for two regions in Crete, Greece". International Journal of Remote Sensing 27 (21). ISSN 0143-1161. Retrieved March 10, 2010.
- Farr, Tom G.; Rosen, Paul A.; Caro, Edward; Crippen, Robert; Duren, Riley; Hensley, Scott; Kobrick, Michael; Paller, Mimi et al. (2007). "The Shuttle Radar Topography Mission". Reviews of Geophysics 45: RG2004. Bibcode:2007RvGeo..45.2004F. doi:10.1029/2005RG000183.
- Hirt, C.; Filmer, M.S.; Featherstone, W.E. (2010). "Comparison and validation of recent freely-available ASTER-GDEM ver1, SRTM ver4.1 and GEODATA DEM-9S ver3 digital elevation models over Australia.". Australian Journal of Earth Sciences 57 (3): 337–347. Bibcode:2010AuJES..57..337H. doi:10.1080/08120091003677553. Retrieved May 5, 2012.
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.
- Official NASA SRTM site (SRTM V1 and V2)
- NASA MEaSUREs Products (SRTM V3 and more)
- NASA's server with SRTM data tiles – Please read the accompanying documentation
- Digital elevation data from Geoscience Australia – arcsecond-resolution derived data covering Australia
- Maps-For-Free.com Free global relief maps
- GeoSAR, the Testbed for SRTM – Currently owned and operated by Fugro EarthData
- 1-Degree SRTM data tiles in GeoTIFF format – UMD's Global Land Cover Facility
- Void filled SRTM data at CGIAR-CSI and Droppr
- USGS HydroSHEDS – Full resolution SRTM-based DEM for hydrological applications
- Software that can read and process SRTM data: 3dem, GRASS GIS, SAGA GIS, MapWindow GIS, DG Terrain Viewer/Void Killer, Virtual Terrain Project
- Mapping data – SRTM with fision tables
- Viewfinder Panoramas – Unofficial SRTM data with voids corrected using topographic maps
- Discussion forum for SRTM data users
- Atlogis Meta-Maps: Online-Viewer for relief maps generated from SRTM-Data.
- http://sourceforge.net/projects/srtm-matlab MATLAB-based SRTM reader
- readhgt.m Matlab function to download/import SRTM data files