Frequently Asked Questions - 

Why Do Many Datum Parameters Exist?

Collection by Stefan A. Voser
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14. August 1999update  

Table of Content

  1. Notes
  2. Datum Transformations
  3. Others


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 surface
  • There exist different definitions of "geodetic datum", but including the same semantics:
    1. 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 surface)
    2. 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 where established using one or many fundamental points (central points) where the datum parameters where fixed based on astronomical, geophysical, terrestrial measurements. These datums are valid and in use for local and regional areas like countries, islands or continents. Such measurements
  • 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 parameters regarding to a global (world-wide) datum.
    • Transformation Paramaters are time-dependent!!!
  • Geographic coordinates need a datum specification
  • Geographic coordinates and map projections may have the same geodetic datum:
    • coordinates of a map projection are converted 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 parameter set needs the knowledge of:
  • Source datum
  • Target datum
  • Mathematical model 
    • formulae
    • parameters
  • Metadata
    • accuracy
    • area of validity
    • epochs of measurements 
    • set of control points


Why do different datum parameters exist?

  • control points (and continents) move because of geodynamic processes (time dependency)
  • different sets of control points are used
  • different mathematical models are used
     

Others


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© by Stefan A. Voser
 
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