1"/3" SRTM Files

Shuttle Radar Topography Mission data was first  released in early 2002, two years after the ten day mission flew on the space shuttle. By early 2004, 3" data for North America, South America, Eurasia, Africa, Australia, and islands was available. Only the polar regions are missing. 1" data for the US was also universally available initially, and has now being released worldwide after initially being available only for the US military.  This is still the best DEM at the global scale.  The mission did not cover the poles, leaving 20% of the landmass vacant.

SRTM is probably of better quality than the ASTER GDEM, but lower quality than the 1" USGS NED/GDEP.  The biggest problem with SRTM is holes, which occur mostly in two settings: very steep mountains, like the Himalyas, when steep canyons and ridge sides often were not seen by the sensors; and dry sand in the regions like parts of the Sahara where there were no reflections

Recommended sources, in order to try:

• EarthExplorer  has the global 1" data  and the version 3, hole filled.  You will have to create an account and log in.
  • If you use the USGS Bulk Download Application (BDA), you can only get the BIL format.
  • If you download files one at a time, you can get DTED or GEOTIFF.
• MERIT DEM
• https://gdex.cr.usgs.gov/gdex/
• HydroSHEDS:  also has drainage basins. Select files by name; the Lat/Long are for the SW corner of the data set, and each covers 5ºx5°. 
• SRTM ftp download for version 2.1, with US 1", and global 3" and 30".  Download directions. 
• CGIAR with holes removed (tends to annoyingly slow)
• SRTM version 3 from NASA: http://opentopo.sdsc.edu/gridsphere/gridsphere?gs_action=lidarDataset&cid=geonlidarframeportlet&opentopoID=OTSRTM.042013.4326.1

SRTM hole filling projects.  Noble goal, but remember that you cannot easily turn lead into gold.  But for visualization or hydrology, removing the holes is important.

• HydroSHEDS:  Probably the best option if you need the holes filled.  These are 5 degree tiles; you want the BIL format.  You can also get stream networks and drainage basin shapefiles.
• http://srtm.csi.cgiar.org/  Slow downloads.
• http://www.viewfinderpanoramas.org/dem3.html  Some DEMs in mountainous regions, from Jonathan de Ferranti.
• MERIT DEM.
• Coming NASADEM, https://earthdata.nasa.gov/community/community-data-system-programs/measures-projects/nasadem

Using  SRTM data:

• Map library can graphically open and merge tiles on the fly 
• Open and merge DEMs on the File, Open submenu.
• Do automatic filling of the voids; pick this, and set the maximum size void to fill, on the Import/Export tab of the Options form. This will remove some smaller holes, but not the large "Swiss cheese" DEMs in many mountainous regions.

• MICRODEM does not perform a vertical datum shift for this data, which you must consider when comparing SRTM data to other DEMs.

SRTM References:

• jpl_nasa siteindex.html
• srtm factsheet
• factsheet_technology

Relation of SRTM3 (DTED 1) to SRTM1 (DTED 2):

• "SRTM data is being processed at the Jet Propulsion Laboratory (JPL) in Pasadena, California into research-quality digital elevation models (DEMs). The data is 3 X 3 averaged to 3-arc second spacing (90 meter posting) from the original 1-arc second data."  Inspection of DEMs (using difference maps, direct inspection of the grids, and topographic profiles) shows that averaging, rather than decimation, was in fact used for this "research grade" data. The SRTM data at the USGS seamless server also has averaged data. Neither of these sources has the water corrections done.
• "SRTM DTED® is a uniform matrix of elevation values indexed to specific points on the ground. The horizontal datum is the World Geodetic System 1984 (WGS84) and the vertical datum is mean sea level as determined by the WGS84 Earth Gravitational Model (EGM 96) geoid. The elevation data are with respect to the reflective surface, which may be vegetation, man-made features or bare earth. DTED® Level 2 elevation values are spaced one arc second apart between 0 degrees and 50 degrees latitude, and spaced one arc second apart in latitude and two arc seconds apart in longitude between 50 degrees and 60 degrees latitude. SRTM DTED® Level 1 values are derived from the SRTM DTED® 2 values such that the DTED® 1 values are identical to the DTED® 2 values at coincident points." This is apparently the data available for sale by USGS.

Last revision 1/3/2017