There are a number of sources of free elevation data, with varying degrees of horizontal accuracy and spatial resolution. One of the coarser datasets comes from the Shuttle Radar Topography Mission (SRTM), which gathered elevation data for 1 arc-second intervals in 2000. [An arc-second is equivalent to approximately 30 meters, depending upon the longitude. Thanks, internets!] These data are available for the United States and its territories, and for everywhere else in the world, the values were interpolated and released for 3 arc-second intervals (or approximately 90 meters).
Why go with SRTM? It is generally agreed that the error for SRTM data is pretty low. For example, the absolute geolocation error in Eurasia is 8.8 m, and the absolute height error is 6.2 m (Farr et al. 2007:21). Not too bad when you’re dealing with 90-m by 90-m swaths of land. Also, if you are working with larger study regions, a single tile provides pretty wide coverage.
SRTM1 and SRTM3 data are provided free, courtesy the National Geospatial-Intelligence Agency and NASA. The 2.1 version is the latest release, and it’s been cleaned of spikes, wells, and pixels with missing data. For a quick overview, check out this file.
These data can be downloaded in one degree lat/long tiles from a few different websites. The two government hosts are http://dds.cr.usgs.gov/srtm/ or http://earthexplorer.usgs.gov/. Tiles downloaded from the first website are called something like “N36022.hgt.” Tiles from the second website are called something like “n36.dt1” (DTED format). Supposedly these are the same data, but values appear to vary slightly (by 1 or 2 meters) around my study region.
A third website, hosted by the CGIAR Consortium for Spatial Information, has SRTM3 data that has been processed even further to remove NoData holes, using an algorithm developed by the CGIAR team. I stick to this version of the SRTM data because I like the idea the idea of a complete dataset. Plus, the website claims: “WE ARE CONFIDENT THIS IS NOW THE HIGHEST QUALITY SRTM DATASET AVAILABLE.” Who could pass that up?
Reference: Farr, T. G., et al. (2007), The Shuttle Radar Topography Mission, Rev. Geophys., 45, RG2004, doi:10.1029/2005RG000183.