State Plane Coordinate System

The State Plane Coordinate System is not a projection. It is a coordinate system that divides all fifty states of the United States, Puerto Rico and the U.S. Virgin Islands into over 120 numbered sections, referred to as zones. Each zone has an assigned code number that defines the projection parameters for the region.

The State Plane Coordinate System (SPCS) is a coordinate system designed for mapping the United States. It was developed in the 1930s by the U.S. Coast and Geodetic Survey to provide a common reference system to surveyors and mappers. The goal was to design a conformal mapping system for the country with a maximum scale distortion of 1 part in 10,000, then considered the limit of surveying accuracy.

Three conformal projections were chosen: the Lambert Conformal Conic for states that are longer in the east-west direction, such as Tennessee and Kentucky, the Transverse Mercator projection for states that are longer in the north-south direction, such as Illinois and Vermont, and the Oblique Mercator projection for the panhandle of Alaska, because it is neither predominantly north nor south, but at an angle.

To maintain an accuracy of 1 part in 10,000, it was necessary to divide many states into zones. Each zone has its own central meridian or standard parallels to maintain the desired level of accuracy. The boundaries of these zones follow county boundaries. Smaller states such as Connecticut require only one zone, whereas Alaska is composed of ten zones and uses all three projections.

This coordinate system is referred to here as the State Plane Coordinate System of 1927 (SPCS 27). It is based upon a network of geodetic control points referred to as the North American Datum of 1927 (NAD27). State Plane and the North American Datum Technological advancements of the last fifty years have led to improvements in the measurement of distances, angles, and the Earth's size and shape. This, combined with moving the origin of the datum from Meades Ranch in Kansas to the Earth's center of mass, for compatibility with satellite systems, made it necessary to redefine SPCS 27. Consequently, the coordinates for points are different for SPCS 27 and SPCS 83. There are several reasons for this. For SPCS 83, all State Plane coordinates published by NGS are in metric units, the shape of the spheroid of the Earth is slightly different, some states have changed the definition of their zones, and values of latitude and longitude are slightly changed.