The State Plane Coordinate System (SPCS), which is only used in the United States, is a plane coordinate system (north-south and east-west lines are perpendicular) in which each individual state has between one to six zones, depending on the state's size and shape. This coordinate system’s high level of accuracy is achieved through the use of relatively small zones.
The State Plane Coordinate System’s 120 different zones generally follow county boundaries (except in Alaska). Larger states are divided into multiple zones, such as the “Colorado North Zone.” States with a long north-south axis (such as Idaho and Illinois) are usually mapped using a Transverse Mercator projection, while states with a long east-west axis (such as Washington and Pennsylvania) are usually mapped using a Lambert Conformal projection. In either case, the projection's central meridian is generally run down the approximate center of the zone.
A Cartesian coordinate system is created for each zone by establishing an origin some distance (usually 2,000,000 feet) to the west of the zone's central meridian and some distance to the south of the zone's southernmost point. This ensures that all coordinates within the zone will be positive. The X-axis running through this origin runs east-west, and the Y-axis runs north-south. Distances from the origin are generally measured in feet, but sometimes are in meters. X distances are typically called eastings (because they measure distances east of the origin) and Y distances are typically called northings (because they measure distances north of the origin).
When the State Plane Coordinate System is shown on a USGS topographic map, the zone is given in the credit legend in the map’s collar. Grid ticks are shown along the map projection line and coordinate values are annotated at one or more corners. However, on most USGS 7.5-minute Historical topographic maps produced 1947-1995, the grid is tied to the outdated North American Datum of 1927 (NAD27), so coordinates on the map might differ by hundreds of meters from coordinates obtained by modern instruments using newer datums.
With the advent of the North American Datum of 1983 (NAD83)--which is virtually identical to the World Geodetic System of 1984 (WGS84) within the United States--State Plane coordinates shifted with it. Offset from the different datums is indicated by a dashed cross in each corner of some topographic maps.
US Topo maps produced 2010-2016 are annotated with NAD83 State Plane Coordinate values, but those ticks and annotations were dropped from US Topo maps in 2017.
Most Geographic Information Systems (GIS) can display SPCS values and grids, and some consumer GPS devices and apps also implement SPCS. Currently, SPCS is mostly used for land surveying and specialized local mapping.