UN - Geographic Information Systems
In cartography, the three dimensional shape used to represent the Earth. The Earth ellipsoid is characterized by a smaller distance from the centre to the poles (semi-minor axis) than that from the centre to the equator (semi-major axis). Also called a spheroid.
United Nations, "Handbook on Geospatial Infrastructure in Support of Census Activities", Department of Economic and Social Affairs, Statistics Division, Studies in Methods, Series F No. 103, New York, 2009