British national grid reference system
- This article is about the map grid references in the Great Britain. For the electric power transmission system in the UK, see National Grid.
The Ordnance Survey (OS) devised the national grid reference system, and it is heavily used in their survey data, and in maps (whether published by the Ordnance Survey or commercial map producers) based on those surveys. Additionally grid references are commonly quoted in other publications and data sources, such as guide books or government planning documents.
Two such systems exist: this article describes the one used for Great Britain and its outlying islands; a similar system, used throughout Ireland (including Northern Ireland), is the Irish national grid reference system (used jointly by the Ordnance Survey of Ireland and Ordnance Survey of Northern Ireland).
Table of contents
The maps are based on the projection called the Airy 1830 ellipsoid, with an origin at 49 ° N, 2 ° W. The ellipsoid is a regional best fit for Britain, more modern mapping tending to use the GRS80 ellipsoid used by the GPS. Over the Airy projection of Britain 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. 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 now 2° 0′ 6″ W.
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 process is called a Helmert datum transformation, the transformation from ETRS89 to OSGB 36 is called the National Grid Transformation OSTN02.
For the first letter the grid is divided into squares of size 500 km by 500 km. There are four of these: S,T,N, and H (plus letter 'O' used for a tiny corner of the North Riding of Yorkshire: OV000000).
For the second letter, each large square is subdivided into 25 squares of size 100 km by 100 km, each with a letter code from A to Z (omitting I) starting with A in the north-west corner to Z in the south-east corner. The accompanying map shows the resultant grid, with the squares containing land lettered.
Within each square, eastings and northings from the origin (south west corner) of the square are given numerically. For example, NH0325 means 'square NH, 3 km east, 25 km north'. A location can be indicated to varying resolutions numerically, usually from two digits in each coordinate (for a 1 km square) through to five (for a 1 m square); in each case the first half of the digits is for the first coordinate and the second half for the other. The most common usage is the six figure grid reference, employing three digits in each coordinate to determine a 100 m square. For example, the grid reference of the 100 m square containing the summit of Ben Nevis, is NN 166 712.
All numeric grid references
Grid references may also be quoted as a pair of numbers: eastings then northings in metres. Note that 13 digits may be required for locations in Orkney and north thereof. For example the grid reference for Sullom Voe oil terminal may be given as HU 397 753 or 439668,1175316.
- Maps of the UK and Ireland
- Templates: Gbmapping, Gbmappingsmall, Gbmaprim. These templates create external links to the Ordnance Survey Get-a-map service and should always be used in a Wikipedia article rather than creating an explicit link. The service accepts the letters + digits form of grid reference with 2, 4, 6, 8 or 10 digits (oddly, with two digits the window covers 4 km while it should be at least 10 km).