Licensing options for this format begin with FME Desktop Professional Edition.
The SRTM HGT Reader provides FME with access to data in the Shuttle Radar Topography Mission Height file format.
Overview
SRTM data are data elevation models (DEM) and are distributed in two levels: SRTM-1 with data sampled at one arc-second intervals in latitude and longitude, and SRTM-3 sampled at three arc-seconds. Three arc-second data are generated by 3x3 averaging of the one arc-second samples.
Data is divided into one by one-degree latitude and longitude tiles in "geographic" projection, which is to say a raster presentation in no projection at all but easy to manipulate and mosaic.
File names refer to the latitude and longitude of the lower left corner of the tile, and FME cannot read files that do not conform to this convention. For example, N37W105 has its lower left corner at 37 degrees North latitude and 105 degrees West longitude. To be more exact, these coordinates refer to the geometric center of the lower left pixel, which in the case of SRTM-3 data, will be about 90 meters in extent.
Height files have the extension .HGT and are signed two-byte integers. Heights are in meters referenced to the WGS84 geoid. Data voids are assigned the value 32768.
SRTM-3 files contain 1201 rows and 1201 columns. 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 height file. SRTM-1 files contain 3601 rows and 3601 columns, with similar overlap.
Reader Overview
FME requires SRTM HGT files to have specific filenames in order to read them. Since these files do not contain typical georeferencing information, this information is inferred from the filename.
The format for the filename is “abbcddd.hgt”, where:
- a indicates the northern (“N” or “n”) or the southern (“S” or “s”) hemisphere;
- b is the two-digit latitude in the given hemisphere;
- c indicates the western (“W” or “w”) or the eastern (“E” or “e”) hemisphere;
- d is the three-digit longitude in the given hemisphere.
Examples of correct filenames include N55E002.hgt, s01w135.hgt, n42W62.hgt.
FME considers a single SRTM HGT file to be a dataset.
FME Raster Features
FME raster features represent raster data and use several concepts that are unlike those used in the handling of vector data. The topics below describe how FME processes raster data.
SRTM HGT only supports rasters with a single numeric band.