The British national grid reference system is a system of geographic grid references commonly used in Great Britain, different from using latitude or longitude. See also Kingdom of Great Britain Great Britain (Breatainn Mhòr Prydain Fawr Breten Veur Graet Breetain is the larger of the two main islands Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement
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. Ordnance Survey (OS is an Executive agency of the United Kingdom government Grid references define locations on Maps using Cartesian coordinates. Additionally grid references are commonly quoted in other publications and data sources, such as guide books or government planning documents. For the government of parliamentary systems see Executive (government.
Two such systems exist: this article describes the one used for Great Britain and its outlying islands (including the Isle of Man); a similar system, used throughout Ireland (including Northern Ireland), is the Irish grid reference system (used jointly by the Ordnance Survey of Ireland and Ordnance Survey of Northern Ireland). See also Kingdom of Great Britain Great Britain (Breatainn Mhòr Prydain Fawr Breten Veur Graet Breetain is the larger of the two main islands The Isle of Man (Ellan Vannin ˈɛlʲən ˈvanɪn or Mann (Mannin) is a self-governing Crown dependency, located in the Irish Sea at the geographical Ireland (pronounced /ˈaɾlənd/ Éire) is the third largest island in Europe, and the twentieth-largest island in the world Northern Ireland (Tuaisceart Éireann Ulster Scots: Norlin Airlann) is a Country within the United Kingdom, lying in the northeast of The Irish grid reference system is a system of geographic Grid references commonly used in Ireland (both Northern Ireland and the Republic of Ireland Ordnance Survey Ireland (OSi Irish: Suirbhéireacht Ordanáis Éireann) is the mapping agency in the Republic of Ireland and together with the Ordnance Ordnance Survey of Northern Ireland (OSNI was the official Mapping agency of Northern Ireland.
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General
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. Geodetic systems or geodetic data are used in Geodesy, Navigation, Surveying by Cartographers and Satellite navigation systems Sir George Biddell Airy FRS (27 July 1801&ndash2 January 1892 was an English Mathematician and Astronomer, Astronomer Royal An ellipsoid is a type of quadric surface that is a higher dimensional analogue of an Ellipse.
The Airy ellipsoid is a regional best fit for Britain; more modern mapping tends to use the GRS80 ellipsoid used by the GPS (the Airy ellipsoid assumes the Earth to be about 1 km smaller in diameter than the GRS80 ellipsoid, and to be slightly less flattened). Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth The maps adopt a Transverse Mercator projection with an origin at 49° N, 2° W. The transverse Mercator projection is an adaptation of the Mercator projection. 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. 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). The World Geodetic System defines a reference frame for the earth for use in Geodesy and Navigation.
OSGB 36 was also used by Admiralty nautical charts until 2000 after which WGS 84 has been used. The Admiralty was formerly the authority in the United Kingdom responsible for the command of the Royal Navy. A nautical chart is a graphic representation of a maritime area and adjacent Coastal regions 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. The World Geodetic System defines a reference frame for the earth for use in Geodesy and Navigation.
A geodetic transformation between OSGB 36 and other terrestrial reference systems (like ITRF2000, ETRS89, or WGS 84) can become quite tedious if attempted manually. Geodesy (dʒiːˈɒdɪsi also called geodetics, a branch of Earth sciences, is the scientific discipline that deals The International Terrestrial Reference System ( ITRS) describes procedures for creating Reference frames suitable for use with measurements on or near the Earth's surface The European Terrestrial Reference System 1989 ( ETRS89) is a ECEF (Earth-Centered Earth-Fixed geodetic Cartesian reference frame, in which The World Geodetic System defines a reference frame for the earth for use in Geodesy and Navigation. The most common transformation is called the Helmert datum transformation, which results in a typical 7 m error from true. Friedrich Robert Helmert ( July 31 1843 &ndash June 15 1917) was a German Geodesist and an important writer on the theory The definitive transformation from ETRS89 that is published by the OSGB is called the National Grid Transformation OSTN02. [1] This models the detailed distortions in the 1936–1962 retriangulation, and achieves backwards compatibility in grid co-ordinates to sub-metre accuracy.
