The orientation of a self-propelled vehicle or a towed vehicle relative to the direction of travel of the vehicle can be determined by mounting two ground speed sensors on the vehicle. The orientation of the vehicle relative to the direction of travel of the vehicle can be used to correct the error in the location of an implement mounted on the vehicle determined by a GPS system mounted on the vehicle or the vehicle towing the implement due to crabbing of the vehicle as the vehicle travels substantially perpendicular to a significant slope.