Georeferencing
The fundamental problem of the location of objects on the terrestrial surface is the curved shape of the relief
and the earth being almost round. To display the information on a flat surface, the characteristics of geometric projection
are used. Different projection methods have been developed.
By georeferencing objects, a referencing system is established for the positioning of the object on the terrestrial
surface. If the reference system is called universal, it is the same for the entire planet, if it is called relative;
it is
related only to a region or a country. To simplify matters, the earth is preferably represented in a flat plain. To do
so,
different map projections have been developed in the way to establish a punctual relation between the terrestrial surface
and its representation in the plain. There are some facts resumed in the following:
- The earth's surface is modeled geometrically by a gravitational equipotential surface, called geoid. The geoid
is approximated by the ellipsoid of revolution, also known as spheroid. Note that the knowledge about the geoid has
been improved with the advent of satellites. In order to ensure consistency, the national mapping systems base on
ellipsoids which have been defined legally.
- In spherical coordinate systems, every point on the earth's surface is expressed by three parameters:
Longitude (l), latitude (p) and distance from the center of the earth (r). If x and y are the coordinates,
a projection is represented by the following function: x = f(r, l, p) et y = g(r, l, p)