Geoid

Part of speech: noun

Pronunciation: /ˈdʒiːɔɪd/

Definitions

1. A geometric representation outlining the gravitational field of the planet while considering mean sea level variations
2. A mathematical surface that describes Earth's gravitational field and aligns with mean sea level while accounting for variations in topography and density
3. A shape representing the Earth where the surface corresponds with mean sea level affected by gravity and variations in mass distribution

Etymology: The geoid is an idealized surface of Earth's gravity: an imaginary sea level that would form over the whole planet if only gravity and rotation shaped the oceans, ignoring winds and tides. The name was coined in German, where "Geoid" appears as a technical term, and it is built from the Greek "gē", meaning earth, plus the suffix "-oid", meaning "resembling" or "having the form of", as in "asteroid" or "android". So the word literally means something like "earth-shaped." The shape is far from a smooth ball. Because Earth's mass is unevenly distributed, with dense mountain ranges and deep ocean trenches, gravity pulls slightly harder in some places than others, and the geoid bulges and dips accordingly by tens of metres. Maps of it, such as those built from the EGM96 gravity model, show these hills and hollows as coloured variations against a smoother reference ellipsoid, the mathematically simple shape used for most positioning work.

Synonyms: physical earth model