NASA Shuttle Radar Topography Mission United States 1 arc second number

PI: Michael Kobrick, Robert Crippen

The SRTMUS1N collection was retired when global SRTM data became available at a resolution of 1 arc second. The data for the United States are included in the SRTMGL1N dataset.


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) Shuttle Radar Topography Mission (SRTM), which includes the United States 1 arc second (~30 meter) number product.

Ancillary one-byte (0 to 255) “NUM” (number) files were produced for NASA SRTM Version 3. These files have names corresponding to the elevation files, except with the extension “.NUM” (such as N37W105.NUM). The elevation files use the extension “.HGT”, meaning height (such as N37W105.HGT). The separate NUM file indicates the source of each DEM pixel; the number of ASTER scenes used (up to 100), if ASTER; and the number of SRTM data takes (up to 24), if SRTM. The NUM file for both 3 arc second products (whether sampled or averaged) references the 3 x 3 center pixel.

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.

read more


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~0.21 MB
Temporal ResolutionMulti-Day
Temporal Extent2000-02-11 to 2000-02-21
Spatial ExtentUnited States
Coordinate SystemGeographic Latitude and Longitude
File FormatNUM
Geographic Dimensions1 degree lat x 1 degree lon


Characteristic Description
Number of Science Dataset (SDS) Layers1
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
NUM Source of Input Data N/A 8-bit unsigned integer N/A N/A 1 to 224 N/A

Source of Input Data

Value Description
1 Water-masked SRTM void
2 Water-masked SRTM non-void
5 GDEM elevation = 0 in SRTM void (used to help correct ocean masking)
11 NGA-interpolated SRTM
21 GMTED2010 oversampled from 7.5 arc second postings
25 SRTM within GDEM
31 NGA fill of SRTM via GDEM
53 Alaska USGS NED via GDEM
72 Canadian Digital Elevation Data (CDED) via GDEM
101 to 200 ASTER granule count (count limited to 100)
201 to 224 SRTM swath count (non-voided swaths), actual maximum = 24

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.


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

Using the Data

Access Data