Geographic Coordinates

The geographic coordinates (or “geographic points of reference”) of a place are its latitude, longitude and altitude in relation to sea level. In order to get one’s bearings on the surface of the earth, another system, known as “cartographical markers” is used.

To get a location on the earth, it is necessary to use a geodesic system from which the geographic coordinates found on maps stem. These coordinates can be expressed in longitude and latitude (so-called geographic coordinates), or as a flat cartographical representation (so-called projection coordinates).

The standard projection in France is a Lambert-type conical shape (NTF geodesic system). With the aim of minimising deformations (linear alterations), France was divided into four zones. A projection known as “extended Lambert 2” covers the whole country for requirements of a national scope. The inconvenience of this extended system is that the linear alteration increases towards the North and South.

The Cartesian coordinates X and Y of a water point are the coordinates used in the projection, which is normally Lambert 2, and is extended to all the water points located in mainland France and in Corsica. The projection is expressed to a maximum scale of one metre. For water points located outside of mainland France, coordinates are expressed in the local projection system.