The longitudinal position (i.e. the position along the road) of a vehicle can typically be estimated quite accurately using GPS (perhaps augmented with odometry to handle GPS signal loss). However, obtaining an accurate estimate of a vehicle's lateral position, i.e. its position in the lane(s), perpendicular to the road tangent, is more challenging, as the (standard) GPS signal is not sufficiently accurate for such measurements. Knowledge of the lateral position is essential for the development of autonomous vehicles.