This paper tackles the problem of vehicle's geolocalisation in urban areas. For this purpose, Global Positioning System (GPS) receiver is the main sensor. But the use of GPS alone is not sufficient in many urban environments. GPS has so to be helped with dead-reckoned sensors, map data, cameras, range finder ... In this paper, we propose a novel approach to compute observation of the absolute pose of the vehicle to back up GPS and to compensate the drift of deadreckoned sensors. This approach uses a new source of information which is a 3D city model i.e. 3D city model of the environment of vehicle evolution. This 3D city model is managed in real-time by a 3D Geographical Information System (3D-GIS). The pose's observation is constructed by using an on-board horizontal laser scanner which provides a set of distances. This set of distances (laser scan data) is matched with depth information (virtual laser scan data), provided by 3D-GIS, using Iterative Closest Point algorithm (ICP). Experimental results performed using real data illustrate the performances of the proposed approach.