Earth & SkyGreek → English
Geoid
/ˈdʒiːɔɪd/noun
The surface of equal gravitational potential that best matches global mean sea level, used as the reference for altitude.
The Greek roots
γῆ
gê
earth, land
εἶδος
eîdos
form, shape
Literally: “earth shaped”
The story of the word
Johann Benedict Listing coined it in 1873 for the shape the Earth would take if the oceans ran through the continents and settled. It is neither a sphere nor a clean ellipsoid but a lumpy figure, with a hollow south of India more than a hundred metres below the ellipsoid and a bulge near New Guinea. Every height on every map is measured from it. Listing had already given us another word still in daily use, topology.
- How it travelled
- Ancient Greek → scientific coinage → English
- First recorded
- 1870s
Same family
geographygeologygeometrygeothermal
In a sentence
Satellite gravimetry has mapped the geoid to within a few centimetres.
Your phone reports height above the ellipsoid; the surveyor wants it above the geoid.