Geographic Coordinate System 

Release 9.2
Last modified August 3, 2007 
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A geographic coordinate system is a method for describing the position of a geographic location on the Earth's surface using spherical measures of latitude and longitude. These are measures of the angles (in degrees) from the center of the Earth to a point on the Earth's surface.
In the geographic coordinate system, the sphere is divided into equal parts usually called degrees; some countries use grads. A circle is 360° or 400 grads. Each degree is subdivided into 60 minutes, with each minute composed of 60 seconds.
The geographic coordinate system consists of latitude and longitude lines. Each line of longitude runs north–south and measures the number of degrees east or west of the prime meridian. Values range from 180 to +180°. Lines of latitude run east–west and measure the number of degrees north or south of the equator. Values range from +90° at the North Pole to 90° at the South Pole.
The equator is at an angle of 0 degrees latitude. Often, the northern hemisphere has positive measures of latitude and the southern hemisphere has negative measures of latitude. Longitude measures angles in an eastwest direction. Longitude measures are traditionally based on the Prime Meridian, which is an imaginary line running from the North Pole through Greenwich, England to the South Pole. This angle is Longitude 0. West of the Prime Meridian is often recorded as negative Longitude and east is recorded as positive. For example, the location of Los Angeles, California is roughly Latitude "plus 33 degrees, 56 minutes" and Longitude "minus 118 degrees, 24 minutes."
Users with global datasets often use geographic coordinates to store and manage their data on a global framework but project the data into a local planar coordinate system for editing and analysis.
Map projections also use latitude and longitude values from geographic coordinate systems to reference parameters, such as the central meridian, the standard parallels, and the latitude of origin.