24.04.2005Datum 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
- 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
related information
