*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: