1. A Projected coordinate system (PCS) is a two-dimensional planar surface. However, the Earth's surface is three-dimensional. Transforming three-dimensional space onto a two-dimensional surface is called projection. Projection formulas are mathematical expressions that convert data from a geographical location (latitude and longitude) on a sphere or spheroid to a corresponding location (x and y) on a flat, two dimensional surface.
2. A projected coordinate system (PCS) is a method of transforming the surface (or a portion of the surface) of the earth into a flat plane.
Coordinate Reference System
http://edndoc.esri.com/arcsde/9.2/concepts/geometry/coordref/coordsys/projected/projected.htm (1.); http://vsp.pnnl.gov/help/Vsample/Projected_Coordinate_System.htm (2.)
Author: Thomas D.