*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 Earth and 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.