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Shuttle Radar Topography Mission (SRTM) Void Filled

  

   These browse images of Mt. Rainier and Mt.
   Adams in the Cascade Mountain Range
   highlight the differences between SRTM
   (left) and SRTM Void Filled (right) data
   (February 11, 2000).

The Shuttle Radar Topography Mission (SRTM) was flown aboard the space shuttle Endeavour February 11-22, 2000. The National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA) participated in an international project to acquire radar data which were used to create the first near-global set of land elevations.

The radars used during the SRTM mission were actually developed and flown on two Endeavour missions in 1994. The C-band Spaceborne Imaging Radar and the X-Band Synthetic Aperture Radar (X-SAR) hardware were used on board the space shuttle in April and October 1994 to gather data about Earth's environment. The technology was modified for the SRTM mission to collect interferometric radar, which compared two radar images or signals taken at slightly different angles. This mission used single-pass interferometry, which acquired two signals at the same time by using two different radar antennas. An antenna located on board the space shuttle collected one data set and the other data set was collected by an antenna located at the end of a 60-meter mast that extended from the shuttle. Differences between the two signals allowed for the calculation of surface elevation.

Endeavour orbited Earth 16 times each day during the 11-day mission, completing 176 orbits. SRTM successfully collected radar data over 80% of the Earth's land surface between 60° north and 56° south latitude with data points posted every 1 arc-second (approximately 30 meters).

SRTM Data Products

The level of processing and the resolution of the data will vary by SRTM data set.

SRTM Non-Void Filled elevation data were processed from raw C-band radar signals spaced at intervals of 1 arc-second (approximately 30 meters) at NASA’s Jet Propulsion Laboratory (JPL). This version was then edited or finished by the NGA to delineate and flatten water bodies, better define coastlines, remove spikes and wells, and fill small voids. Data for regions outside the United States were sampled at 3 arc-seconds (approximately 90 meters) using a cubic convolution resampling technique for open distribution.

SRTM Void Filled elevation data are the result of additional processing to address areas of missing data or voids in the SRTM Non-Void Filled collection. The voids occur in areas where the initial processing did not meet quality specifications. Since SRTM data are one of the most widely used elevation data sources, the NGA filled the voids using interpolation algorithms in conjunction with other sources of elevation data. The resolution for SRTM Void Filled data is 1 arc-second for the United States and 3 arc-seconds for global coverage. 

SRTM 1 Arc-Second Global elevation data offer worldwide coverage of void filled data at a resolution of 1 arc-second (30 meters) and provide open distribution of this high-resolution global data set. Some tiles may still contain voids. The SRTM 1 Arc-Second Global (30 meters) data set will be released in phases starting September 24, 2014. Users should check the coverage map in EarthExplorer to verify if their area of interest is available. Please note that tiles above 50° north and below 50° south latitude are sampled at a resolution of 2 arc-second by 1 arc-second.

SRTM File Formats

EarthExplorer offers SRTM data with a regularly spaced grid of elevation points in three file formats:

  • Digital Terrain Elevation Data (DTED®) is a standard mapping format designed by the NGA. Each file or cell contains a matrix of vertical elevation values spaced at regular horizontal intervals measured in geographic latitude and longitude units. File size is approximately 25 MB for 1-arc-second data files and approximately 3 MB for 3-arc-second data files.
  • Band interleaved by line (BIL) is a binary raster format with an accompanying header file which describes the layout and formatting of the file. File size is approximately 7 MB for 1-arc-second data files and approximately 1 MB for 3- arc-second data files.
  • Georeferenced Tagged Image File Format (GeoTIFF) is a TIFF file with embedded geographic information. This is a standard image format for GIS applications. File size is approximately 25 MB for 1-arc-second data files and approximately 3 MB for 3-arc-second data files.

SRTM elevation data are intended for scientific use with a Geographic Information System (GIS) or other special application software.

Product Specifications

Projection Geographic
Horizontal Datum WGS84
Vertical Datum EGM96 (Earth Gravitational Model 1996)
Vertical Units Meters
Spatial Resolution 1 arc-second for global coverage (~30 meters)
3 arc-seconds for global coverage (~90 meters)
Raster Size 1 degree tiles
C-band Wavelength 5.6 cm

Additional SRTM Products are available through collaborating agencies:

Research grade SRTM (C-band) data are available through NASA JPL. These data were sampled at 3 arc-seconds using a nearest neighbor resampling technique for global coverage.

The German and Italian space agencies operated the X-band hardware and processed the data independently into a separate elevation data set. The SRTM/X-SAR data may be obtained through the German Aerospace Center (DLR).

Data Use and Citation Policies

For information on how to cite USGS data plus guidance on usage restrictions,
please see our Data Citations and Use page at https://lta.cr.usgs.gov/citation.

Any use of trade, firm, or product names is for descriptive purposes only and
does not imply endorsement by the U.S. Government.

Contact Information for User Assistance

Long Term Archive User Services
U.S. Geological Survey (USGS)
Earth Resources Observation and Science (EROS) Center
47914 252nd Street
Sioux Falls, SD 57198-0001

Phone Number: 605-594-6151
Toll Free: 800-252-4547
Fax: 605-594-6589

Email:lta@usgs.gov
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