Abstract: The vehicle is operated over land which is described by a grid coordinate system having a north direction offset from geographic north. The vehicle has a pick-up to determine its heading speed along its longitudinal center line. The spin axis of a free gyro establishes a reference axis. A meridian gyro determines the angle, if any, between said reference axis and geographic north at the start of a mission. At the start of the mission a computer is supplied with information as to the starting point in said grid coordinate system, the angle between geographic north and grid north, and an estimated corrective factor (based on the type of terrain to be traversed) to be applied to the measured heading speed. The computer continuously determines, for display, the position of the vehicle in said grid system.