Often, this is desirable profiles made with DEM data are usually 'smoother' looking than GPS, and typically contain fewer gaps or suspicious readings. Note that the elevation-adding feature will erase any existing altitude data (for example, from a GPS) that might already be in your file.
Complete copies of the SRTM3 and USGS NED databases, as well as a large number of SRTM1 and ASTER tiles, are stored on GPS Visualizer's server - that's more than 250GB of raw data. Or, look in GPS Visualizer's various input forms for the menu called 'Add DEM elevation data,' and choose one of the elevation databases (described in the table below).
