The
**Ordnance Survey**

Ordnance Survey National
**Grid reference**

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

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**

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: this article describes the
system created solely for Great Britain and its outlying islands
(including the Isle of Man); the
**Irish grid reference system**

Irish grid reference system was a
similar system created by the
**Ordnance Survey**

Ordnance Survey of Ireland and the
**Ordnance Survey**

Ordnance Survey of Northern Ireland for the island of Ireland. The
**Universal Transverse Mercator coordinate system**

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**

Channel Islands and Ireland (since 2001).
European-wide agencies also use UTM when mapping locations, or may use
the
**Military Grid Reference System**

Military Grid Reference System (MGRS) system, or variants of it.

Contents

1 General

1.1 Datum shift between OSGB 36 and WGS 84
1.2 Datum shift between OSGB 36 and ED 50

2 Grid letters
3 Grid digits
4 All-numeric grid references
5 Summary parameters of the coordinate system
6 See also
7 Notes
8 References
9 External links

General[edit]

Grid square TF. The map shows
**The Wash**

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**

OSGB36 datum (
**Ordnance Survey**

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**

English Channel which lies
between the island of
**Jersey**

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**

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[edit]
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**

Scotland and the greatest in Kent.
Datum shift between OSGB 36 and ED 50[edit]
These two datums are not both in general use in any one place, but for
a point in the
**English Channel**

English Channel halfway between
**Dover**

Dover and Calais, the
**ED50**

ED50 longitude lines are about 20 m east of the OSGB36
equivalents, and the
**ED50**

ED50 latitude lines are about 150 m south of
the
**OSGB36**

OSGB36 ones.[citation needed]

Illustration of the
**Ordnance Survey**

Ordnance Survey National Grid coordinate system,
with
**Trafalgar Square**

Trafalgar Square as an example

Grid letters[edit]

For the first letter, the grid is divided into squares of size
500 km by 500 km, outlined in dark grey on the map to the
right. There are four of these which contain significant land area
within Great Britain: S, T, N and H. The O square contains a tiny area
of North Yorkshire, almost all of which lies below mean high tide.[5]
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 (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 map, with those containing land lettered. The central (2° W)
meridian is shown in red.
Grid digits[edit]
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 is NN 166 712. (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**

OSGB36 National Grid location for Ben
Nevis is at 216600, 771200.
All-numeric grid references[edit]
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**

Sullom Voe oil
terminal in the Shetland Islands may be given as HU396753 or
439668,1175316.
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.
Summary parameters of the coordinate system[edit]

Datum: OSGB36
Map projection:
**Transverse Mercator projection**

Transverse Mercator projection using Redfearn series
True Origin: 49°N, 2°W
False Origin: 400 km west, 100 km north of True Origin
Scale Factor: 0.9996012717+[6]
**EPSG**

EPSG Code: EPSG:27700

Ellipsoid: Airy 1830[7]
**Semi-major axis**

Semi-major axis a: 6377563.396 m
**Semi-minor axis**

Semi-minor axis b: 6356256.909 m
**Flattening**

Flattening (derived constant): 1/299.3249646

See also[edit]

World Geodetic System
Irish grid reference system
United States National Grid
Tetrad, Hectad, Myriad
International Map of the World#Map Indexing System

Notes[edit]

^ "OSGB 1936 / British National Grid:
**EPSG**

EPSG Projection -- Spatial
Reference". spatialreference.org.
^ "Coordinate systems and projections for beginners".
^ OS Net, The true origin
^ "Surveying guidelines". Ordnance Survey.
^ Standing, 2006
^ Scale factor is defined by its base-10 logarithm of (0.9998268 −
1) exactly.
^ The defining Airy dimensions are a = 20923713 "feet", b = 20853810
"feet". In the Retriangulation the base-10 logarithm of the number of
metres in a "foot" was set at (0.48401603 − 1) exactly and the Airy
metric dimensions are calculated from that. The flattening is exactly
69903 divided by 20923713.

References[edit]

**Ordnance Survey**

Ordnance Survey A guide to coordinate systems in Great Britain: An
introduction to mapping coordinate systems and the use of GPS datasets
with
**Ordnance Survey**

Ordnance Survey mapping; Version 2.2 December 2013 [accessed 13
February 2014].
Ordnance Survey's Grid script: a brief introduction to the National
Grid Reference; Version November 2011 [accessed 13 February 2014].
Standing, Peter (2006) OV0000 a unique grid square at Beast Cliff,
Geograph Project, UK, web article [accessed 11 June 2007]
"The National Grid FAQs". Ordnance Survey. Retrieved 13 February
2014.

External links[edit]

**Ordnance Survey**

Ordnance Survey Guide to the National Grid
**Ordnance Survey**

Ordnance Survey Guide to coordinate systems
**Ordnance Survey**

Ordnance Survey coordinate transformer
Interactive
**Ordnance Survey**

Ordnance Survey Mapping showing grid references
FieldenMaps.info Co-ordinate Converter - Multiple-format co-ordinate
transformer for Great Britain & Channel Islands
Programs to convert
**Ordnance Survey**

Ordnance Survey grid references
Open Source Javascript Conversion Library
GPL Java Conversion Library
Perl Conversion Library
The sole part of Great Britain that lies in the OV square
JavaScript source code for OS coordinate transforms
.Net library to convert between lat/lon in various coordinate systems
and grid reference
UK Grid Reference Finder. Web utility to find a