In February of 2000 the Space Shuttle mapped most of the land surfaces of the Earth, from 60° north to 56° south, to create the highest resolution global elevation dataset available to date. Global data were released at a 3 arc-second (~90 m) resolution. Data covering the United States were also released at a 1 arc-second (~30 m) resolution. The Version 1 data had many data voids and irregular water surfaces and coastlines.
Version 2 SRTM Data
NASA released a “finished” version of these data called Version 2. These data had most voids filled in by interpolation, lake surfaces were corrected, and coastlines properly defined and aligned. These data are available at the USGS Earth Explorer site. Data are available at 3 arc-second (~90 m) resolution globally, and also at 1 arc‑second (~30 m) in the U.S. Care should be taken when using SRTM data in areas with extreme topographical change as there are still data voids in some of these areas.
Version 3 SRTM Data
In November 2013 NASA released Version 3 of the SRTM data. These data had voids filled using other data sources such as the ASTER GDEM2, GMTED2010, and NED. These are the highest quality SRTM data available to date. Get these data at the NASA Reverb | Echo site. As with the Version 2 data on the USGS site, these data are available at 3 arc-second (90 m) resolution globally, and also at 1 arc‑second (30 m) in the U.S. and most of the world.
To obtain these data on the Reverb site zoom in to your area of interest then enter the keyword SRTM in the Search Term box in the upper right. Under Step 2: Select Datasets select the V003 dataset you wish, either NASA Shuttle Radar Topography Mission Global 3 arc second V003 or NASA Shuttle Radar Topography Mission Global 3 arc second V003, then click Search for Granules. (Do not select the datasets that have the phrase “second number V003” as part of their tittle.) Data are packaged in 1° x 1° tiles. Use the Browse button to view the granule(s) and put those you want into your Cart. From the Cart click on the Download button and a text file will be generated that can be used to download your data. Simply paste this line of text into your browser to get the data.
These integer data use the Geographic “projection” (latitude/longitude) and the datum is WGS84. The data are in “height” format with a file extension of .HGT. To open these data in ENVI Standard, from the main menu select File | Open As | Digital Elevation | SRTM DEM.
Global 1 Arc-Second Data - best choice for 30 m resolution
Beginning in September of 2014 the USGS began distributing 1 arc-second Version 3 void-filled data globally. Unlike the original Version 3 release, these data will be distributed through the USGS Earth Explorer site. Click on the Data Sets tab then Digital Elevation | SRTM. Data are packaged in 1° x 1° tiles and distributed in three formats; DTED, BIL, and GeoTIFF. GeoTIFF data may be the easiest to use in most geospatial software programs and is the recommended format.