`24.04.2005`
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 parameters*regarding 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 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 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