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SRTM

SRTM stands for Shuttle Radar Topography Mission; is a digital elevation model that is derived from radar interferometry data obtained during a space shuttle mission.

Coverage:
The dataset covers most of the globe, except polar regions, i.e. between 56° S and 60° N, in a resolution of 3 arc seconds. However, it contains several holes that need to be filled using interpolation.
License:
The original data belongs to the public domain. However, reprocessed datasets can have different licenses; e.g. the CIGAR dataset is free only for non-commercial use.
Author:
It is an international project under the lead of NGA and NASA.

Obtaining the Data

The data is freely available, e.g. here:

http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp

There is not a global data file, but individual tiles (5°×5°) have to be downloaded.

Converting to xyz Format

If the data is downloaded in the ArcInfo ASCII format, it can be converted to the usual xyz format (lon, lat, elevation) using the following script:

esri-asc2xyz.pl

 ./esri-asc2xyz.pl < srtm_38_03.asc > srtm_38_03.xyz

Conversion to GMT grd

 xyz2grd srtm_38_03.xyz -R5/10/45/50 -Gsrtm_38_03.grd -I3c

The region, given in the -R parameter, can be determined using GMT's minmax tool:

minmax srtm_38_03.xyz

Finally, pairs of adjacent tiles can be combined. This is useful e.g. for an illumination map created using grdgradient, where all grid tiles should be normalized in the same way:

grdpaste grid1.grd grid2.grd -Gcombined.grd
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