Map-Matching Procedures at Urban Grade Intersections Lakakis K. and Savvaidis P. ABSTRACT Laboratory of Geodesy and Geomatics of Aristotle University of Thessaloniki has developed and operated a closed vehicle navigation, guidance and positioning system for vehicles in the city of Thessaloniki. The VECON (VEhicles COntrol and Navigation) system produces its own spatial and temporal data, using GNSS (Global Navigation Satellite System) and GIS (Geographic Information System) technologies. A very important issue, relating to the navigating performance of the system, is the quality of the procedures used for the matching of these data to the base map. This procedure is internationally called “Map-Matching”, and this paper is a presentation of the integrated methodology used in VECON, focused to the solutions used by the system at urban grade intersections. Key words : GPS/GNSS, GIS, Map-Matching, Grade Intersections, Vehicle Navigation Systems. INTRODUCTION A major problem concerning the vehicle navigation systems in urban areas is the map- matching quality of spatial and temporal data produced for each road segment. The exact positioning of a vehicle on a digital base map of the road network is very important to all vehicle navigation applications, such as vehicle fleet management, stand-alone navigation systems, etc. Usually, vehicle fleet management systems use GNSS technology (stand-alone GPS with EGNOS services, or - local or wide - differential GPS methods) to initially determine the position of each vehicle. Moreover, a GIS software platform is used, to enable the use of techniques such as map-matching, graphic analysis (e.g. optimum path, “active” area etc.), real time monitoring, geostatistical analysis of spatial and temporal changes etc. Stand-alone GPS in kinematic measurements can provide horizontal accuracy of about 5-25 m. Differential GPS can increase the accuracy of measurements to 1 – 5 m, using differential corrections received by a GPS base station. Real time differential GPS used in vehicle navigation requires radio linkage between GPS receivers in vehicles and the base station. In the case of urban navigation of vehicles, implementation of the aforementioned techniques is subjected to certain limitations, resulting mainly from the reduced optical contact with the satellite constellation (stand-alone and differential GPS), and problems relating to the transmission of differential corrections via the local radio link (differential GPS). In the general framework of navigation systems, mainly in urban navigation, satellite systems should be enhanced by the use of other systems, such as inertial or dead reckoning systems. The systems used more often, mainly in relation to their lower cost, are the dead reckoning systems, using digital compasses and odometers [1]. These systems are used in order to “connect” and “fill” the areas, where spatial tracking of a moving vehicle from the GPS is impossible, due to restricted visibility. During, approximately, the last two years, the