Geographic coordinate systems

A geographic coordinate system is a reference system for identifying locations and measuring features on the curved surface of the earth. It consists of a network of intersecting lines called a graticule. The intersecting lines of the graticule are probably familiar terms to you—longitude and latitude.

The graticule is made up of vertical lines, called lines of longitude, and horizontal lines, called lines of latitude. Because the earth is spherical, these lines form circles.

In a geographic coordinate system, measurements are expressed in degrees, minutes, and seconds. A degree is 1/360th of a circle. Each degree can be divided into 60 minutes, and each minute can be divided into 60 seconds.

Lines of longitude are called meridians. Measures of longitude begin at the prime meridian (which defines the zero value for longitude) and range from 0° to 180° going east and from 0° to -180° going west.

Lines of latitude are called parallels. Measures of latitude begin at the equator and range from 0° to 90° from the equator to the north pole and from 0° to -90° from the equator to the south pole.

The prime meridian (green line) is the starting point for longitude and has a value of 0. The equator (red line) is the starting point for latitude and has a value of 0. It runs midway between the north and south poles, dividing the earth into northern and southern hemispheres.

More about prime meridians

Choosing which meridian will be the prime meridian is completely arbitrary. For example, the Spanish have used a meridian passing through Madrid, the French a meridian passing through Paris, and during the American Revolution, the fledgling U.S. states used either London or Philadelphia as the starting point.

Later, the United States adopted Washington, D.C. as the prime meridian. Finally, in 1884, an International Meridian Conference was held in Washington, D.C., and the Observatory of Greenwich in Great Britain was adopted as the "official" prime meridian.

Longitude and latitude are actually angles measured from the earth's center to a point on the earth's surface. For example, consider the location referenced by the following coordinates:

Longitude: 60 degrees East (60° 00' 00")
Latitude: 55 degrees, 30 minutes North (55° 30' 00")

The longitude coordinate refers to the angle formed by two lines, one at the prime meridian and the other extending east along the equator. The latitude coordinate refers to the angle formed by two lines, one on the equator and the other extending north along the 60° meridian.

Longitude and latitude are angles measured from the earth's center to a point on the earth's surface.