State Plane Coordinate System
The State Plane Coordinate System (SPCS) divides each US state into one or more zones, each with a Lambert Conformal Conic, Transverse Mercator or (in the Alaska Panhandle) Oblique Mercator projection chosen to keep distortion below 1:10,000. Engineering drawings, county GIS and property surveys usually use State Plane on NAD83.
All 124 SPCS 83 zones are here: the 50 states, Puerto Rico and the US Virgin Islands. Results come in each state's legal unit, as listed by NGS: US survey feet in most states, international feet in Arizona, Michigan, Montana, North Dakota, Oregon and South Carolina, and metres in Alabama, Alaska, Hawaii, Missouri and Puerto Rico. A metric version of every zone is in the list of other systems. The projections were checked against 348 NGS datasheet positions in 120 zones, to better than 1 mm.
Zones follow county lines. The converter picks the zone from the official zone outlines published in the EPSG registry, downloaded only when needed, and asks you to confirm only near a boundary. In Kentucky it suggests the single statewide zone (1600) that NGS publishes; the older North and South zones can still be chosen from the list.
Frequently asked questions
US survey feet or international feet?
It depends on the state. NGS lists the legal unit for SPCS 83 in each state: US survey feet (1 ft = 1200/3937 m) in most of them, international feet (0.3048 m) in AZ, MI, MT, ND, OR and SC, and metres only in AL, AK, HI, MO and PR. The converter uses that unit and shows it next to the result. The two feet differ by 2 parts per million, about 1 ft per 100 miles, so check the unit on your drawing.
How do I know which State Plane zone to use?
Zones follow county lines. The converter uses the official zone outlines, so the suggested zone is right almost everywhere; near a county boundary it asks you to confirm, and you can always pick another zone from the list.