The first BNG designator defines a 500 km square. The second designator defines a 100 km square. NAD 27 coordinates are based on the foot. NAD 83 coordinates are based on the meter. State plane systems were developed in order to provide local reference systems that were tied to a national datum.
Some smaller states use a single state plane zone.
Larger states are divided into several zones. State plane zone boundaries often follow county boundaries. Back to Top Public Land Rectangular Surveys Public Land Rectangular Surveys have been used since the 1790s to identify public lands in the United States. The system is based on principal meridians and baselines. Townships, approximately six miles square, are numbered with reference to baseline and principal meridian.