Features – Digital Elevation Models

Use of Digital Elevation Maps (SRTM and ASTER)

1. General

The HERALD user needs to download SRTM or ASTER files covering the region where the radio network is (will be) deployed.

Downloaded files are in zip format and are to be decompressed to get the *.hgt (SRTM) or *.tif (ASTER) file (see below about file formats). It is suggested to check the (uncompressed) file size, which is expected to be 2.884.802 bytes for 3 arcsec resolution SRTM maps, 25.934.402 bytes for 1arcsec resolution SRTM maps, and 25.963.722 bytes for ASTER maps (HERALD does not accept any other file size).

It is suggested that all SRTM map files be copied in a unique folder and all ASTER map files be copied in a different folder. During program execution, the operator can select / modify the map folder path.

2. SRTM Maps

The Shuttle Radar Topography Mission (SRTM) is an international project organized by the National Geospatial-Intelligence Agency (NGA) and the National Aeronautics and Space Administration (NASA). The SRTM measurements were collected during an 11-day mission of the Space Shuttle Endeavour in February 2000 and produced elevation data on a near-global scale (from 56°S to 60°N) For additional information on SRTM measurement and processing techniques, see the official NASA SRTM site at http://www2.jpl.nasa.gov/srtm/.

2.1. Data Format and File Naming

SRTM terrain elevation data are organized in ‘tiles’, each tile covering a 1deg x 1deg region, with 3arcseconds resolution; for some regions also 1arcseconds resolution data are available. Each tile corresponds to a data file, with file extension *.hgt.

File names make reference to the latitude and longitude of the lower left (south western) corner of the tile – e.g. N37W105 has its lower left corner at 37 degrees north latitude and 105 degrees west longitude. Note that a point in the western hemisphere with longitude Wxxx is found in the tile with lower left corner at longitude W(xxx+1); similarly, a point in the southern hemisphere with latitude Syy is found in the tile with lower left corner at latitude S(yy+1). Some examples :

(N25 51 12) (E80 46 21) is found in file N25E080.hgt.
(N25 51 12) (W80 46 21) is found in file N25W081.hgt.
(S00 51 12) (W80 46 21) is found in file S01W081.hgt.
(N00 51 12) (W80 46 21) is found in file N00W081.hgt.

Find more about *.hgt files at http://www.viewfinderpanoramas.org/dem3.html#hgt.

2.2. Data Sources

The original source of SRTM NASA files is at the US Geological Survey website at http://dds.cr.usgs.gov/srtm/version2_1/SRTM1/ (for 1arcsec resolution, when available) and http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/ (for 3arcsec resolution). NASA files incorporate some inaccuracies in water bodies and coastlines contours as well as some random spikes and wells (single pixel errors) and areas of missing data (‘voids’). Most of the above problems have been removed in the above referred Version2 release.

An alternative source is http://www.viewfinderpanoramas.org/dem3.html, where most NASA no-data areas are fixed by incorporating data from other accurate sources and better accuracy is expected, particularly in mountainous regions. This site provides also a number of alternative sources, with useful suggestions. North America is not covered by viewfinderpanoramas, but suitable links are given.

Check the above mentioned websites for updated information.

3. ASTER Maps

The ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) project is organized by the Ministry of Economy, Trade, and Industry (METI) of Japan and the United States National Aeronautics and Space Administration (NASA).
The first version of the ASTER GDEM, released in June 2009, was generated using stereo-pair images collected by the ASTER instrument onboard Terra. ASTER GDEM coverage spans from 83 degrees north latitude to 83 degrees south (99% of Earth’s landmass), with 1arcsecond resolution.

The improved GDEM V2 (released Nov. 2011) adds 260,000 additional stereo-pairs, improving coverage and reducing the occurrence of artifacts. The refined production algorithm provides improved spatial resolution, increased horizontal and vertical accuracy, and superior water body coverage and detection. For additional information on the ASTER project, see the NASA ASTER web page at http://asterweb.jpl.nasa.gov

3.1. Data Format and File Naming

ASTER terrain elevation data are organized in ‘tiles’, each tile covering a 1deg x 1deg region, with 1arcsecond resolution. Each tile corresponds to a data file, in GeoTIFF format, with file extension *.tif.

The file name format is “ASTGTM2_<coord>_dem”, where <coord> makes reference to the latitude and longitude of the lower left (south western) corner of the tile, with the same convention as in naming SRTM files (see above).

3.2. Data Sources

The original sources of ASTER files are:
NASA’s EOS Data Archive at http://asterweb.jpl.nasa.gov/gdem-wist.asp
Japan’s ASTER GDEM Website at http://www.jspacesystems.or.jp/ersdac/GDEM/E/index.html

Free registration is required at both websites. Access to the Japan’s data archive appears to be simpler than to the NASA’s archive.

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