1.0 Introduction In the process of map making ellipsoidal or spherical surfaces are used to represent the surface of the Earth. These curved reference surfaces are then projected on a map formed into a cylinder, cone or flat plane (figure below). Since a map is a small-scale representation of the Earth's surface it is necessary to apply some kind of scale reduction.
1.1 Reference surfaces Two main reference surfaces
(or Earth figures) are used to approximate the shape of the Earth. One is called the ellipsoid, the other is the Geoid. The or height datums.local vertical datums The f) and longitude (l), of points of interest. These locations on the ellipsoid are then projected onto a mapping plane. There are many different ellipsoids defined in the world, some well-known are the WGS84, GRS80, International 1924 (also known as Hayford), Krasovsky, Bessel, or
the Clarke 1880 ellipsoid.
To measure locations accurately, the selected ellipsoid
should fit the area of interest. Therefore a ) is established, which is an ellipsoid but positioned and
oriented in such a way that it best fits to the area or country of interest.
There are a few hundred of these local horizontal datums defined in the
world.
Recent years have seen that globalisation is leading to the definition of global (or geocentic) datums, such as the ITRF or WGS84.geodetic datum
1.2 Map projections To produce a map the curved reference surface of the Earth,
approximated by an ellipsoid or a sphere, is transformed to the flat plane
of the map by means of a map projection. In other
words, each point on the reference surface of the Earth with (Cartesian coordinatesx, y) or map coordinates representing positions on the map plane.
Hundreds of map projections
are developed in order to accurately represent a particular map or to best suit a particular type of map. Examples of
map projections are Transverse Mercator (also known as Gauss-Krüger), equidistant cylindrical and conic projection, Lambert's azimuthal, conic and cylindrical projection, stereographic projection, and various others. Map projections are typically classified according to the geometric surface from which they are derived: cylinder, cone or plane. The three classes of map projections are respectively and conical .azimuthalThree classes of map projections.
Furthermore map projections are typically classified according to the distortion properties of a map. The three distortion properties of map projections are respectively: or equidistant. Equal-area projections correctly represent area sizes, equidistant map projections correctly represent distances (in certain directions), while conformal map projections correctly represent angles and shapes (of small areas).conformal
1.3 Map coordinate systems A map coordinate system can be created by choosing a projection and then tailoring its parameters to fit any region on the Earth. An example is the coordinate system used in the Netherlands. It is called The coordinate system of the Netherlands is derived from an oblique azimuthal stereographic projection.
An overview of map coordinate systems used in the different European countries (external link):
Standard coordinate systems have been developed to simplify the process of choosing a system. The most important standard map coordinate system used is the Universal Transverse Mercator (UTM). Recent years have seen that globalisation is leading to the establishment of global 3D coordinate systems. These spatial reference systems can be realized thanks to advances in satellite-based positioning. The most important standard 3D system for the GIS community is the |