Dynamic navigation system

A computer software product comprising a computer-readable medium in which program instructions are stored, which instructions, when read by a computer, cause the computer to access map data in a memory, the map data comprising vector information delineating roads in a map, and to download to a mobile client device a portion of the vector information corresponding to an area in which a user of the client device is traveling,

• the instructions further causing the computer to download a program to the client device, which causes the client device to find approximate position coordinates of the client device using the location providing device, and to correct the approximate position coordinates using the downloaded vector information, so as to determine a location of the user on one of the roads in the map and to provide a navigation aid to the user based on the determined location,
• wherein the program causes the client device to determine, based on the approximate position coordinates, respective probabilities that the user is located on two or more of the roads, and to determine the location of the vehicle on the one of the roads responsively to the probabilities.