site search by freefind

follow me


On Map Projections and Geodetic Reference Systems Library/Publications
graphic11. May 2007
Coordinate Reference System (CRS)
A combination of at least one coordinate system together with its spatial datum. In a CRS, positions or locations of geographic information are described by coordinates.
Geodetic Reference System/Geodetic Coordinate Reference System graphic
A coordinate reference system, based on shape, size, position and orientation of a (mathematical) reference surface, (e.g. sphere or spheroid). It normally is defined in a superior, geocentric three- dimensional coordinate system (for example WGS84, ETRS89). 
Map Projection/Map Projection Coordinate Reference System graphic
A map projection defines the mapping from geographic coordinates on a sphere or the geodetic coordinates on a spheroid to a plane. It may define
  • a map projection CRS
  • a conversion between a geodetic CRS and a map projection CRS.
Reference Surface
Mathematically (e.g. sphere or spheroid) or physically (geoid) defined surface to approximate the shape of the earth for referencing the horizontal and/or vertical position. Each coordinate system has a reference surface. Map projections have a mapping surface (an extrinsic characteristic) like cylinder, cone, plane.
(Coordinate) Reference Frame
A set of control points to realize a coordinate reference system.
Spatial Datum
Describes the fixing (position and orientation etc.) of a CRS regarding to the real world.
Geodetic Datum
Traditional term for a geodetic reference system. Now: a spatial datum of a geodetic reference system.
  • A map projection needs information about a reference surface.
  • A geodetic datum fixes the reference surface in space (superior coordinate reference system).
  • National Mapping Agencies defined their own Reference System. Different Countries may use the same parameters of the reference surface, but with different position and orientation!
  • National Mapping Agencies may use different map projections, based on the same reference  system. Different map projections depend on scale, area and distortion- charateristic needed.
  • Changing map projection between countries normally need a geodetic datum transformation.
  • Some countries use a double projection for large and medium scale mapping (e.g. the Netherlands, Switzerland, Hungary):
    • conformal projection from the reference ellipsoid to a sphere
    • conformal projection from the sphere to the plane.
  • The process of changing map projection: