Thus, to locate the points, we must reference them to a geographic coordinate or grid system, the most common being that of latitudes and longitudes. Latitude lines run parallel to the equator, each one being a successively smaller circle as we move toward a pole. Longitude lines are meridians, great circles of equal radius that pass through the poles and are perpendicular to all latitude lines. Cartographers address this problem by using a transformation between spherical and planar geometry. Any transformation of an area on the sphere to a plane results either in distortion of the areal scale or of the shape.
Different projections strive to minimize one or the other distortion.
Depending on the projection type, the map scale can be constant or may vary. The most widely used surface-shape representation is the topographic map. We choose this datum plane by convention to be mean sea level, closely approximating the equipotential surface that marks the oceanic segments of the geoid. They measure lines along levels and calculate trigonometrically the vertical differences in elevation between points. They develop nets of baselines and extensions anchored by these benchmarks using triangulation methods and other surveying procedures, and gradually the nets extend throughout a region and ultimately may cover a continent.