1. Apparatus for providing information to improve the accuracy of tracking a vehicle movable over streets in a given area, comprising: (a) first means for providing data identifying respective positions of the vehicle, each position having an accuracy relative to an actual location of the vehicle and one of said positions being a current position, and for providing data identifying a measured heading of the vehicle; (b) second means for providing a map data base of the streets; and (c) means for deriving any of a plurality of parameters in dependence on one or more respective positions of the vehicle and the streets of the map data base to determine if a more probable current position exists, wherein one of said parameters is the measured heading of the vehicle as compared with the headings of the streets of the map data base.