Answer 4

GPS elevation is given by most receivers relative to a theoretical surface called the "geoid", which is the shape the Earth would have without tides. It starts with the basic oblate spheroid caused by the Earth's rotation and incorporates certain distortions that are caused by the nonuniform distribution of mass in the Earth's crust and mantle (e.g., oceans and mountain ranges). The technical definition of this surface is "the equipotential gravitational surface" and it corresponds to mean sea level, even on land.

Contributor: Dave Tweed

Published March 2003