The Ordnance Survey National Grid reference system (OSGB) (also known as British National Grid (BNG)) is a system of geographic

grid reference A projected coordinate system, also known as a projected coordinate reference system, a planar coordinate system, or grid reference system, is a type of spatial reference system that represents locations on the Earth using cartesian coordin ...

s used in Great Britain, distinct from

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

and

longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...

. The

Ordnance Survey Ordnance Survey (OS) is the national mapping agency for Great Britain. The agency's name indicates its original military purpose (see ordnance and surveying), which was to map Scotland in the wake of the Jacobite rising of 1745. There was a ...

(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 by commercial map producers. Grid references are also commonly quoted in other publications and data sources, such as guide books and government planning documents. A number of different systems exist that can provide grid references for locations within the

British Isles The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isles, ...

: this article describes the system created solely for Great Britain and its outlying islands (including the

Isle of Man ) , anthem = "O Land of Our Birth" , image = Isle of Man by Sentinel-2.jpg , image_map = Europe-Isle_of_Man.svg , mapsize = , map_alt = Location of the Isle of Man in Europe , map_caption = Location of the Isle of Man (green) in Europe ...

); the

Irish grid reference system The Irish grid reference system is a system of geographic grid references used for paper mapping in Ireland (both Northern Ireland and the Republic of Ireland). The Irish grid partially overlaps the British grid, and uses a similar co-ordinate sy ...

was a similar system created by the Ordnance Survey of Ireland and the

Ordnance Survey of Northern Ireland {{Unreferenced, date=April 2021 Ordnance Survey of Northern Ireland (OSNI) was the official mapping agency of Northern Ireland. The agency ceased to exist separately on 1 April 2008 when it became part of Land and Property Services, an executiv ...

for the island of Ireland. The

Universal Transverse Mercator coordinate system The Universal Transverse Mercator (UTM) is a map projection system for assigning coordinates to locations on the surface of the Earth. Like the traditional method of latitude and longitude, it is a horizontal position representation, which means i ...

(UTM) is used to provide grid references for worldwide locations, and this is the system commonly used for the

Channel Islands The Channel Islands ( nrf, Îles d'la Manche; french: îles Anglo-Normandes or ''îles de la Manche'') are an archipelago in the English Channel, off the French coast of Normandy. They include two Crown Dependencies: the Bailiwick of Jersey, ...

and Ireland ( since 2001). European-wide agencies also use UTM when mapping locations, or may use the Military Grid Reference System (MGRS), or variants of it.

Grid letters

The first letter of the British National Grid is derived from a larger set of 25 squares of size 500 km by 500 km, labelled A to Z, omitting one letter (I) (refer diagram below), previously used as a military grid. Four of these largest squares contain significant land area within Great Britain: S, T, N and H. The O square contains a tiny area of

North Yorkshire North Yorkshire is the largest ceremonial counties of England, ceremonial county (lieutenancy area) in England, covering an area of . Around 40% of the county is covered by National parks of the United Kingdom, national parks, including most of ...

Beast Cliff Beast Cliff is a steep sea cliff situated about halfway between Whitby and Scarborough on the coast of North Yorkshire in northeastern England. Beast Cliff is at the southern end of a coastal region designated as a Special Area of Conservation ...

at , almost all of which lies below mean high tide. For the second letter, each 500 km square is subdivided into 25 squares of size 100 km by 100 km, each with a letter code from A to Z (again omitting I) starting with A in the north-west corner to Z in the south-east corner. These squares are outlined in light grey on the "100km squares" map, with those containing land lettered. The central (2° W) meridian is shown in red.

Grid digits

Within each square, ''eastings'' and ''northings'' from the south west corner of the square are given numerically. For example, NH0325 means a 1 km square whose south-west corner is 3 km east and 25 km north from the south-west corner of square NH. 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 Ben Nevis ( ; gd, Beinn Nibheis ) is the highest mountain in Scotland, the United Kingdom and the British Isles. The summit is above sea level and is the highest land in any direction for . Ben Nevis stands at the western end of the Grampian ...

is . (Grid references may be written with or without spaces; e.g., also NN166712.) NN has an easting of 200 km and northing of 700 km, so the OSGB36 National Grid location for Ben Nevis is at 216600, 771200. Ordnance Survey National Grid

All-numeric grid references

Grid references may also be quoted as a pair of numbers: eastings then northings in metres, measured from the southwest corner of the SV square. Note that 14 digits may be required for locations in Orkney and further north. For example, the grid reference for Sullom Voe oil terminal in the Shetland Islands may be given as or . Another, distinct, form of all-numeric grid reference is an abbreviated alphanumeric reference where the letters are simply omitted, e.g. 166712 for the summit of Ben Nevis. Unlike the numeric references described above, this abbreviated grid reference is incomplete; it gives the location relative to an OS 100×100 km square, but does not specify which square. It is often used informally when the context identifies the OS 2-letter square. For example, within the context of a location known to be on OS Landranger sheet 41 (which extends from NN000500 in the south-west to NN400900 in the north-east) the abbreviated grid reference 166712 is equivalent to NN166712. If working with more than one Landranger sheet, this may also be given as 41/166712. Alternatively, sometimes numbers instead of the two-letter combinations are used for the 100×100 km squares. The numbering follows a grid index where the tens denote the progress from West to East and the units from South to North. In the north of Scotland, the numbering is modified: the 100 km square to the north of 39 is numbered N30; the square to the north of 49 is N40, etc.

