*11. May 2007*
*Definitions*

**Coordinate Reference System (CRS)**

A combination of at least one coordinate system together with its spatial datum. In a CRS,
positions or locations of geographic information are described by coordinates.

**Geodetic Reference System/Geodetic Coordinate Reference System **

A coordinate reference system, based on shape, size, position and orientation of a (mathematical)
reference surface, (e.g. sphere or spheroid). It normally is defined in a superior, geocentric three-
dimensional coordinate system (for example WGS84, ETRS89).
**Map Projection/Map Projection Coordinate Reference System**

A map projection
defines the mapping from geographic coordinates on a sphere or the geodetic
coordinates on a spheroid to a plane. It may define
- a map projection CRS
- a conversion between a geodetic CRS
and a map projection CRS.

**Reference Surface**

Mathematically (e.g. sphere or spheroid) or physically (geoid) defined surface to approximate the
shape of the earth for referencing the horizontal and/or vertical position. Each coordinate system
has a reference surface. Map projections have a mapping surface (an extrinsic characteristic) like
cylinder, cone, plane.

**(Coordinate) Reference Frame**

A set of control points to realize a coordinate reference system.

**Spatial Datum**

Describes the fixing (position and orientation etc.) of a CRS regarding to the real world.

**Geodetic Datum**

Traditional term for a geodetic reference system. Now: a spatial datum of a geodetic reference
system.

*Notes*

- A map projection needs information
about a reference surface.
- A geodetic datum fixes the reference
surface in space (superior coordinate reference system).
- National Mapping Agencies defined
their own Reference System. Different Countries may use
the same parameters of the reference surface, but with different position and orientation!
- National Mapping Agencies may use
different map projections, based on the same reference
system. Different map projections depend on scale, area and distortion- charateristic needed.
- Changing map projection between countries
normally need a geodetic datum transformation.
- Some countries use a double projection
for large and medium scale mapping (e.g. the
Netherlands, Switzerland, Hungary):
- conformal projection from the reference
ellipsoid to a sphere
- conformal projection from the sphere
to the plane.

- The process of changing map projection: