The Shuttle Radar Topography Mission (SRTM) data products result from a collaborative mission by the National Aeronautics and Space Administration (NASA), the National Imagery and Mapping Agency (NIMA), the German space agency (DLR) and Italian space agency (ASI), to generate a near-global digital elevation model (DEM) of the Earth using radar interferometry.
Figure 1: SRTM data compared to GTOPO30
SRTM data is distributed in two levels: SRTM-1 (USA only ) with data sampled at 1 arc-second intervals in latitude and longitude, and SRTM-3 (for rest of the World) sampled at 3 arc-seconds.
The original digital elevation data is unedited and was intended for scientific use and evaluation. For example it may contain numerous voids (areas without data), water bodies may not appear flat, and coastlines may be ill-defined.
Original data sets are available either from the USGS server:
or from JPL at
MadMappers 'SRTM-3 Mad' Data File Structure and format
The original data had the following limitations:
www.madmappers.com has attended to some of the limitations mentioned in point 1. and has produced a corrected set for the African continent.
SRTM-3 data is sampled at three arc-seconds intervals in latitude and longitude (in practical terms roughly equivalent to a 90m grid) and published into 1x1 degree latitude and longitude tiles in WGS84 "geographic-lat/lon" projection, a projection easy to manipulate and mosaic.
Void-filling was achieved by an extrapolation method. This method is satisfactory for filling smaller voids but has obvious limitations for extremely large voids such as those present in equatorial regions. Furthermore the method does not fully address voids at the edge of a tile.
File format is the standard SRTM .hgt format. Files are in a compressed format.
File names refer to the latitude and longitude of the lower left corner of the tile
- e.g. S33E25_Mad has its lower left corner at 33 degrees South latitude and 25 degrees East longitude. Suffix "Mad" has been used to differentiate this set from the original data.
Feel free to contact firstname.lastname@example.org for further assistance or to point out anomalies which might require further processing.
Void filling is the main feature of the SRTM-3 Mad Set. In the comparison below tile S43E018.hgt of the unprocessed Standard Set, located the Table Bay region of Cape Town, South Africa, clearly shows elevation anomalies illustrated as large red patches. These elevation voids have been replaced by extrapolated values in the SRTM-3 MadMappers Set.
Elevation irregularities over water bodies and ill-defined coastlines remain uncorrected.
Future Dataset Developments
In the months to come MadMappers is to undertake two major tasks:
A SRTM index map (ozf2 format) is available.
Files must be un-zipped for use in Oziexplorer and Ozi3D. The Oziexplorer Help Menu contains detailed set-up instructions:
Both compressed and uncompressed files can be loaded.
ARC/INFO or ArcView Users
Users of ARC/INFO or ArcView can display the DEM data directly after renaming the file extension from .HGT to .BIL. However, if a user needs access to the actual elevation values for analysis in ARC/INFO the DEM must be converted to an ARC/INFO grid with the command IMAGEGRID.
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