On Map Projections and Geodetic Reference Systems

Collection by Stefan A. Voser		 MapRef
MapRef
17. December 2000update  

Table ot Content

  1. Short Definitions
  2. Notes
  3. Others
Map Projection, Reference Surface and Geodetic Datum 
Short Definitions
Map Projection 
A Map Projection defines the mapping from geographic coordinates on a sphere or the geodetic coordinates on a spheroid to a plane. 
Reference System 
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). 
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. 
Reference Frame 
A set of control points to realize a Reference System. 
Geodetic Datum 

traditional Term for Reference System. 

Notes

  • A map projection needs information about a Reference Surface 
  • A Reference Surface is based on a Geodetic Datum. 
  • A Geodetic Datum fixes the Reference Surface in space (superior coordinate 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. 
  • The Process of Changing Map Projection: GIF, 16 kbytes 
  • Some countries use a double projection for large and medium scale mapping (e.g. the Netherlands, Switzerland, Hungary): 
    1. conformal projection from the Reference Ellipsoid to a sphere 
    2. conformal projection from the sphere to the plane. 

Others 

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© by Stefan A. Voser
 
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