Projected coordinate system
A projected coordinate system (PCS) describes a two-dimensional planar surface. It inherits the components of a geographic coordinate system and also has:

• Projection—The mathematical transformation used to convert from geographic coordinates to planar (projected) coordinates.
• Parameters—Used in the transformation. These parameters are specific to the projection.
• Units—Linear measurement for coordinates on the plane.

Geographic coordinate system
A geographic coordinate system (GCS) describes a three-dimensional reference system that locates points on the Earth's surface. A GCS has the following components:

• Angular units—The unit of measure on the spherical reference system.
• Spheroid—The reference spheroid for the coordinate system.
• Datum—Defines the relationship of the reference spheroid to the Earth's surface.
• Prime Meridian—The longitude origin of the spherical reference system.

Parameters
Parameters are required by projection and geographic transformation objects

Projections
A map projection is a set of mathematical equations to convert from longitude and latitude (l,j) to planar coordinates (x,y). Converting from three to two dimensions causes distortions. A map projection is designed to minimize the distortion caused by flattening the Earth's surface.

Units of Measure
A unit object may be an angular or linear definition.

Methods of transformation
Changing coordinate systems usually includes transforming between datums. Because datums are based on spheroids, a datum transformation always converts geographic (longitude-latitude) coordinates. Some methods convert the geographic coordinates to geocentric (xyz) coordinates, transform the xyz coordinates, and convert the new values to geographic coordinates.

Spheroids
A spheroid defines the shape and size of a geographic coordinate system's surface. Although the Earth is best represented by a spheroid, it is sometimes treated as a sphere to make mathematical calculations easier. A spheroid consists of a name, the semimajor axis value in meters, and the inverse flattening.

Geographic transformations
A geographic (datum) transformation object is comprised of a name, PeGeographicCS for the from geographic coordinate system. PeGeogprahicCS for the to geographic coordinate system, PeMethod for the method used to transform the geographic coordinate systems and PeParameters for any parameters needed by the transformation method.