Map projections require a point of reference on the Earth’s surface. Most often this is the center, or origin, of the projection. This point is defined in two coordinate systems:
Geographical, or spherical, coordinates are based on the network of latitude and longitude (Lat/Lon) lines that make up the graticule of the Earth. Within the graticule, lines of longitude are called meridians, which run north and south, and the prime meridian at 0° (Greenwich, England). Meridians are designated as 0° to 180°, east or west of the prime meridian. The 180° meridian (opposite the prime meridian) is the International Dateline.
Lines of latitude are called parallels, which run east and west. Parallels are designated as 0° at the equator to 90° at the poles. The equator is the largest parallel. Latitude and longitude are defined with respect to an origin located at the intersection of the equator and the prime meridian. Lat/Lon coordinates are reported in degrees, minutes, and seconds. Map projections are various arrangements of the Earth’s latitude and longitude lines onto a plane.
Planar, or Cartesian, coordinates are defined by a column and row position on a planar grid (X,Y). The origin of a planar coordinate system is typically located south and west of the origin of the projection. Coordinates increase from 0,0 going east and north. The origin of the projection, being a false origin, is defined by values of false easting and false northing. Grid references always contain an even number of digits, and the first half refers to the easting and the second half the northing.
In practice, this eliminates negative coordinate values and allows locations on a map projection to be defined by positive coordinate pairs. Values of false easting are read first and may be in meters or feet.