Ordnance Survey National Grid
As National Grid coordinate system used in the maps of the UK Ordnance Survey is called.
OSGB36
Basis of the survey of Great Britain is the Geodetic Datum OSGB36. It has been defined as part of the re-measurement of the country in the years from 1936 to 1953.
As a reference ellipsoid, the ellipsoid Airy 1830 is used, which is named after George Biddell Airy and the earth in the area of Great Britain approaches optimal. His semi-axes are a = 6377563.396 m and b = 6,356,256.910 m; the flattening is f = ( ab) / a = 1:299,3249753.
Map projection
As a map projection is a transverse Mercator projection is used. Their origin is 2 ° West and 49 ° N, ie approximately in the middle between Jersey and Saint Malo. It should be noted that the zero meridian at OSGB36 significantly different from the zero meridian used in WGS84 ( in London, he lies about 120 m west ), which is to be considered in coordinate conversions between the different systems.
Coordinate system
In the (real) origin of the map projection, a Cartesian coordinate system is defined, whose one direction to the east (English: easting, German: Easting ) and the other direction to the north (English: northing, German: Northing ) shows. These values are given in meters. To avoid negative coordinates, 400 km will be added to the easting, northing of 100 km can be deducted. The result is a false origin about 80 km west of the Scilly Islands, and throughout the UK receives positive coordinates, their legal value is a maximum of about 650,000 and their high- value maximum of approximately 1,200,000.
Example Conwy Castle: This castle is located in the WGS84 at 53 ° 16 '48 " North 3 ° 49 ' 32 " West. This corresponds to the coordinates 278 380, 377 446 in the National Grid. ( Note: for transverse Mercator coordinates of easting and then the high value is always specified first, while numbers of degrees usually only the latitude and then the length is called. )
Since these coordinates can be quite large and confusing, the map is divided into grid squares: squares with side lengths of 500 km, points H, N, O, S, and T; within which squares are defined with the side length 100 km from A to Z, with the large I is omitted. Within these squares then the mileage from 00 to 99 are numbered. This kilometer grid is printed on the maps of the Ordnance Survey in the scales of 1:50,000 or larger.
For Conwy Castle, this results in the coordinate specification SH 78380 77446th
Instead of this meter exact coordinates but it is common in the UK to provide a grid square. So you could say, Conwy Castle is located in the grid square SH 78 or SH 77 783 774 - the first square has a side length of 1 km, the second is 100 m long. Especially those six -digit information is often found in British sources.