# National Grid (Ordnance Survey)

The **National Grid** is the coordinate system used on the British Ordnance Survey maps . It is also known colloquially as the **British National Grid** (BNG).

## OSGB36

The geodetic datum OSGB36 is the basis for surveying Great Britain . It was defined in the course of the re-surveying of the country between 1936 and 1953.

The ellipsoid *Airy 1830 is* used as a reference ellipsoid , which is named after George Biddell Airy and which approximates the earth in the UK optimally. Its semi-axes are a = 6377563.396 m and b = 6356256.910 m; the flattening is f = (ab) / a = 1: 299.3249753.

## Map projection

A transverse Mercator projection is used as a map projection . Its origin lies at 2 ° west and 49 ° north, i.e. roughly in the middle between Jersey and Saint-Malo . It should be noted that the prime meridian in OSGB36 differs significantly from the prime meridian used in WGS84 (in London it is about 120 m to the west), which must be taken into account when converting coordinates between the different systems.

## Coordinate system

In the (real) origin of the map projection is a Cartesian coordinate system defined, the 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 are added to the easting value, 100 km can be deducted from the high value. The result is a *wrong origin* about 80 km west of the Isles of Scilly , and the whole of Great Britain receives positive coordinates with a maximum eastward value of approx. 650,000 and a maximum value of approx. 1,200,000.

**Example Conwy Castle :** This castle is located in WGS84 at 53 ° 16 ′ 48 ″ North 3 ° 49 ′ 32 ″ West. This corresponds to coordinates 278.380, 377.446 on the National Grid. (Attention: with transversal Mercator coordinates, first the right value and then the high value is given, while with degrees, the latitude is usually given first and then the longitude.)

Since these coordinates can be quite large and confusing, the map is divided into grid squares: squares with sides of 500 km are given the letters H, N, O, S, and T; within it, squares with a side length of 100 km from A to Z are defined, whereby the capital I is omitted. Within these squares, the kilometers are numbered from 00 to 99. This kilometer grid is printed on the Ordnance Survey maps at a scale of 1: 50,000 or larger.

For Conwy Castle this results in the coordinates SH 78380 77446.

Instead of these exact coordinates, however, it is common in Great Britain to specify a grid square. So you could say that Conwy Castle is located in grid square SH 78 77 or SH 783 774 - the first square is 1 km long, the second is 100 m long. Such six-digit information is particularly common in British sources.

## literature

- Ordnance Survey: A guide to coordinate systems in Great Britain . December 2010 ( ordnancesurvey.co.uk [PDF; accessed February 17, 2012]).