ABSTRACT
Plane surveying has traditionally relied on an imaginary flat reference surface, or datum , with Cartesian axes. This rectangular system is used to describe mea sured positions by ordered pairs, usually expressed in northings and eastings, or x-and ^-coordinates. Even though surveyors have always known that this as sumption 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 irregu larity of the earth, but the whole system is always undone by its inherent inaccu racy as surveys grow large.
