Ordnance Survey National Grid

The Ordnance Survey National Grid reference system is a system of geographic grid references used in Great Britain, different from latitude and longitude. It is often called British National Grid (BNG). [1] [2]

The Ordnance Survey (OS) devised the national grid reference system, and it is heavily used in their survey data, and in maps based on those surveys, whether published by the Ordnance Survey or commercial map producers. Grid references are also commonly quoted in other publications and data sources, such as guide books or government planning documents.

A number of different systems exist that can provide grid references for locations within the British Isles: this article describes the system created solely for Great Britain and its outlying islands (including the Isle of Man); the Irish grid reference system is a similar system created by the Ordnance Survey of Ireland for the island of Ireland. The Universal Transverse Mercator coordinate system (UTM) is used to provide grid references for worldwide locations and this is the system commonly used for the Channel Islands. European-wide agencies also use UTM when mapping locations, or may use the Military Grid Reference System (MGRS) system, or variants thereof.

General

Grid square TF. The map shows The Wash and the North Sea, as well as places within the counties of Lincolnshire, Cambridgeshire and Norfolk.

The grid is based on the OSGB36 datum (Ordnance Survey Great Britain 1936,[ citation needed] based on the Airy 1830 ellipsoid),[ citation needed] and was introduced after the retriangulation of 1936–1962.[ citation needed] It replaced the previously used Cassini Grid which,[ citation needed] up to the end of World War Two, had been issued only to the military.[ citation needed]

The Airy ellipsoid is a regional best fit for Britain; more modern mapping tends to use the GRS80 ellipsoid used by the Global Positioning System (the Airy ellipsoid assumes the Earth to be about 1 km smaller in diameter than the GRS80 ellipsoid, and to be slightly less flattened). The British maps adopt a Transverse Mercator projection with an origin (the "true" origin) at 49° N, 2° W (an offshore point in the English Channel which lies between the island of Jersey and the French port of St. Malo). [3] Over the Airy ellipsoid a straight line grid, the National Grid, is placed with a new false origin to eliminate negative numbers, creating a 700 km by 1300 km grid. This false origin is located south-west of the Isles of Scilly. The distortion created between the OS grid and the projection is countered by a scale factor in the longitude to create two lines of longitude with zero distortion rather than one. Grid north and true north are only aligned on the 400 km easting of the grid which is 2° W (OSGB36) and approx. 2° 0′ 5″ W ( WGS 84).

OSGB 36 was also used by Admiralty nautical charts until 2000[ citation needed] after which WGS 84 has been used.

A geodetic transformation between OSGB 36 and other terrestrial reference systems (like ITRF2000, ETRS89, or WGS 84) can become quite tedious if attempted manually. The most common transformation is called the Helmert datum transformation, which results in a typical 7 m error from true. The definitive transformation from ETRS89 that is published by the OSGB is called the National Grid Transformation OSTN02. [4] This models the detailed distortions in the 1936–1962 retriangulation, and achieves backwards compatibility in grid coordinates to sub-metre accuracy.

Datum shift between OSGB 36 and WGS 84

The difference between the coordinates on different datums varies from place to place. The longitude and latitude positions on OSGB 36 are the same as for WGS 84 at a point in the Atlantic Ocean well to the west of Great Britain. In Cornwall, the WGS 84 longitude lines are about 70 metres east of their OSGB 36 equivalents, this value rising gradually to about 120 m east on the east coast of East Anglia. The WGS 84 latitude lines are about 70 m south of the OSGB 36 lines in South Cornwall, the difference diminishing to zero in the Scottish Borders, and then increasing to about 50 m north on the north coast of Scotland. (If the lines are further east, then the longitude value of any given point is further west. Similarly, if the lines are further south, the values will give the point a more northerly latitude.) The smallest datum shift is on the west coast of Scotland and the greatest in Kent.

Datum shift between OSGB 36 and ED 50

These two datums are not both in general use in any one place, but for a point in the English Channel halfway between Dover and Calais, the ED50 longitude lines are about 20 m east of the OSGB36 equivalents, and the ED50 latitude lines are about 150 m south of the OSGB36 ones.[ citation needed]

Illustration of the Ordnance Survey National Grid coordinate system, with Trafalgar Square as an example