A geographic coordinate system enables every location on the Earth to be specified in three coordinates, using mainly a spherical coordinate system.
The Earth is not a sphere, but an irregular shape approximating an ellipsoid; the challenge is to define a coordinate system that can accurately state each topographical feature as an unambiguous set of numbers.
Latitude (abbreviation: Lat. or (φ) pronounced phi) is the angle from a point on the Earth's surface and the equatorial plane, measured from the centre of the sphere. Lines joining points of the same latitude are called parallels, which trace concentric circles on the surface of the Earth, parallel to the equator. The north pole is 90° N; the south pole is 90° S. The 0° parallel of latitude is designated the equator. The equator is the fundamental plane of all geographic coordinate systems. The equator divides the globe into Northern and Southern Hemispheres.
Longitude (abbreviation: Long. or (λ) pronounced lambda) is the angle east or west of a reference meridian between the two geographical poles to another meridian that passes through an arbitrary point. All meridians are halves of great circles, and are not parallel. They converge at the north and south poles.
A line passing near the Royal Observatory, Greenwich (near London in the UK) has been chosen as the international zero-longitude reference line, the Prime Meridian. Places to the east are in the eastern hemisphere, and places to the west are in the western hemisphere. The antipodal meridian of Greenwich is both 180°W and 180°E. The choice of Greenwich is arbitrary, and in other cultures and times in history other locations have been used as the prime meridian.
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° North, and a longitude of 76.6° West. So, a vector drawn from the center of the Earth to a point 39.3° north of the equator and 76.6° west of Greenwich will pass through Baltimore.
This latitude/longitude "webbing" is known as the conjugate graticule.
In defining an ellipse, the vertical diameter is known as the conjugate diameter, and the horizontal diameter — perpendicular, or "transverse", to the conjugate — is the transverse diameter. With a sphere or ellipsoid, the conjugate diameter is known as the polar axis and the transverse as the equatorial axis. The graticule perspective is based on this designation: As the longitudinal rings — geographically defined, all great circles — converge at the poles, it is the poles that the conjugate graticule is defined. If the polar vertex is "pulled down" 90°, so that the vertex is on the equator, or transverse diameter, then it becomes the transverse graticule, upon which all spherical trigonometry is ultimately based (if the longitudinal vertex is between the poles and equator, then it is considered an oblique graticule).
DMS is the most common format, and is standard on all charts and maps, as well as Global Positioning Systems (GPS) and geographic information systems (GIS). DD is the most convenient if a need for calculation or computation might arise, avoiding the complexity and likely introduction of errors by mixed radix degree minute second arithmetic.
Though early navigators thought of the sea as a flat surface that could be used as a vertical datum, this is far from reality. The Earth can be thought to have a series of layers of equal potential energy within its gravitational field. Height is a measurement at right angles to this surface, and though gravity pulls mainly toward the centre of the Earth, the geocentre, there are local variations. The shape of these layers is irregular but essentially ellipsoidal. The choice of which of these layers to choose is arbitrary. The reference height we have chosen is the one closest to the average height of the world's oceans. This is called the geoid.
The Earth is not static as points move relative to each other due to continental plate motion, subsidence, and diurnal movement caused by the moon and the tides. The daily movement can be as much as a metre. Continental movement can be up to a year, or in a century. A weather system 'high' pressure area can cause a sinking of . Scandinavia is rising by a year as a result of the melting of the ice sheets of the last ice age, but neighbouring Scotland is only rising by . These changes are insignificant if a local datum is used. But these changes are significant if the global GPS datum is used.
The width of one longitudinal degree on latitude can be calculated by this formula (to get the width per minute and second, divide by 60 and 3600, respectively):
|51° 28' 38" N||Greenwich||69.29km||1.155km||19.24m||6.93m|
In popular GIS software, data projected in latitude/longitude is often represented as a 'Geographic Coordinate System'. For example, data in latitude/longitude if the datum is the North American Datum of 1983 is denoted by 'GCS North American 1983'.
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