This article is about the map grid references in the UK. For the electric power transmission system in the UK, see National Grid.

The British national grid reference system is a system of geographic grid references commonly used in Great Britain, different from using latitude or longitude.

The national grid reference system was devised by the Ordnance Survey, and 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.

There are two such systems and 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).

General

The grid is technically known as OSGB36 ™ (Ordnance Survey Great Britain 1936), and was introduced after the retriangulation of 1936-1953.

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 400 km by 100 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. The produced maps contain a small variation between true north and grid north.

A geodetic transformation between OSGB36 and other terrestrial reference systems (like ITRF2000, ETRS89, or WGS84) can be quite tedious, if attempted manually. The process is called a Helmert datum transformation, the transformation from ETRS89 to OSGB36 is called the National Grid Transformation OSTN02.

Grid letters

For the first letter the grid is divided into 25 squares of size 500 km by 500 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. As this is much larger than the islands mapped, only four grid squares actually contain land - S,T,N, and H.

For the second letter, each large square is subdivided into 25 squares of size 100 km by 100 km, using the same lettering system. The created grid is shown on the accompanying map, with the squares containing land lettered.

Grid digits

Within each square, eastings and northings from the origin (south west corner) of the square are given numerically. For example, HL0305 means 'square HL, 3 km east, 5 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 100m square containing the summit of Ben Nevis, is NN166712.

Maps

The URL http://getamap.ordnancesurvey.co.uk/getamap/frames.htm?mapAction=gaz&gazName=g&gazString= followed by the grid reference (two letters and 0, 4, or 6 digits) gives a map (oddly, with two digits the window width is 4 km while it should be at least 10 km). See also Template:Gbmapping.