Datum shift between OSGB 36 and WGS 84
The difference between the co-ordinates 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. Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the The World Geodetic System defines a reference frame for the earth for use in Geodesy and Navigation. 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. Cornwall ( Kernow ˈkɛɹnɔʊ is the most southwesterly county of England, on the Peninsula that lies to the west of the River Tamar East Anglia is often used as a shorthand for the Kingdom of the East Angles. 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. Cornwall ( Kernow ˈkɛɹnɔʊ is the most southwesterly county of England, on the Peninsula that lies to the west of the River Tamar The Scottish Borders, often referred to simply as the Borders, is one of 32 local government council areas of Scotland. Scotland ( Gaelic: Alba) is a Country in northwest Europethat occupies the northern third of the island of Great Britain. (NB. 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 (1400 AM) is a Radio station broadcasting a Adult Standards/MOR format
For other co-ordinate systems, the shifts are different again. For example, Universal Transverse Mercator co-ordinates differ by many hundreds of metres, as UTM northings count from the Equator, and the notional OSGB 36 position of the Equator is many hundred metres north of that on WGS 84. The Universal Transverse Mercator ( UTM) Coordinate system is a grid-based method of specifying locations on the surface of the Earth The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the Attempting to calculate even relative British National Grid co-ordinate offsets from WGS 84 UTM data without first adjusting from WGS 84 to OSGB 36 can give similarly discrepant results, at least for Eastings, to those discussed above.
Datum shift between OSGB 36 and ED 50
These two datums are not really 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. Dover is a town and major ferry port in the county of Kent, England. Calais (kaˈlɛ in English often kæˈleɪ traditional English pronunciation /ˈkælɨs/ Kales is a town in northern France. ED 50 ( European Datum 1950) is a geodetic datum which was defined after World War II for the international connection of Geodetic networks
Grid letters
For the first letter the grid is divided into squares of size 500 km by 500 km. 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. North Yorkshire is a non-metropolitan or shire county located in the Yorkshire and the Humber region of England, and a ceremonial county in [2])
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, and the central meridian marked in red.
It would be possible to extend the grid system over Ireland, completing the S and N squares and introducing what would become the R and M squares (with the arrangement of first letters following the same pattern as for the second letter). Ireland (pronounced /ˈaɾlənd/ Éire) is the third largest island in Europe, and the twentieth-largest island in the world However, there is no motion for this at the moment, and the accuracy of the projection would start to diminish in the west of Ireland, more than 8 degrees from the central meridian. Theoretically, the system extends far over the Atlantic Ocean and well into Western Europe with square AA near Iceland and square ZZ in northern Italy. Western Europe at its most general meaning means 'all the countries in the West of Europe ' In fact, Rockall is mapped by the Ordnance Survey, but is usually shown as an inset without gridlines on a mainland sheet. Rockall is a small uninhabited rocky Islet in the north Atlantic Ocean, and one of the sea areas named in the Shipping Forecast broadcast on BBC However, the grid can be extended to put Rockall in grid square MC as shown in this 1:50,000 mockup.
Grid digits
Within each square, eastings and northings from the origin (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 metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International 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. Ben Nevis ( Gaelic: Beinn Nibheis, peˈɲivəʃ is the highest mountain in the British Isles. (Grid references may be written with or without spaces, e. g. also NN166712. )
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 13 digits may be required for locations in Orkney and further north. Orkney (also known as the Orkney Islands or incorrectly the Orkneys) is an Archipelago in northern Scotland, situated 10 miles (16 km north For example the grid reference for Sullom Voe oil terminal may be given as HU396753 or 439668,1175316. Sullom Voe is an Inlet between North Mainland and Northmavine on Shetland in Scotland.
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 does not contain enough information to specify a 100m square uniquely without additional context, and is therefore less useful. However, it is often used informally when the context already limits the location to within an area of less than 100 km in each direction. 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.
Summary parameters of the British National Grid coordinate system:
- Datum: OSGB1936,
- Map projection: Transverse Mercator,
- Latitude of Origin: 49,
- Longitude of Origin: -2,
- Scale Factor: 0. 9996012717,
- False Easting: 400000 m,
- False Northing: -100000 m
- EPSG Code: EPSG:27700
Notes
- ^ GPS and Positioning Services. European Petroleum Survey Group or EPSG ( 1986 – 2005) was a scientific organization with ties to the European Petroleum industry consisting Ordnance Survey.
- ^ Standing, 2006
References
- Standing, Peter (2006) OV0000 a unique grid square at Beast Cliff, Geograph Project, UK, web article [accessed 11 June 2007]
See also
- World Geodetic System
- Maps of the UK and Ireland
- Irish grid reference system
- United States National Grid
- Tetrad, Hectad, Myriad
External links
- Ordnance Survey Guide to the National Grid
- Ordnance Survey Guide to coordinate systems
- Programs to convert Ordnance Survey grid references
- Open Source Javascript Conversion Library
- GPL Java Conversion Library
- The sole part of Great Britain that lies in the OV square
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