SRTM

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Screenshot of GpsDrive showing OpenCycleMap tiles with relief shading and contour lines based on SRTM

The Shuttle Radar Topography Mission (SRTM) (Wikipedia article) is a NASA mission conducted in 2000 to obtain elevation data for most of the world. It is the current dataset of choice for digital elevation model data (DEM) since it has a fairly high resolution (1 arc-second, or around 25 meters, for the United States, and 3 arc-second, or around 90 meters at the equator, for the rest of the world), has near-global coverage (from 56°S to 60°N), and is in the public domain.

Many OpenStreetMap-based projects use SRTM data to provide topography information, relief shading, and elevation profiles for trails and routes. An example is the OpenCycleMap rendering which shows contours and relief shading derived from SRTM data.

Versions

Official versions

There are three versions of the official SRTM dataː

  • Version 1.0 is the (almost) raw data obtained during the mission and its quality is considered research-grade. Data for this version can be downloaded from this USGS directory page.
  • Version 2.1 (aka Non-Void Filled) is the data from Version 1 cleaned-up to correct processing errors and to clip data to water boundaries. This version still contains "void" areas for which there is no elevation data. These void areas are due to problems obtaining data using the radar methodology, such as in areas with steep terrain, and areas of low reflectivity such as flat deserts. Data for this version can be downloaded from this USGS directory page.

    Note: USGS is currently in the process (as of late 2014) of releasing the 1-arc-second data for the whole world.

  • Version 3.0 (aka SRTM Plus or Void Filled) removes all of the void areas by incorporating data from other sources such as the ASTER GDEM. More information about Version 3.0 (PDF).<p>The data comes in 7 related data sets:
    • SRTMGL3 is the default 3-arc-second data for the world obtained by averaging the 1-arc-second raw data.
    • SRTMGL3S is the sampled 3-arc-second data for the whole world obtained by getting the middle 1-arc-second raw data sample out of a 3×3 matrix.
    • SRTMGL3N is the meta-data for the previous 2 data sets explaining the source of each data point.
    • SRTMGL30 is the 30-arc-second data for the whole world.
    • SRTMSWBD is the tiled-vector data of the world's coastlines.
    • SRTMUS1 is the 1-arc-second data for the United States.
    • SRTMUS1N is the meta-data for the United States data set.

Other third-party versions

A few other projects have post-processed the official Version 2.1 data to fill in the data voids and in some cases improves accuracy or resolution in some places. The two known projects that covers the whole world are:

  • CIAT SRTM. The data is only available for non-commercial use only. Therefore this data cannot be mixed used with OSM!
  • Viewfinder Panoramas. The data is not under an open license but can be freely used in open source projects according to a note here based from the author's response. However, some of the data was obtained from Soviet topographical maps that is not available for licensing, according to a blog post comment. Please use with caution.

In addition, since 25 May 2011, the German Aerospace Center ("Deutsches Zentrum für Luft- und Raumfahrt") also offers SRTM-based data that is more accurate than SRTM but only covers 40% of the world and is free for scientific use only].

Format

The official 3-arc-second and 1-arc-second data for versions 2.1 and 3.0 are divided into 1°×1° data tiles. The tiles are distributed as zip files containing HGT files labeled with the coordinate of the southwest cell. For example, the file N20E100.hgt contains data from 20°N to 21°N and from 100°E to 101°E inclusive.

The HGT files have a very simple format. Each file is a series of 16-bit integers giving the height of each cell in meters arranged from west to east and then north to south. Each 3-arc-second data tile has 144201 integers representing a 1201×1201 grid, while each 1-arc-second data tile has 12967201 integers representing a 3601×3601 grid. The outermost rows and columns of each tile overlap with the corresponding rows and columns of adjacent tiles.

Recent versions of GDAL support the HGT files natively (as long as you don't rename the files; the names they come with are the source of their georeferencing), but the srtm_generate_hdr.sh script can also be used to create a GeoTIFF from the HGT zip files. (Note that the script has SRTM3 values hardcoded; if you're using SRTM1, you'll have to change the number of rows and columns to 3601, the number of row bytes to 7202, and the pixel dimensions to 0.000277777777778.)

Data in OSM format (XML)

The Hochschule für Technik Stuttgart has converted contours calculated from SRTM data to OSM format (XML) and has it ready for download at [1]
The lines are having the tags contour=elevation + contour_ext=elevation_minor every 40m + contour_ext=elevation_major every 1000m + ele=nn (elevation_medium is not used)
NB. This data use OSM Protocol Version 0.5, which was deprecated in 2009. Newer software/editors might not be able to use these files.

You can also do a manual conversion to OSM format using:

  • Srtm2Osm to write contours as OSM ways
  • Phyghtmap to write contours as OSM ways
  • Groundtruth (see below) to write contours as OSM ways

other Tools that are using SRTM data:

  • Srtm2wayinfo to calculate the altitude differences along each way
  • Srtm to Nodes as a plugin for Osmosis to add a height tag to each node

In use

  • OSM-3D - uses preprocessed OSM SRTM
  • OpenDEM - collects reliefs and started with SRTM
  • OSM2World - reads raw SRTM to improve OSM-based 3D scenes

See also