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Why do many Datum Parameters exist? Help/FAQ Geodetic Reference System
graphic24.04.2005
Datum parameters mentioned here describe a relationship between Coordinate Reference Systems with a different datum as a transformation.
A transformation has a specific method an the parameters depend on the controll points used to determinde the parameters. The parameters indicate also the accuracy of the transformation.
Notes
  • A geodetic datum delivers the spatial semantics of an earth model (sphere, ellipsoid, geoid, others):
    • shape, size, position and orientation in relation to the earth's surface
  • There exist different definitions of "geodetic datum", but including the same semantics:
    • A geodetic datum describes a geodetic reference system (an earth model) together with its realization, meaning its shape, size, position and orientation in relation to the earth's surface)
    • The geodetic datum describes the position, orientation and scale of an earth model in relation to the earth surface
  • Traditionally, two different types of geodetic datums are used:
    • a horizontal datum using a sphere or ellipsoid as a reference surface and describing the position on it by geographic coordinates
    • a vertical datum using a (known or unknown) geoid. The geoid is representing the levelling surface called Mean Sea Level.
    • These datums were established using one or many fundamental points (central points) where the datum parameters were fixed based on astronomical, geophysical, terrestrial measurements. These datums are valid and in use for local and regional areas like countries, islands or continents.
  • In modern Geodesy, established by using artificial satellites, full three-dimensional systems and geocentric (earth centred) systems (like WGS84, ITRS89, ETRS89) are used:
    • three- dimensional datum definitions are established
    • horizontal and vertical datums are linked together (compound datums)
    • Datums are described by their transformation parametersregarding to a global (world-wide) datum.
    • Transformation Paramaters are time- dependent!!! e.g. due to geodynamical processes
  • Geographic coordinates need a datum specification
  • Geographic coordinates and map projections may have the same geodetic datum: 
    • coordinates of a map projection areconverted geographic coordinates (defined method and parameters)
    • Changing geographic coordinates between different datums is called a transformation.
Datum Transformations
A datum transformation changes coordinates between two datums (including earth models). The required transformation parameters are derived by using control points (points with fixed coordinates, known in both systems) and a mathematical model describing the mapping between the different systems.
A datum transformation parameter set needs the knowledge of [Voser 2007]:
  • Source datum
  • Target datum
  • Mathematical model  
    • formulae
    • parameters
  • Metadata 
    • accuracy
    • area of validity
    • epochs of measurements
    • set of control points
Summary
  • control points (and continents) move because of geodynamic processes (time dependency)
  • different sets of control points are used
different mathematical models are used