Geodesy, initially included gravity measurements in the field and over the last 40 years have depended on refinements resulting from variations in satellite orbits responding to variations in the earths gravitational field. Each point on the map corresponds to a point on the Earth surface. These surface points, however, are distributed on curved surfaces that make up the near-sphere of the globe. This must be addressed by some type of transformation between spherical and planar geometry. A particular transformation of an area on the sphere to a plane will result either in distortion of the areal scale or of the shape.
Different projections strive to minimize one or the other distortion.
Depending on projection type, the scale applying to the map can be constant or will vary. The most widely used surface-shape representation is the topographic map. This datum plane by convention is chosen to be mean sealevel, a close approximation to the equipotential surface marking the oceanic segments of the geoid. Nets of baselines and extensions anchored by these benchmarks are developed using triangulation methods and other surveying procedures, and gradually become extended throughout a region up to continental dimensions. These maps display the usual planimetric features including latitude-longitude coordinates, specific scale used, and projection type.