to determine the position of the vehicle on a geographical map preferably a road map one or more road maps enhanced with data representing a road network in a geographical area within which the vehicle is located are indicated as a whole by 16 these may be permanently stored in a mass storage medium on board the vehicle or downloaded from a remote supplier via on board communication system or personal communication system in possession of a user in the latter case with a predetermined frequency for example relating to the current location of the vehicle an electronic processing system 18 is arranged for integrating the vehicle geographical positioning data with the supplementary information obtainable from the data representing the road network in the geographical positioning area and for building an electronic horizon of the vehicle an electronic horizon typically forms a database that includes a road map referenced to the geographical coordinates of the road network that it represents in which a plurality of attributes is associated with each road segment including geometrical data such as for example the length the radius of curvature the slope and other data such as the class motorway suburban road urban road country road the number of lanes depending on the road class a presumed standard width is.