CivTAK and ATAK can use elevation data in DTED Format. Digital Terrain Elevation Data is both a format and data developed by the United States Department of Defense (DoD). The DoD data is not publically available, except possibly for the lowest resolution version.
NASA has also collected elevation data of the earth as part of the Shuttle Radar Terrain Measurements (SRTM) program and publishes it in several formats available for public use, at the http://earthexplorer.usgs.gov/ web site.
Note: DTED data from the DoD attempts to model the level of the ground. SRTM attempts to measure the height of the top cover (trees, rooftops, etc)
To make it easy to use, I’ve collected some of that data and formatted it so that it is easily imported into ATAK.
You an find SRTM, broken into regions here.