When you click on the map, the altitude is displayed on the left side of the mapping page.
This height is computed from GTopo30 database of USGS and SRTM3 database. US government gives a free access to thesedatabases.

The grid size is about 900 meters. This size gives a general indications of the altitude but cannot be a precise data, especially in mountainous regions.

USGS has other databases with different precisions. SRTM (Shuttle Radar Topography Mission) database has a grid size of 90 meters or 30 merters and is worldwide. NED (National Elevation Dataset) database has a grid size of 10 or 30 meters, and coverage is USA.

SRT (Shuttle Radar Topography Mission) has several available formats.
We use version 2 (called "finalized" version). Data has been edited to erase some errors (sea, cost lines, erratic single pixels).
Grid size is 3 seconds of arc. It is about 90 meters.