NASA Shuttle Radar Topography Mission Combined Image Data Set

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 1 arc second (~30 meter) combined (merged) image data product. (See User Guide Section 2.2.2)

The combined image data set contains mosaicked one degree by one degree images/tiles of uncalibrated radar brightness values at 1 arc second. To create a smooth mosaic image, each pixel in an output is an average of all the image pixels for a location. Pixels with a value of zero (voids) were not counted. Because SRTM imaged a given location with two like-polarization channels (VV = vertical transmit and vertical receive, and HH = horizontal transmit and horizontal receive) and at a variety of look and azimuth angles, the quantitative scattering information was lost in the pursuit of a smoother image product unlike the SRTM swath image product SRTMIMGR, which preserved the quantitative scattering information.

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 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

  • Version 3.0 products are 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~6.92 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 FormatIMG and NUM
Geographic Dimensions1° x 1°


Characteristic Description
Number of Science Dataset (SDS) Layers2
Columns/Rows3601 x 3601
Pixel Size~30 m

Layers / Variables

SDS Name Description Units Data Type Fill Value No Data Value Valid Range Scale Factor
IMG Radar Brightness Values N/A 8-bit signed integer 0 N/A N/A N/A
NUM¹ Number of Averaged Pixels Number 8-bit signed integer 0 N/A 0 to 10 N/A

¹In some instances the number of averaged pixels may exceed 10.

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 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, 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.

About the image

NASA Shuttle Radar Topography Mission Combined Image Data Set Radar Brightness Values from the SRTMIMGM product over the Democratic Republic of the Congo, 2000.

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User Guide
Algorithm Theoretical Basis Document (ATBD)
SRTM Quick Guide
Shuttle Radar Topography Mission (JPL) Webpage

Using the Data

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