Shuttle Radar Topography Mission

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The Shuttle Radar Topography Mission (SRTM) is a research effort that obtained digital elevation models on a near-global scale from 56 °S to 60 °N, to generate the most complete high-resolution digital topographic database of Earth to date. SRTM consisted of a specially modified radar system that flew onboard the Space Shuttle Endeavour during the 11-day STS-99 mission in February of 2000. To acquire topographic (elevation) data, the SRTM payload was outfitted with two radar antennas. One antenna was located in the Shuttle's payload bay, the other on the end of a 60-meter (200-foot) mast that extended from the payload bay once the Shuttle was in space. The technique employed is known as interferometric Synthetic Aperture Radar.

The elevation models are arranged into tiles, each covering one degree of latitude and one degree of longitude, named according to their south western corners. It follows that "n45e006" stretches from 45°00′N 6°00′E to 46°00′N 7°00′E and "s45w006" from 45°00′S 6°00′W to 44°00′S 5°00′W. The resolution of the cells of the source data is one arc second, but this data has only been released over United States territory; for the rest of the world, only three arc second data is available. Each three arc second tile has 1,201 rows, each consisting of 1,201 16 bit bigendian cells.

The elevation models derived from the SRTM data are used in Geographic Information Systems. They can be downloaded freely over the internet, and their file format (.hgt) is supported by several software developments.

The Shuttle Radar Topography Mission is an international project spearheaded by the National Geospatial-Intelligence Agency (NGA) and the National Aeronautics and Space Administration (NASA).

[edit] No-data areas

SRTM void filling with spline interpolation in GRASS GIS.

The elevation datasets are affected by mountain and desert no-data areas. These amount to no more than 0.2% of the total area surveyed, but can be a problem in areas of very high relief. They affect all summits over 8,000 metres, most summits over 7,000 metres, many Alpine and similar summits and ridges, and many gorges and canyons. There are some SRTM data sources which have filled these data voids, but some of these have used only interpolation from surrounding data, and may therefore be very inaccurate. If the voids are large, or completely cover summit or ridge areas, no interpolation algorithms will give satisfactory results. Other developers, including NASA World Wind and Google Earth, have tried to improve their results by using 30 arc second data in the interpolation process, but, due to the poor resolution of these data, and very poor quality of some of them, the results tend to show little improvement over pure interpolation. Readers with Google Earth software can examine the effect of larger voids by clicking on 27°59′14″N, 86°55′31″E (Everest) and tilting the image.