What is Google source for United States topographic data?

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

Source: http://www.opentopography.org/blog/lidar-beginning-appear-google-maps-terrain-layer

In the US, it appears that Google is using LiDAR data where it is available. I ran some tests in two areas. One where I had actual LiDAR data, the other where I had field survey data. Google Elevation data was amazingly accurate.

You can read more on my blog - https://chasmtech.wordpress.com/2017/01/17/civil-site-design-satellite-to-surface-feature-is-amazingly-accurate/