Glossary for Geospatial Science

ellipsoid:  An ellipse rotated about its shorter axis.  In the context of map projections, an ellipsoid is a geometric reference surface that closely approximates the geoid.  Since the geoid (that depicts the earth) is an irregular spheroid characterized by polar flattening (or ellipticity), many methods have been developed to describe its ellipsoidal deviations.  Cartographers have a selection of ellipsoids from which to choose; most of which have “best-fit” properties for certain portions of the globe.  In 1924, an International ellipsoid was defined which described the earth ellipsoid as a flattening of 1 part in 297.  Historically, ellipsoids were derived from careful surface measurements.  More recently, satellite data has been used to construct ellipsoid models that relate coordinate measurements to the earth’s center of mass.  A few common standards: GRS 1980 (North America), WGS 72 (NASA/satellite), Australian (Australia), Krasovsky (Soviet Union), Clarke-1880 (Africa), Clarke-1886 (North America, Philippines), Airy (Great Britain), Bessel (Central Europe, Chile, Indonesia), Everest (India, Burma, Pakistan, Afghan, Thailand, and other southern Asian countries), and International (most other areas).