World Geographic Reference System: Difference between revisions

Revision as of 04:36, 12 July 2014

Revision as of 03:48, 17 February 2015

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'''Georef''' ('''World Geographic Reference System''') is a grid-based method of specifying locations on the surface of the [[Earth]]. Georef is essentially based on the geographic system of [[latitude]] and [[longitude]], but using a simpler and more flexible [[notation]]. Georef was used primarily for [[air navigation]], particularly in military or inter-service applications, but it is rarely seen today. However, Georef can be used with any map or chart that has latitude and longitude printed on it.

== Operation ==

Georef is based on the standard system of [[latitude]] and [[longitude]], but uses a simpler and more concise notation. Georef divides the Earth's surface into successively smaller quadrangles, with a notation system used to identify each quadrangle within its parent. Unlike latitude / longitude, Georef runs in one direction horizontally, east from the 180° [[meridian (geography)|meridian]]; and one direction vertically, north from the [[South Pole]]. Georef can easily be adapted to give co-ordinates with varying degrees of precision, using a 2-122–12 character [[geocode]].

Georef co-ordinates are defined by successive divisions of the Earth's surface, as follows:

* The first level of Georef divides the world into quadrangles each measuring 15 degrees of longitude by 15 degrees of latitude; this results in 24 '''zones''' of longitude and 12 '''bands''' of latitude. A longitude zone is identified by a letter from A to Z (omitting I and O) starting at 180 degrees and progressing eastward through the full 360 degrees of longitude; a latitude band is identified by a letter from A through M (omitting I) northward from the south pole. Hence, any 15 degree quadrangle can be identified by two letters; the easting (longitude) is given first, followed by the northing (latitude). These two letters are the first two characters of a full Georef coordinate.

* Each 15-degree quadrangle is further divided into smaller quadrangles, measuring 1 degree of longitude by 1 degree of latitude. These quadrangles are lettered A to Q (omitting I and O), running from west to east for longitude; and A to Q (omitting I and O), running south to north for latitude. These letters form the third and fourth characters of a full Georef coordinate. Four letters thus identify any 1-degree quadrangle in the world.

* Each of the 1-degree quadrangles is further subdivided into 60 1-minute longitude zones, numbered 00 through 59 from west to east, and 60 1-minute latitude bands, numbered 00 to 59 from south to north. These numbers are always written as two digits, with a leading zero if necessary, and the easting is always followed by the northing. Thus, 4 letters and 4 digits give the position of any 1-minute quadrangle.

* Each of the 1-minute quadrangles may be further divided into 10 or 100 smaller divisions both north-south and east-west, permitting the identification of 0.1-minute or 0.01-minute quadrangles. The Georef coordinate for any 0.1-minute quadrangle consists of four letters and six numbers; the Georef coordinate for any 0.01-minute quadrangle consists of four letters and eight numbers.

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[[Image:georef-local.jpg|thumb|256px|right|The GJ local grid]]

For example, on a GEOREF chart, [[Naval Air Station Patuxent River]] is located (to the nearest minute) at position GJPJ3716. <!-- Figure needed! In figure 12-10, the blacked-out square (PJ) within the enlarged 15° square (GJ) indicates the 1° area that contains Patuxent. -->

[[Image:Georef Example Patuxent.png|Breakdown of Georef co-ordinate GJPJ3716 into 15-degree quadrangle (GJ), 1-degree quadrangle (PJ), minutes east (37), and minutes north (16).]]

To locate the position from the coordinates, proceed as follows:

* Right from 180° longitude to longitude zone G

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== Designation of area ==

Extensions to the above notation allow the Georef system to be used to designate an area around a reference point. This is achieved by adding an area designation to a base Georef co-ordinate. The area designation can be the letter S, to specify the sides of a rectangle (separated by the letter X); or the letter R, to specify the radius of a circle. In both cases the units are nautical miles. In addition, the letter H can be added, followed by an altitude in thousands of feet.

For example, the reference GJQJ0207S6X8 designates a rectangle centered around Deal Island (GJQJ0207), running {{convert|6|nmi|km|0}} east-west and {{convert|8|nmi|km|0}} north-south. Designation GJPJ4103R5 means a circle around Point Lookout (GJPJ4103) with a radius of {{convert|5|nmi|km|0}}. Designation GJPJ3716H17 means a height of 17,000 feet over GJPJ3716.