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NASA Shuttle Radar Topography Mission Global 30 arc second (SRTMGL3)

    lpdaac image
 
    Sample image of SRTMGL3
 

The NASA SRTM data sets result from a collaborative effort by the National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA - previously known as the National Imagery and Mapping Agency, or NIMA), as well as the participation of the German and Italian space agencies, to generate a near-global digital elevation model (DEM) of Earth using radar interferometry.  The SRTM instrument consisted of the Spaceborne Imaging Radar-C (SIR-C) hardware set modified with a Space Station-derived mast and additional antennae to form an interferometer with a 60-meter long baseline (Kobrick, 2006). A description of the SRTM mission can be found in Farr and Kobrick (2000) and Farr et al. (2007), and radar interferometry is explained in Rosen et al. (2000).

Synthetic aperture radars are side-looking instruments and acquire data along continuous swaths.  The NASA SRTM swaths extended from about 30 degrees off-nadir to about 58 degrees off-nadir from an altitude of 233 km, and thus were about 225 km wide. During the data flight the instrument was operated at all times the orbiter was over land and about 1000 individual swaths were acquired over the ten days of mapping operations. The length of the acquired swaths ranges from a few hundred to several thousand km. Each individual data acquisition is referred to as a "data take."

SRTM was the primary (and virtually only) payload on the STS-99 mission of the Space Shuttle Endeavour, which launched February 11, 2000 and flew for 11 days. Following several hours for instrument deployment, activation and checkout, systematic interferometric data were collected within a 222.4-hour period. The instrument operated almost flawlessly and imaged 99.96% of the targeted landmass at least one time, 94.59% at least twice and about 50% at least three or more times. The goal was to image each terrain segment at least twice from different angles (on ascending, or north-going, and descending, or south-going, orbit passes) to fill in areas shadowed from the radar beam by terrain.

This 'targeted landmass' consisted of all land between 56 degrees south and 60 degrees north latitude, which comprises almost exactly 80% of Earth’s total landmass.  The coverage reaches somewhat further north than south because the side-looking radar looked toward the north side of the Shuttle.

NASA SRTM data have undergone a sequence of processing steps resulting in data versions that have different characteristics.  All processing has occurred at one-arc-second (about 30 meters) postings.  Three-arc-second (about 90 meters) data are freely available for worldwide coverage.  One-arc-second data coverage is freely available for only the United States and its territories.  NASA SRTM version 1.0 included.

NASA SRTM data have undergone a sequence of processing steps resulting in data versions that have different characteristics.  All processing has occurred at one-arc-second (about 30 meters) postings.  Three-arc-second (about 90 meters) data are freely available for worldwide coverage.  One-arc-second data coverage is freely available for only the United States and its territories.  NASA SRTM version 1.0 included radar echo data that were processed in a systematic fashion using the SRTM Ground Data Processing System (GDPS) supercomputer system at the NASA Jet Propulsion Laboratory. This processor transformed the radar echoes into strips of digital elevation data, one strip for each of the 1000 or so data swaths. These strips were eventually mosaicked into 14,278 one degree by one degree tiles.  The data were processed on a continent-by-continent basis beginning with North America and proceeding through South America, Eurasia, Africa, Australia and Islands, with data from each continent undergoing a “block adjustment” to reduce residual errors.  Each NASA SRTM data tile contains a mosaic and blending of elevations generated by averaging all data takes that fall within that tile. Since the primary error source in synthetic aperture radar data is speckle, which has the characteristics of random noise, combining data through averaging reduced the error by the square root of the number of data takes used. NASA SRTM data takes ranged from a minimum of one (about 5% of the coverage) up to as many as 24 (very little of the coverage).  Typical coverage was 2-3 data takes.  For NASA SRTM Version 2.0 NGA applied several post-processing procedures to the NASA SRTM data including editing, spike and pit removal, water body leveling and coastline definition. Following these "finishing" steps data were returned to NASA for distribution to the scientific and civil user communities as well as the public. NASA SRTM Version 2.1 corrected some minor errors found in the original Version 2.0 three-arc-second product.

Version 3:  Elimination of the voids in the NASA SRTM DEM was the primary goal of a project under the NASA MEaSUREs (Making Earth System Data Records for Use in Research Environments) Program.  Ultimately this was achieved by filling the voids with elevation data primarily from the ASTER GDEM2 (Global Digital Elevation Model Version 2) and secondarily from the USGS GMTED2010 elevation model or the USGS National Elevation Dataset (NED). 

NASA SRTM height data are distributed in two detail levels: NASA SRTM1 (for the U.S. and its territories and possessions) with data postings at one-arc-second intervals in latitude and longitude, and NASA SRTM3 (for the world) sampled at three arc-seconds.  NASA SRTM V3.0 three-arc-second data are provided in two forms: (1) by three-by-three averaging of the one arc-second samples, and (2) by extracting the middle sample of those same three-by-three samples.

File names refer to the latitude and longitude of the southwest (lower left) corner of the tile.  For example, N37W105 has its lower left corner at 37 degrees north latitude and 105 degrees west longitude and covers the area 37-38˚N and 104-105˚W. To be more exact, the file name coordinates refer to the geometric center of the lower left pixel, and all edge pixels of the tile are centered on full-degree lines of latitude and/or longitude.

Height files have the extension .HGT and are signed two-byte (16-bit) integers. The bytes are in Motorola "big-endian" order with the most significant byte first. Most PCs and Macintosh computers built after 2006 use Intel ("little-endian") order so byte swapping may be necessary. Some software performs the swapping during ingest.  Heights are in meters referenced to the WGS84/EGM96 geoid.

SRTMGL3 files contain 1201 lines and 1201 samples. The rows at the north and south edges as well as the columns at the east and west edges of each tile overlap, and are identical to, the edge rows and columns in the adjacent tile. The data are stored in row major order, meaning all the data for the northernmost row (row 1) is followed by all the data for row 2, etc. There are no header or trailer bytes embedded in the file.

Data Set Characteristics

Tile Size 1201 x 1201 (3 degree by 3 degree)
Pixel Size 3 arc second
Geographic coordinates Geographic latitude and longitude
DEM output format HGT, signed 16 bits, in units of vertical meters
Geoid reference WGS84/EGM96
Special DN values N/A No voids in V3
Tile volume 20GB
Coverage North 60 degrees to south 56 degrees, 22,702 tiles

Additional Information:

Shuttle Radar Topography Mission information:  http://www2.jpl.nasa.gov/srtm/

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