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# Geographic coordinate system

(Redirected from Geographic coordinates)

Map of Earth showing lines of latitude (vertically) and longitude (horizontally); large version (pdf)

The geographic (earth-mapping) coordinate system expresses every horizontal position on Earth by two of the three coordinates of a spherical coordinate system which is aligned with the spin axis of the Earth. It defines two angles measured from the center of the Earth:

• the latitude measures the angle between any point and the equator
• the longitude measures the angle along the equator from an arbitrary point on the Earth. Greenwich in London is the accepted zero-longitude point in most modern societies.

By combining these two angles, the horizontal position of any location on Earth can be specified. For example, Baltimore, Maryland (in the USA) has a latitude of 39.3 degrees North, and a longitude of 76.6 degrees West (). So, a vector drawn from the center of the Earth to a point 39.3 degrees north of the equator and 76.6 degrees west of Greenwich will pass through Baltimore.

The equator is obviously an important part of this coordinate system, it represents the zeropoint of the latitude angle, and the halfway point between the poles. The equator is the fundamental plane of the geographic coordinate system. All spherical coordinate systems define such a fundamental plane.

Lines of constant latitude are called parallels. They trace circles on the surface of the Earth, but the only parallel that is a great circle is the equator (latitude=0 degrees). Lines of constant longitude are called meridians. The meridian passing through Greenwich is the Prime Meridian (longitude=0 degrees). Unlike parallels, all meridians are great circles, and meridians are not parallel: they intersect at the north and south poles.

This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml

To be merged:

Latitude and longitude are arbitrary measurements used to describe any point on the Earth, or similar globe. Borrowing from theories of ancient Babylonians, later expanded by the famous Greek thinker and geographer Ptolemy, a full circle is assigned 360 degrees. Latitude is the term for the distance from the middle of the circle, or, in the case of the Earth, the equator. The equator is designated 0 degrees, with each pole being 90 degrees. Longitude is the horizontal measurement--current convention places the zero degree point at Greenwich, England (also known as the Prime Meridian), with 180 degrees being on the opposite point on the globe.

Lines of longitude are all of the same length whereas those of latitude vary in length.

## Third dimension

To completely specify a location on, in, or above the Earth, one has to specify also the elevation / height position. This can e.g. be expressed relative to a datum such as mean sea level (above mean sea level) or the geoid. The distance to the Earth's center is a practical coordinate both for very deep positions and for positions in space.

The elevation specifies the vertical position of the Earth surface.

Various elevation / height coordinates either with respect to the surface or some other datum are altitude, height, and depth.