Compatibility with related systems

The grid is based on the ''OSGB36'' datum (Ordnance Survey Great Britain 1936, based on the Airy 1830 ellipsoid), and was introduced after the retriangulation of 1936–1962. It replaced the

Cassini Grid The Cassini Grid was a grid coordinate system used on British military maps during the first half of the twentieth century, particularly during World War II. The referencing consists of square grids drawn on a Cassini projection. For a period af ...

which had previously been the standard projection for Ordnance Survey maps. 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 Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...

(the Airy ellipsoid assumes the Earth to be about 1 km smaller in diameter than the

GRS80 The Geodetic Reference System 1980 (GRS 80) is a geodetic reference system consisting of a global reference ellipsoid and a normal gravity model. Background Geodesy is the scientific discipline that deals with the measurement and representation ...

ellipsoid, and to be slightly less flattened). The British maps adopt a

transverse Mercator projection The transverse Mercator map projection (TM, TMP) is an adaptation of the standard Mercator projection. The transverse version is widely used in national and international mapping systems around the world, including the Universal Transverse Mercat ...

with an origin (the "true" origin) at 49° N, 2° W (an offshore point in the

English Channel The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" (Cotentinais) or ( Jèrriais), (Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Kana ...

which lies between the island of

Jersey Jersey ( , ; nrf, Jèrri, label= Jèrriais ), officially the Bailiwick of Jersey (french: Bailliage de Jersey, links=no; Jèrriais: ), is an island country and self-governing Crown Dependency near the coast of north-west France. It is the l ...

and the French port of St. Malo). 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. In order to minimize the overall scale error, a factor of 2499/2500 is applied. This creates two lines of longitude about 180 km east and west of the central meridian along which the local scale factor equals 1, i.e. map scale is correct. Inside these lines the local scale factor is less than 1, with a minimum of 0.04% too small at the central meridian. Outside these lines the local scale factor is greater than 1, and is about 0.04% too large near the east and west coasts. Grid north and true north are only aligned on the central meridian (400 km easting) of the grid which is 2° W (OSGB36) and approx. (

WGS 84 The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also descri ...

). OSGB 36 was also used by Admiralty

nautical chart A nautical chart is a graphic representation of a sea area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land ( topographic map), natural features of the seabed, details of the co ...

s until 2000 after which

has been used. A

geodetic Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...

transformation between OSGB 36 and other terrestrial reference systems (like ITRF2000,

ETRS89 The European Terrestrial Reference System 1989 (ETRS89) is an ECEF (Earth-Centered, Earth-Fixed) geodetic Cartesian reference frame, in which the Eurasian Plate as a whole is static. The coordinates and maps in Europe based on ETRS89 are not su ...

, or

) 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 Ordnance Survey is called the National Grid Transformation OSTN15. 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

and

positions on OSGB 36 are the same as for

at a point in the Atlantic Ocean well to the west of Great Britain. In

Cornwall Cornwall (; kw, Kernow ) is a historic county and ceremonial county in South West England. It is recognised as one of the Celtic nations, and is the homeland of the Cornish people. Cornwall is bordered to the north and west by the Atlantic ...

, 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

, the difference diminishing to zero in the Scottish Borders, and then increasing to about 50 m north on the north coast of

Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...

. (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 Kent is a county in South East England and one of the home counties. It borders Greater London to the north-west, Surrey to the west and East Sussex to the south-west, and Essex to the north across the estuary of the River Thames; it faces ...

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

halfway between Dover and Calais, the

ED50 ED50 ("European Datum 1950", EPSG:4230) is a geodetic datum which was defined after World War II for the international connection of geodetic networks. Background Some of the important battles of World War II were fought on the borders of Ger ...

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.

Summary parameters of the coordinate system

* Datum: OSGB36 * Map projection:

Transverse Mercator projection The transverse Mercator map projection (TM, TMP) is an adaptation of the standard Mercator projection. The transverse version is widely used in national and international mapping systems around the world, including the Universal Transverse Mercat ...

using Redfearn series * True Origin: 49°N, 2°W * False Origin: 400 km west, 100 km north of True Origin *

Scale Factor In affine geometry, uniform scaling (or isotropic scaling) is a linear transformation that enlarges (increases) or shrinks (diminishes) objects by a '' scale factor'' that is the same in all directions. The result of uniform scaling is similar ...

: 0.9996012717 * EPSG Code: EPSG:27700 * Ellipsoid: Airy 1830 * Semi-major axis a: *

Semi-minor axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the lo ...

b: * Flattening (derived constant): 1/299.3249646

Notes

References

External links

* Ordnance Surve
A guide to coordinate systems in Great Britain
An introduction to mapping coordinate systems and the use of GPS datasets with Ordnance Survey mapping; Version 3.6, 2020 etrieved 19 February 2022 * Ordnance Survey'
Grid script
a brief introduction to the National Grid Reference; Version November 2011 etrieved 13 February 2014 * * * * * - Multiple-format co-ordinate transformer for Great Britain & Channel Islands * * * * * * (JavaScript source code) * * Web utility to find a UK grid reference
LatLong <> OS Grid Ref
converts & presents in many formats, generates specific links to that location for several useful map web pages - 1840–present. LatLong WSG84 <> GB, Ireland (inc NI) and Chanel Islands (30U) GR formats recognised. Distance measure for dog-leg routes & area calculations. * Open source dataset (in GeoPackage format) of the British National Grids at various resolutions, available for download from Ordnance Survey's GitHub. {{Geocoding-systems Geography of the United Kingdom Maps from Ordnance Survey Geographic coordinate systems Land surveying systems Geodesy Geocodes