[shagie]

Another system from back in the days of pre-very accurate gps: The advanced snowplow program http://www.path.berkeley.edu/sites/default/files/documents/i... and from archive.org: http://web.archive.org/web/20070112182654/http://www.path.be...

Picture driving a snow plow in the California sierras in whiteout conditions. On one side of you is a steep cliff with no guard rails (not that guard rails would - the snow is higher than the rails).

So, they embedded magnets in the road that had data in them (the way they were organized):

> Two primary technologies are being used in the ASP: detecting the plow's position relative to the center of the lane, to assist the operator in steering; and detecting obstacles, for collision warning. PATH's magnetic marker guidance system provides guidance information. This system was developed for automated vehicle guidance and control applications. A single magnetometer array comprised of seven magnetic sensors was installed at the front of the snowplow. Signal processing of the magnetometers provides lateral position measurement relative to the center of the lane, longitudinal position relative to mileposts, and yaw angle estimate. Binary coding of the magnetic markers when installed (north pole up vs. south pole up) also provides information about roadway characteristics, e.g. the direction and radius of the curves. The obstacle detection system uses a commercially available Eaton-Vorad radar, incorporating a digital interface developed by AHMCT in conjunction with Eaton Vorad.