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) product. The 3 arc second data was derived from the 1 arc second using averaging 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 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.
The SRTMGL3 data were generated from SRTM1GL data that fall within that tile. These elevation files use the extension “.HGT”, meaning height (such as N37W105.SRTMGL3.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 global 3 arc second SRTM product is also available in NetCDF4 format as the SRTMGL3_NC dataset with the source of each elevation pixel in the corresponding SRTMGL3_NUMNC product.
|File Size||~0.98 MB|
|Temporal Extent||2000-02-11 to 2000-02-21|
|Spatial Extent||Global (60°N to 56°S, 180°W to 180°E)|
|Coordinate System||Geographic Latitude and Longitude|
|File Format||HGT or NetCDF4|
|Geographic Dimensions||1° x 1°|
|Number of Science Dataset (SDS) Layers||1|
|Columns/Rows||1201 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|
Scientists used a method called Kinematic Global Positioning System Geodetic field surveying to validate 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 were deployed with precisely measured coordinates and served as control points in the SRTM data.
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. https://doi.org/10.14358/PERS.72.3.249