NASA Shuttle Radar Topography Mission Global 3 arc second sub-sampled

PI: Michael Kobrick, Robert Crippen


The Land Processes Distributed Active Archive Center (LP DAAC) is responsible for the archive and distribution of NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) SRTM, which includes the global 3 arc second (~90 meter) sub-sampled product. The 3 arc second data was derived from the 1 arc second using sampling methods. (See Figure 3 in the User Guide)

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. This collaboration aims to generate a near-global digital elevation model (DEM) of Earth using radar interferometry. 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.

The SRTMGL3 data were sub-sampled from SRTM1GL data that fall within that tile. These elevation files use the extension “.HGT”, meaning height (such as N37W105.SRTMGL3S.HGT). The primary goal of creating the Version 3 data was to eliminate gaps, or voids, that were present in earlier versions of SRTM data. In areas with limited data, existing topographical data were used to supplement the SRTM data to fill the voids. The source of each elevation pixel is identified in the corresponding SRTMGL3N product (such as N37W105.SRTMGL3N.NUM).

The SRTM swaths extended from ~30 degrees off-nadir to ~58 degrees off-nadir from an altitude of 233 kilometers (km), creating swaths ~225 km wide, and consisted of all land between 60° N and 56° S latitude to account for 80% of Earth’s total landmass.

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Improvements/Changes from Previous Versions

  • Voids in the Version 3.0 products have been filled with ASTER Global Digital Elevation Model (GDEM) Version 2.0, the Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010), and the National Elevation Dataset (NED).

Collection and Granule


Characteristic Description
CollectionMEaSUREs SRTM
File Size~1.02 MB
Temporal ResolutionMulti-Day
Temporal Extent2000-02-11 to 2000-02-21
Spatial ExtentGlobal (60°N to 56°S, 180°W to 180°E)
Coordinate SystemGeographic Latitude and Longitude
File FormatHGT
Geographic Dimensions1° x 1°


Characteristic Description
Number of Science Dataset (SDS) Layers1
Columns/Rows1201 x 1201
Pixel Size~90 m


SDS Name Description Units Data Type Fill Value No Data Value Valid Range Scale Factor
DEM Elevation Meters 16-bit signed integer N/A N/A -32767 to 32767 N/A

Product Quality

Scientists used a method called Kinematic Global Positioning System Geodetic field surveying to validate the SRTM data. This method facilitates the very rapid long lines of precise positions from a moving vehicle. Several entities conducted the actual survey work, including private contractors, NGA geodesists and NASA Jet Propulsion Laboratory (JPL) scientists. In all, about 70,000 kilometers of survey lines were collected in support of this mission. The data were used to model long-wavelength error sources.

In addition, NASA’s JPL deployed corner reflectors during the mission. These are highly reflective structures that appear as a bright point in the radar image. These reflectors deployed with precisely measured coordinates, served as control points in the Shuttle Radar Topography Mission data.

Known Issues

Known issues in the NASA SRTM are described in the following publication: Rodriguez, E., C. S. Morris, and J. E. Belz (2006), A global assessment of the SRTM performance, Photogramm. Eng. Remote Sens., 72, 249–260.


Using the Data

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About the Image

NASA Shuttle Radar Topography Mission Global 3 arc second sub-sampled elevation data from the SRTMGL3S product over the Democratic Republic of the Congo, 2000.

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