Georef

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The GEOREF Grid

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.

1 Operation
- 1.1 Example
2 Designation of area
3 References

[edit] 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; 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 2-12 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.

The initial two letters of a Georef reference, designating the 15 degree quadrangle, can be omitted, if it is clear which 15 degree quadrangle the reference applies to (e.g., when working within a restricted geographical area).

[edit] Example

The GJ local grid

For example, on a GEOREF chart, Naval Air Station Patuxent River is located (to the nearest minute) at position GJPJ3716.

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

Right from 180° longitude to longitude zone G

Up from the South Pole to latitude zone J

Right in zone GJ to the lettered 1° column P

Up in zone GJ to the lettered 1° row J

Right in the 1° horizontal zone to 37 minutes

Up in the 1° vertical zone to 16 minutes

[edit] 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 6 nautical miles east-west and 8 nautical miles north-south. Designation GJPJ4103R5 means a circle around Point Lookout (GJPJ4103) with a radius of 5 nautical miles. Designation GJPJ3716H17 means a height of 17,000 feet over GJPJ3716.