The State Plane Coordinate System (SPCS) was developed in the 1930s by the U.S. Coast and Geodetic Survey to provide a common reference system for surveyors and mappers. The goal was to design a conformal mapping system for the country with a maximum scale distortion of one part in 10,000, which at the time was considered the limit of surveying accuracy. The State Plane Coordinate System (SPCS) is used for local surveying and engineering applications, but isn’t used if crossing state lines.
The State Plane grid system is very similar to that used with the UTM system, with the exception of where the origin for the grids are located. The easting origin for each zone is always placed an arbitrary number of feet west of the western boundary of the zone, eliminating the need for negative easting values. The northing origin, however, is not at the equator as in UTM, but rather it is placed at an arbitrary number of feet south of the state border.
To maintain the accuracy of one part in 10,000 and minimize distortion, large states were divided into zones, and depending on their orientation different projections were chosen. The three conformal projections used are listed below.
Lambert Conformal Conic… for states that are longer east–west, such as Tennessee and Kentucky.
Transverse Mercator projection… for states that are longer north–south, such as Illinois and Vermont.
The Oblique Mercator projection… for the panhandle of Alaska, because it lays at an angle.
The number of zones in a state is usually determined by the area the state covers and ranges from one to as many as ten in Alaska. Each zone has a unique central meridian.
Idaho uses a transverse Mercator projection and is divided into three zones – east, central and west