Rivix.com: SRTM DEMs

SRTM (Shuttle Radar Topography Mission)

Description

The Shuttle Radar Topography Mission (SRTM) was a 10-day mission which flew on the Space Shuttle Endeavour in February 2000. It was a cooperative project between NASA and the National Geospatial Intelligence Agency (NGA, formerly NIMA and DMA) that collected high-resolution topographic data for most of the land surfaces between -56 and 60 degrees latitude. Data was collected using a method called single-pass radar interferometry, with two imaging radar systems on the Shuttle separated by a 60 meter mast. This technology is based on the Spaceborne Imaging Radar - C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) that was used on two previous Shuttle missions.

There are three different SRTM data sets that differ in their grid spacing:

SRTM1 has 1-arcsecond grid spacing and is available only for the USA.
SRTM3 has 3-arcsecond grid spacing and is available for the entire planet between latitudes of -56 and 60.
SRTM30 has 30-arcsecond grid spacing and is also available for the entire planet between latitudes of -56 and 60.

For each of these resolutions, there are now three official versions of the data, known as SRTM V1, V2 and V3. SRTM Version 1 had many data quality issues including vertical errors, water area "noise", single-pixel errors and nodata "holes" or voids (especially in areas of high elevation or steep slopes.) SRTM Version 2, known as the "finished version" fixed some issues, including the water area "noise" issue, but still had "gaps" or "holes". SRTM Version 3 (or SRTM Plus) was released by LPDAAC (Land Processes Distributed Active Archive Center) in November 2013 with all voids eliminated.

Other groups have also created void-filled versions of SRTM data. One of these is CGIAR-CSI (Consultative Group on International Agricultural Research, Consortium for Spatial Information), and their versions are called "Version 3" or "Version 4". The US Geological Survey also created their own void-filled and "hydrologically-sound" version to support the HydroSHEDS project.

Grid Information

Coord. System:	Geographic decimal degrees, fixed-angle
Cell Size:	SRTM1: 1 arcsecond (1/3600 of a degree; USA only) SRTM3: 3 arcseconds (1/1200 of a degree)
Tile Dimensions:	SRTM1: 3601 columns x 3601 rows (USA) SRTM3: 1201 columns x 1201 rows
Tile Span:	1 degree x 1 degree
Data Type:	2-byte integer, signed
Horizontal Datum:	WGS 84
Vertical Datum:	mean sea level
Vertical Units:	meters (EGM96 geoid)
Nodata Value:	-32768
Formats:	SRTM tiles are usually provided as simple binary files, with signed, 2-byte integer data given in row major format using big-endian byte order, without any header. There is no separate georeferencing information file, but the filename of each tile includes the latitude and longitude of the southwest corner of the tile and ends with the extension ".hgt". Other attributes are as described above. SRTM tiles are sometimes redistributed in the GeoTIFF format (e.g. various void-filled versions).

How to Import

Choose Import DEM → SRTM DEM (.hgt) from the File menu and then select the file, which will usually have the filename extension "*.hgt". If the DEM is in GeoTIFF format, instead choose GeoTIFF.

Data Source

The primary SRTM data (V1 and V2) is distributed by the EROS Data Center (EDC) of the US Geological Survey (USGS) and can be downloaded. SRTM V3 is currently available from LPDAAC.

Data Availability

Coverage includes most land surfaces between -56 and 60 degrees latitude. Within the USA, the full-resolution data will be released without restrictions. Outside the USA, the grid spacing is reduced from 1 to 3 arcseconds.

Web Sites

SRTM Home: http://www.jpl.nasa.gov/srtm

Data Products: http://www.jpl.nasa.gov/srtm/dataproducts.html

Data at the USGS: http://srtm.usgs.gov

Data for the Americas, Eurasia and Africa: ftp://edcsgs9.cr.usgs.gov/pub/data/srtm