A geographic coordinate system is a coordinate system that enables every location on the Earth to be specified by a set of numbers. The coordinates are often chosen such that one of the numbers represent vertical position, and two or three of the numbers represent horizontal position. A common choice of coordinates is latitude, longitude and elevation.
1 Geographic latitude and longitude
2 Latitude and longitude in practice
3 UTM and UPS systems
4 Stereographic coordinate system
5 Geodetic height
6 Cartesian coordinates
7 Shape of the Earth
8 Expressing latitude and longitude as linear units
9 Datums often encountered
10 Geostationary coordinates
11 See also
12 Notes
13 References
14 External links
The geographic latitude (abbreviation: Lat., φ, or phi) of a point on the Earth's surface is the angle between the equatorial plane and a line that passes through that point and is normal to the surface of a reference ellipsoid which approximates the shape of the Earth.[n 1] This line passes a few kilometers away from the center of the Earth except at the poles and the equator where it passes through Earth's center.[n 2] Lines joining points of the same latitude trace circles on the surface of the Earth called parallels, as they are parallel to the equator and to each other. The north pole is 90° N; the south pole is 90° S. The 0° parallel of latitude is designated the equator, the fundamental plane of all geographic coordinate systems. The equator divides the globe into Northern and Southern Hemispheres.
The Longitude (abbreviation: Long., λ, or lambda) of a point on the Earth's surface is the angle east or west from a reference meridian to another meridian that passes through that point. All meridians are halves of great ellipses (often improperly called great circles), which 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 zero/zero point is located in the Gulf of Guinea about 625 km south of Tema, Ghana.
In 1884 the United States hosted the International Meridian Conference and twenty-five nations attended. Twenty-two of them agreed to adopt the location of Greenwich as the zero-reference line. The Dominican Republic voted against the adoption of that motion, while France and Brazil abstained.[2] To date, there exist organizations around the world which continue to use historical prime meridians which existed before the acceptance of Greenwich became common-place.[n 3]
The combination of these two components specifies the position of any location on the planet, but does not consider altitude nor depth.
This latitude/longitude "webbing" is known as the conjugate graticule.
In defining an ellipse, the short (vertical) diameter is known as the conjugate diameter, and the long (horizontal) diameter—perpendicular, or "transverse", to the conjugate—is the transverse diameter.[3] 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).
^ The surface of the Earth is closer to an ellipsoid than to a sphere, as its equatorial diameter is larger than its north-south diameter.
^ The greatest distance between an ellipsoid normal and the center of the Earth is 21.9 km at a latitude of 45°, using Earth radius#Radius at a given geodetic latitude and Latitude#Comparison of selected types: (6367.5 km)×tan(11.67')=21.9 km.
^ The French Institut Géographique National (IGN) maps still use longitude from a meridian passing through Paris, along with longitude from Greenwich.
Latitude and longitude in practice
Say you set up your Wild T4 next to the water tank north of the airport at Hilo, Hawaii, intending to determine its latitude and longitude by the stars. NGS predicts you will find the tank to be at 19.7323 deg North, 155.0412 deg West.[4]
You cross the island and set the T4 next to the Keahole Point lighthouse; NGS estimates that by the stars the lighthouse will turn out to be 19.7244 N 156.0787 W. [5] Calculating the distance from the water tank to the lighthouse using those lat-lons we get about 108.8 km, but if we measure the actual distance it turns out to be 105.5 km. What went wrong?
Hawaii is an extreme case of a problem that exists everywhere: when trying to measure latitude and longitude by the stars we can only orient our measuring device by gravity. We'd like the T4's axis to point to the center of the Earth, but the T4's level vials don't know where that is — all they know is the direction of gravity, which is much affected by that 4000-meter mountain 50 km away. So when we measure the lat-lons for two points the relationship between those two points can be distorted, which renders their lat-lons fairly useless for most people. When we measure the lat-lons of two points we want to be able to use those lat-lons to calculate the distance and direction from one to the other; we want to be able to draw a scale map and plot points on it by their lat-lons, and the distance between any pair of points on the map is supposed to closely match the actual distance we would measure on the ground.
So we need a different plan — a different definition of latitude and longitude. What they did in Hawaii circa 1930 was call the marker "Oahu West Base"[6] 21 deg 18 min 13.889 sec North, 157 deg 50 min 55.796 West, and define the lat-lon of every other point by its distance and direction from there.[7] NGS now says that in 1993 that point was 21-18-02.54891 N 157-50-45.90280 W in the present NAD83 system. Was the old lat-lon off by 300+ meters? Well, yes, but the relationships between points in the islands were much more accurate than that. C&GS triangulated from island to island, calculating each successive point's lat-lon by its distance and direction from the previous points in the chain. Eventually they deemed the Hilo water tank to be at 19-43-54.526 N 155-03-26.463 W, which would make it 339191.7 meters from Oahu West Base on the Clarke 1866 spheroid. NGS now figures those two points are 339192.8 meters apart.
Similarly in North America. If in 1980 you had asked NGS for the lat-lons for the Empire State Building and a certain water tank in Anchorage, the NAD27 lat-lons they would have given you would be different from the current ones, but the distance you would have calculated then is 8.2 meters different from now. A transcontinental triangulation cannot do better than that
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