The altimetry permits to determine the height of one point that is expressed by the difference between the point and a surface of reference as the sea level. The geoids are the surface of reference for the observations of altimeter. The measure of the heights called levelling makes itself by lines, whose intermediate points are marked with the help of ankles implanted in the facades of the buildings. The set of these reference marks constitutes the network of levelling of the studied geographical zone. All information on these reference marks is listed by the direction of the land-registry of the wilaya.
The plane coordinates of one point are calculated near to the centimetre with the help of triangles, where we measured the angles and the distances with tachometers. To orient their observations, the geometers determined the coordinates of the mosques minarets or the water castles that represent very visible reference marks from afar. The administration of the land-registry and the topography manages the geodesic basis network composed of score thousand points. The system of positioning by GPS satellites (Global Positioning System) permits the precise determination of the coordinates of one point quickly on earth. To be able to produce a card or a plan from the measured data, we conduct calculations of a mathematical projection. Several geodesic systems exist. An important system is the WGS4 that is the basis of the GPS system (Laurini R. & Thompson D, 1992).