ABSTRACT
Plane surveying has traditionally relied on an imaginary, flat reference surface, or datum, with Cartesian axes. This rectangular system is used to describe measured positions by ordered pairs, usually expressed in northings and eastings or y and x coordinates. Even though surveyors have always known that this assumption of a flat Earth is fundamentally unrealistic, it provided, and continues to provide, an adequate arrangement for small areas. The attachment of elevations to such horizontal coordinates somewhat acknowledges the topographic irregularity of the Earth, but the whole system is always undone by its inherent inaccuracy as surveys grow large.
Development of State Plane Coordinate Systems Designed in the 1930s, the purpose of the state plane coordinate system was to overcome some of the limitations of the horizontal plane datum when they are applied over large areas while avoiding the imposition of geodetic methods and calculations on local surveyors. Using the conic and cylindrical models of the Lambert and Mercator map projections respectively, the flat datum was curved but only in one direction. By curving the datums and limiting the area of the zones the distortion can be limited to a scale ratio of about 1 part in 10,000 without disturbing the traditional system of ordered pairs of Cartesian coordinates.
