There is more than one source depending on the area of interest around the globe.
Google Maps is the terrain layer, which provides a shaded relief (aka
hillshade) view of the topography derived from a digital elevation
model. Google has done a nice job generating a visually pleasing
terrain layer, and we use it for all of our Google Maps-based
interfaces in OpenTopography. Google appears to use a range of
digital elevation model data sources to derive the terrain layer.
Throughout the US, it appears that the terrain layer comes from either
10 or 30 m (1/3 arc-second & 1 arc-second respectively) DEMs from the
USGS National Elevation Dataset (NED). Globally the terrain data
appear to be derived from either Shuttle Radar Topography Mission
(SRTM) data or something like the NOAA Global Land One-km Base
Elevation Project (GLOBE) dataset. Recently however, I’ve begun to
notice higher resolution data in the Google Maps terrain layer. These
higher-resolution data are localized, and are patched into the terrain
layer among the standard 10 and 30 m derived images, but they are
impressive when you find them. Based on the location of these patches
of high-resolution terrain, and their appearance, it is pretty easy to
deduce that Google is now incorporating bare earth LiDAR digital
elevation models into the Google Maps terrain layer.
Thus far, I’ve noticed high-resolution data in downtown Portland, OR,
the greater Los Angeles area, and Mount Saint Helens in Washington.
There may be other examples of LiDAR-derived terrain