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.

[

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.^[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). 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. 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.

[

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 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.)

[

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. For example the grid reference for Sullom Voe oil terminal 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 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

[

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

BCUZ.com FACTS Encyclopedia content is licensed under the GFDL as approved by Wikipedia.
For more information review our copyright contact and privacy policy. © 1996 - BCUZ.COM - We have all the FACTS you need about Small Business Financing, Behavior Disorder, Having Too Many Bills, Needing Cash Fast, Structured Settlements, Frequent Flier Programs, Top Steak Houses, The Mayan Indians, Norfolk and Suffolk England, Growing Longer Hair and a full reference English Encyclopedia and Spanish Encyclopedia.Privacy Policy