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 On Map Projections and Geodetic Reference Systems
11. May 2007
Definitions
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
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
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.
Notes
• 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:
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