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Encyclopedia > British national grid reference system

The British national grid reference system is a system of geographic grid references commonly used in Great Britain, different from using latitude or longitude. Latitude, sometimes denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... Map of Earth showing lines of longitude, which appear curved and vertical in this projection, but are actually halves of great circles Longitude, sometimes denoted by the Greek letter λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ...


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. Image produced from the Ordnance Survey Get-a-map service. ... A grid reference is a standard method for the location of a point on a map. ...


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). Dieu et mon droit (Royal motto) (French for God and my right)3 Northern Irelands location within the UK Official languages English, Irish, Ulster Scots Capital and largest city Belfast First Minister Office suspended Area  - Total Ranked 4th 13,843 km² Population  - Total (2001)  - Density Ranked 4th 1,685... The Irish national grid reference system is a system of geographic grid references commonly used in Ireland. ... Ordnance Survey Ireland (OSI; Irish: Suirbhéireacht Ordanáis na hÉireann) is the mapping agency in the Republic of Ireland. ... Ordnance Survey of Northern Ireland is the mapping agency in Northern Ireland, part of the United Kingdom. ...

Contents


General

The grid is based on the OSGB36datum (Ordnance Survey Great Britain 1936, based on the Airy 1830 ellipsoid), and was introduced after the retriangulation of 1936-1962. Geodetic systems or map datums are used in geodesy, navigation, surveying and the Global Positioning System to indicate the datum or reference point to which a measured position is relative. ... George Biddell Airy Sir George Biddell Airy (July 27, 1801–January 2, 1892) was British Astronomer Royal from 1835 to 1881. ... In 1935, the new Director General of the Ordnance Survey, Major-General Malcolm MacLeod, started the retriangulation of Great Britain, an immense task which involved erecting concrete triangulation pillars (trig points) on prominent hilltops throughout Great Britain. ...


The ellipsoid is a regional best fit for Britain, more modern mapping tending 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.) The maps adopt a Transverse Mercator projection with an origin at 49 ° N, 2 ° W. 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 (WGS84). Over fifty GPS satellites such as this NAVSTAR have been launched since 1978. ...


OSGB 36 was also used by Admiralty nautical charts until 2000 after which WGS 84 has been used. Old Admiralty House, Whitehall, London, Thomas Ripley, architect, 1723-26, was not admired by his contemporaries and earned him some scathing couplets from Alexander Pope The Admiralty was historically the authority in the United Kingdom responsible for the command of the Royal Navy. ... Portion of chart of Bering Strait, site of former land bridge between Asia and North America. ... This article is about the year 2000. ...


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 7m error from true. The definitive transformation from ETRS89 to OSGB36 thats published by the OSGB is called the National Grid Transformation OSTN02, and promises sub-metre accuracy. The World Geodetic System defines a fixed global reference frame for the Earth, for use in geodesy and navigation. ...


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. 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. (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. Map of Earth showing lines of longitude, which appear curved and vertical in this projection, but are actually halves of great circles Longitude, sometimes denoted by the Greek letter λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... Latitude, sometimes denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... Motto: Onen hag oll (Cornish: One and all) Geography Status Ceremonial and (smaller) Non-metropolitan county Region South West England Population - Total (2004 est. ... Norfolk and Suffolk, the core area of East Anglia. ... Royal motto: Nemo me impune lacessit (English: No one provokes me with impunity) Scotlands location within the UK Languages English, Gaelic, Scots Capital Edinburgh Largest city Glasgow First Minister Jack McConnell Area - Total - % water Ranked 2nd UK 78,782 km² 1. ... Kent is a county in England, south-east of London. ...


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. Similarly, attempting to give British National Grid co-ordinates on the WGS 84 datum may give wildly discrepant results. A transverse Mercator projection is a map projection similar to the Mercator projection in that it is a projection of Earth on a tangent cylinder by rays radial with respect to the cylinder. ... The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. ...


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. Satellite view of the English Channel The English Channel (French: La Manche, IPA: , the sleeve), also for some time known in England as the British Sea, is the part of the Atlantic Ocean that separates the island of Great Britain from northern France, and joins the North Sea to the... Map sources for Dover at grid reference TR315415 Arms of Dover Borough Council This article is about the English port town. ... Location within France The Burghers of Calais, by Rodin, with Calais Hotel de Ville behind J.M.W. Turner: Calais Pier Calais (Dutch: Kales) is a town in northern France, located at 50°57N 1°52E. It is in the département of Pas-de-Calais, of which it is... 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 the North Riding of Yorkshire, almost all of which lies below mean high tide: OV000000) Image File history File links OS_grid_key_Britain. ... The North Riding of Yorkshire is one of the three traditional subdivisions of the English county of Yorkshire. ...


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.


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). 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 southern Germany or thereabouts. In fact, Rockall is mapped by the Ordnance Survey, but is usually shown as an inset without gridlines on a mainland sheet. However, the grid can be extended to put Rockall in grid square MC as shown in this 1:50,000 mockup. Western Europe is distinguished from Eastern Europe by differences of history and culture rather than by geography. ... Rockall, a small, isolated rocky islet in the North Atlantic Ocean Rockall is a small, rocky islet in the North Atlantic but is probably better known as one of the British Sea Areas named in the Shipping Forecast broadcast on BBC Radio 4. ...


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 six (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.) The metre (Commonwealth English) or meter (American English) (symbol: m) is the SI base unit of length. ... Map sources for Ben Nevis at grid reference NN166713 Ben Nevis is the highest mountain in the British Isles. ...


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 HU396753 or 439668,1175316. The Orkney Islands form one of 32 unitary council regions in Scotland, and are a Lieutenancy Area. ... Sullom Voe is an inlet between North Mainland and Northmavine on Shetland in Scotland, and an oil terminal sited on its shore. ...


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: 0.999601272000000040,
False Easting: 400000 m,
False Northing: -100000 m
EPSG Code: EPSG:27700

See also

  • Maps of the UK and Ireland
  • Irish national grid reference system
  • United States national grid (proposed [1])
  • Tetrad, Hectad
  • 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).

Maps of the UK and Ireland are available in various media. ... The Irish national grid reference system is a system of geographic grid references commonly used in Ireland. ... A tetrad is the term used to describe an area 2km x 2km square. ... A hectad is an area 10km x 10km square. ...

External links

  • Detailed information about GPS and the National Grid from the UK Ordnance Survey

 
 

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