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).