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About Geographic Coordinates Library/Publications
graphic8. October 1998
  • Geographic co-ordinates are not planar coordinates!
  • Geographic co-ordinates are based on a specific reference system (e.g. sphere or ellipsoid).
  • Geographic co-ordinates define the perpendicular direction on the surface of the reference system.
  • Geographic co-ordinates are ambiguous!
    • Geographic co-ordinates may define different points on the earth's surface. This depends on their reference system (the geodetic datum problem).
    • One point on the earth's surface may have many geographic co- ordinates, depending on their reference system (geodetic datum).
  • Changing geographic co- ordinates means:
    • Map Projection
      Conversion from the curved surface to a plane.
    • Geodetic Datum Transformation
      Changing geographic co- ordinates between different reference systems or reference surfaces.
Terminology confusion with geographic coordinates
3 types of co-ordinates define different perpendiculars:
  • astronomical coordinates
    physically: perpendicular, based on the gravity
  • geodetic coordinates
    mathematically: perpendicular, based on the reference surface, specifically ellipsoid, used for large and medium scale mapping, and in geodesy.
  • geographic coordinates
    mathematically: perpendicular, based on the reference surface, typically spheres, used for small scale mapping