The geoid is defined as the equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level. The surface of the ocean bulges outward and inward mimicking the topography of the ocean floor. These ocean bumps, too small to be seen, can be measured by a radar altimeter aboard a satellite. See ngdc.noaa.gov for a more complete explanation.    This image highlights tectonic features, such as ocean trenches and ridges, and landforms at plate boundaries such as the Himalayas. Only closer inspection shows the continental landmasses of Africa and Australia. South of Africa, the rift valley and the fracture zones along the Southwest Indian Ridge are especially clear. 2008 Geoid Map Bathymetry Sonar:Multibeam MGDS