British National Grid

British National Grid

The basis for co-odinates in Great Britain is the British National Grid

(Image courtesy of Ordnance Survey - Crown Copyright)

British National Grid 

British National Grid (BNG)

As the Earth is round it is impossible to accurately represent its curved surface on a flat piece of paper. Geographers and cartographers spent many decades overcoming the issue, and a solution is to adopt a separate map projection that is limited in its geographical size. That way, distortions caused by the Earth’s curvature are minimised. In the case of Great Britain, the mapping projection used is called the British National Grid (BNG), which is used to cover Great Britain (i.e. Northern Ireland is not included, as it is included in the Irish National Grid). For the purposes keeping it simple in respect of explaining the BNG, the Earth is flat, which makes a convenient way to draw paper maps.

In technical terms, the British National Grid is a Transverse Mercator Projection, with it’s origin at 49°N, 2° W, and based on the Airy 1930 ellipsoid using the OSGB36 datum. The grid extends 700km East and 1,000km north from the origin, which is near to the Scilly Isles. (You don’t need to know this info, but have a read of the links if interested.)

The purpose of the National Grid is to create a system from which you can pin-point any land based feature by quoting a set of co-ordinates. Without the National Grid we'd have to use Latitude & Longitude, which is a more cumbersome system to use.

It's important that you understand the basic construction of the British National Grid so that you can correctly quote a grid reference, which you may need to do when planning or case of emergency when out in the hills.


This video from Ordnance Survey is about the British National Grid and it explains further how the grid is made up. Some of the information is a little detailed, but have a look anyway...

Key Points

  • The grid is made up of 25 squares measuring 500km by 500km each, and are allocated a letter. Only squares S, T, N & H fall over the land mass of Great Britain

  • The 500km by 500km squares are further sub-divided into 25 more squares, each being 100km by 100km, and allocated a letter of the alphabet (the letter 'I' is excluded)

  • The combination of the above two letters uniquely identify an area of 100km square, for example SD, which covers north west England

  • When quoting co-ordinates (see section on grid references) you must also prefix the numeric co-ordinates with the two character alphas (SD in the example above) so that you uniquely identify the correct 100km by 100km larger grid square. If you don't quote the two digit alpha, your grid reference could relate to a number of locations in GB

Interactive Guide

The Ordnance Survey produce two guides on the British National Grid

Grid North

An anomaly created by using a separate grid system is that it points to a slightly different version of North, being called Grid North. (The other two being True North and Magnetic North – more here.) This information is important, as you have to make an adjustment to compass readings to allow for the difference between Grid North and Magnetic North, see Magnetic Variation.

What's more important is that Grid North is applicable to Great Britain (not Northern Ireland & Republic of Ireland). In many other parts of the world maps are oriented to True North

Converting Co-ordinates

If you have a Longitude/Latitude co-ordinate and want to convert to British National Grid, or vice versa, you can do that here

Northern Ireland

In Northern Ireland, maps use the Irish National Grid, which is similar to the British grid in that it is not aligned to either true or magnetic north, but its own 'grid' north. However, the Irish grid north is different to the grid north of the British grid. In practical terms, this means that the grid prefixes are different (the alpha characters), and that the magnetic variation used you have to take account of in Northern Ireland is different to mainland UK - it's larger. 

More info on the Irish National Grid can be found here.


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