The Shuttle Radar Topography Mission (SRTM) is a collaborative effort from NASA (National Aeronautics and Space Administration) and NGA (National Geospatial-Intelligence Agency) as well as DLR (Deutsches Zentrum für Luft-und Raumfahrt) and ASI (Agenzia Spaziale Italiana). SRTM was flown aboard the Endeavour space shuttle in February 2000 to provide a high-resolution Digital Elevation Model (DEM). The SRTM instrumentation consisted of the Spaceborne Imaging Radar-C (SIR-C) with an additional antenna to form a 60 meters long baseline. As a result of the SRTM mission, several DEM versions have been released since 2003, which differ in terms of data processing and procedures applied for the filling of voids (areas not or poorly observed by the SRTM radar observations).

SRTM v3.0 (SRTM Plus) is the newest version, published in 2015 by NASA as a part of NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) project, which incorporates topographic data to fill the gaps or voids in earlier versions of SRTM data. For the void filling with the Delta Surface Fill algorithm, ASTER DEMs have been used as auxiliary data source, or interpolations have been applied. Many variants of DEM are available in SRTM v3.0, with SRTMGL1 being one of the key products from SRTM v3.0. ‘GL1’ on its name stands for “Global 1-arc second”. It provides regularly spaced DEM grids of 1 arc-second (approximately 30 meters) and covering 80% of Earth’s landmass, between 60° North and 56° South. This product is divided into 1° x 1° latitude and longitude tiles in “geographic” projection, as shown here.

A typical file of the SRTMGL1 dataset requires 25 MB memory (without compression) and stores exactly one 1°x1° tile; it contains 3,601 lines and 3,601 columns, which sum up to around 100 GB (compressed) and 350 GB (uncompressed) for the global data set of 14297 tiles. Individual tile names refer to the latitude and longitude of southwest (lower left) corner of the tile, e.g., tile N20W030 has lower left corner at 20°N and 30°W, covering area of 20-21°N and 30-29°W. The absolute vertical accuracy for SRTM heights has been found to be ~9 m (90 % confidence) or better (Rodriguez et al. 2005).

Geodetic information: The SRTM GL1 DEMs are vertically referenced to the EGM96 geoid and horizontally referenced to the WGS84 (World Geodetic System 1984).

Further notes: The SRTM DEM represents bare ground elevations only where vegetation cover and buildings are absent. Over most areas, the DEM elevations reside between the bare ground (terrain) and top of canopies (surface), so are technically a mixture of terrain and surface models. Few artefacts, e.g., pits or spikes may still be present in the data set.

Data access: The homepage of SRTM mission is http://www2.jpl.nasa.gov/srtm/. SRTM v3.0 datasets can be searched in MEASURES webpage and acquired freely from USGS website (http://earthexplorer.usgs.gov/) and USGS data pool (http://e4ftl01.cr.usgs.gov/SRTM/).

References:

Farr, T.G., E. Caro, R. Crippen, R. Duren, S. Hensley, M. Kobrick, M. Paller, E. Rodriguez, P. Rosen, L. Roth, D. Seal, S. Shaffer, J. Shimada, J. Umland, M. Werner, 2007, The Shuttle Radar Topography Mission. Reviews of Geophysics, volume 45, RG2004, doi:10.1029/2005RG000183.

NASA, The Shuttle Radar Topography Mission (SRTM) Collection User Guide. Available on https://lpdaac.usgs.gov/sites/default/files/public/measures/docs/NASA_SRTM_V3.pdf

Rodriguez, E., C.S. Morris, J.E. Belz, E.C. Chapin, J.M. Martin, W. Daffer, S.Hensley, 2005, An assessment of the SRTM topographic products, Technical Report JPL D-31639, Jet Propulsion Laboratory, Pasadena, California, 143 pp. available on http://www2.jpl.nasa.gov/srtm/SRTM_D31639.pdf

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