Geoid

The geoid is the equipotential surface of Earth’s gravity field that best matches global mean sea level, the shape a worldwide ocean at rest would take. It undulates by up to about 100 m above and below the reference ellipsoid because of mass-density variations. Heights above the geoid are orthometric (the everyday sense of elevation), while GPS measures height above the ellipsoid, so the geoid-ellipsoid separation must be applied to convert between them.