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
