*31.12.2000*
*"There exist various coordinate reference
systems in which a geographic location may be described
mathematically by coordinates. In each system, the position gets
its own coordinate values. These values differ in a numerical
sense, but represent the same geographic position.*

*When collecting data stored in different
coordinate reference systems, each definition has to be known
together with its geometric relationship to a standard system. Only
then, it is possible to transfer all data into a standard
coordinate reference system."*

*Stefan A.
Voser*

**Fig. 1: From the
earth surface to the plane of a map
projection**

The main coordinate
reference systems for describing a geographic position
mathematically are *geodetic reference systems* and *map
projections* (Fig. 1).

**Geodetic reference
systems** are used for describing the figure of the
Earthand positions on it:
*ellipsoids* (and the *sphere*) are used for describing
the horizontal position, whereas *geoids* are the gravity
related model for referencing the elevation. Geodetic reference
systems have a *datum*, describing the position and
orientation of the model in relation to the Earth and its
surface.
Not considering the
correct definition of the systems together with their relations may
effort positional errors of hundreds of meters or
more.