HGT NASA SRTM Grid Data File Description

Didger can import HGT binary grid files. HGT Files are produced by the Shuttle Radar Topography Mission, of NASA.


The SRTM data sets result from a collaboration between NASA, National Geospatial-Intelligence Agency, German space agency, and the Italian space agency. The files contain near-global DEM files of the Earth using radar interferometry. The SRTM instrument consisted of the Spaceborne Imaging Radar-C (SIR-C) hardware set modified with a Space Station-derived mast and additional antennae to form an interferometer with a 60 meter long baseline.


The SRTM data have undergone a sequence of processing steps resulting in several data versions having slightly different characteristics. In addition, the different naming conventions used by the NGA and NASA can lead to some confusion. In the first step raw SRTM radar echo data were processed in a systematic fashion using the SRTM Ground Data Processing System (GDPS) supercomputer system at the Jet Propulsion Laboratory. This processor transformed the radar echoes into strips of digital elevation data, one strip for each of the 1000 or so data swaths. These strips were then mosaicked into just less than 15,000 one degree by one degree cells and formatted according to the Digital Terrain Elevation Data (DTED) specification for delivery to NGA, who are using it to update and extend their DTED products. The DTED specification can be found in MIL-PDF-89020b.pdf on this server.


The data were processed on a continent-by-continent basis beginning with North America and proceeding through South America, Eurasia, Africa, Australia and Islands, with data from each continent undergoing a “block adjustment” to reduce residual errors. These data were also reformatted into the SRTM format, detailed in Section 3 below, and placed on this server as Version 1.0.


In the next step NGA applied several post-processing procedures to these data including editing, spike and well removal, water body leveling and coastline definition as described in the document SRTM_Edit_Rules.doc on this server. Following these "finishing" steps data were returned to NASA for distribution to the scientific and civil user communities as well as the public. These data were also reformatted into the SRTM format and are referred to as Version 2. The figure below shows a portion of cell N34W119.hgt, demonstrating the difference between the edited and unedited data.


Data Formats

The names of individual data tiles refer to the longitude and latitude of the lower-left (southwest) corner of the tile (this follows the DTED convention). For example, the coordinates of the lower-left corner of tile N40W118 are 40 degrees north latitude and 118 degrees west longitude. To be more exact, these coordinates refer to the geometric center of the lower left sample, which in the case of SRTM3 data will be about 90 meters in extent.


SRTM1 data are sampled at one arc-second of latitude and longitude and each file contains 3601 lines and 3601 samples. The rows at the north and south edges as well as the columns at the east and west edges of each cell overlap and are identical to the edge rows and columns in the adjacent cell.


SRTM3 data are sampled at three arc-seconds and contain 1201 lines and 1201 samples with similar overlapping rows and columns. This organization also follows the DTED convention. Unlike DTED, however, 3 arc-second data are generated in each case by 3x3 averaging of the 1 arc-second data - thus 9 samples are combined in each 3 arc-second data point. Since the primary error source in the elevation data has the characteristics of random noise this reduces that error by roughly a factor of three.


This sampling scheme is sometimes called a "geographic projection", but of course it is not actually a projection in the mapping sense. It does not possess any of the characteristics usually present in true map projections, for example it is not conformal, so that if it is displayed as an image geographic features will be distorted. However it is quite easy to handle mathematically, can be easily imported into most image processing and GIS software packages, and multiple cells can be assembled easily into a larger mosaic (unlike the pesky UTM projection, for example.)


Refer to NASA/JPL SRTM web site for additional information.


Import Options Dialog

No import options dialog is displayed.


Export Options Dialog

Didger does not currently export this file format.


See Also

File Format Chart