The State Plane Coordinate System (SPCS) is not a projection; rather it is a system for specifying positions of geodetic stations using plane rectangular coordinates. This 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.
There are four possible projections for SPCS. The geometric direction of each state determines the projection utilized. For states that are longer in the east-west direction, the Lambert Conformal Conic is used. States which are longer in the north-south direction use the Transverse Mercator Projection. The panhandle of Alaska, which the sole distinction of lying at an angle, garners the use of the Oblique Mercator Projection. While Guam uses a Polyconic projection
The formulae for these calculations are based on Publication 62-4, "State Plane Coordinates by Automatic Data Processing", U.S. Department of Commerce 1968. These projections should only be used for data that has been computed using this method. For all other state plane calculations use Exact Methods. The parameters for these coordinate systems are defined in Publication 62-4. For further information contact the U.S. Department of Commerce.
The "Guam27" Projection does not require any Parameters.