Generating a terrain profile for use with terrain modeling tools like TERA or HFTA requires an accurate Digital Elevation Model (DEM). The DEM is a grid of points, usually organized by latitude and longitude, with the elevation of each point in meters above Mean Sea Level (MSL).
The DEM must be a 'bare earth' model providing the height of the ground, not of buildings, vegetation, tree cover etc.
For US locations, K6TU.NET uses data from the US Geological Survey (USGS) which provides data for all US States and Territories (such as Guam, Puerto Rico etc.). USGS data is accessible via the internet and is regularly updated. Data is available at different resolutions; 10m & 30m resolution is typical but quickly being augmented with LIDAR derived data at 1m resolution.
Models for international locations are more complicated. After using NASA SRTM and the NASA/Japan MITI ASTER dataset, K6TU.NET now uses the Global Ensemble Digital Terrain Model (GEDTM) at the 1 arc second resolution (approximately 30m) - aka, GEDTM30. You can learn more this model here.
GEDTM30 is the current "best and free" dataset available and has excellent coverage. K6TU.NET holds this dataset locally (about 400 GB), enabling fast generation of terrain profiles for international locations.