page_title: Map projection
section_title: Metric properties of maps
hierarchical_section_title: Map projection / Metric properties of maps
caption_attribution_description: English: Condensed explanation of Albers conic-projection maps, rearranged as taller image, from larger, wider USGS file of GIF format. See enlarged view below, using a wikitable to expand beyond original size.
context_page_description: In cartography, a map projection is a way to flatten a globe's surface into a plane in order to make a map. This requires a systematic transformation of the latitudes and longitudes of locations from the surface of the globe into locations on a plane.
  All projections of a sphere on a plane necessarily distort the surface in some way and to some extent. Depending on the purpose of the map, some distortions are acceptable and others are not; therefore, different map projections exist in order to preserve some properties of the sphere-like body at the expense of other properties. Every distinct map projection distorts in a distinct way, by definition. The study of map projections is the characterization of these distortions. There is no limit to the number of possible map projections.
  Projections are a subject of several pure mathematical fields, including differential geometry, projective geometry, and manifolds. However, "map projection" refers specifically to a cartographic projection.
context_section_description: Many properties can be measured on the Earth's surface independent of its geography:
  Area
  Shape
  Direction
  Bearing
  Distance
  Map projections can be constructed to preserve some of these properties at the expense of others. Because the curved Earth's surface is not isometric to a plane, preservation of shapes inevitably leads to a variable scale and, consequently, non-proportional presentation of areas. Vice versa, an area-preserving projection can not be conformal, resulting in shapes and bearings distorted in most places of the map. Each projection preserves, compromises, or approximates basic metric properties in different ways. The purpose of the map determines which projection should form the base for the map. Because many purposes exist for maps, a diversity of projections have been created to suit those purposes.
  Another consideration in the configuration of a projection is its compatibility with data sets to be used on the map. Data sets are geographic information; their collection depends on the chosen datum (model) of the Earth. Different datums assign slightly different coordinates to the same location, so in large scale maps, such as those from national mapping systems, it is important to match the datum to the projection. The slight differences in coordinate assignation between different datums is not a concern for world maps or other vast territories, where such differences get shrunk to imperceptibility.
caption: An Albers projection shows areas accurately, but distorts shapes.