The position of any point in Great Britain can be described by its National Grid Reference. These references are often used in conjunction with Ordnance Survey maps, and are used to give the names of individual Ordnance Survey data tiles.

Grid References are made up of:

- two letters, optionally followed by
- an even number of digits, optionally followed by
- a quadrant name: "NW", "NE", "SW" or "SE", standing for North West, North East, South West and South East.

These components are usually written without spaces. However, spaces are included here for legibility.

Examples of Grid References are: SK 07, SK 07 SW, SK 042366.

Two letters together identify a 100km square, and are derived as follows:

The first letter identifies a 500km square, from the diagram shown here. The letters are allocated in alphabetical order from left to right and top to bottom, and omitting the letter "I".Ordnance Survey's products lie completely within four of these 500km squares: H, N, S and T.

Each 500km square is then divided into twenty five 100km squares, and these squares are then labelled using the same pattern of letters. The resulting 100km squares covering Great Britain are shown here.

The digits identify a location within a 100km square. The first half of the digits give the Easting, or distance East from the edge of the 100km square. The second half of the digits give the Northing, or distance North from the edge of the 100km square.

If there are:

- 2 digits: the Easting and Northing are each in units of 10km;
- 4 digits: the Easting and Northing are each in units of 1km;
- 6 digits: the Easting and Northing are each in units of 100m;
- 8 digits: the Easting and Northing are each in units of 10m;
- 10 digits: the Easting and Northing are each in units of 1m.

If there are no digits, the Grid Reference applies to the complete 100km square.

If there is a quadrant name present, it indicates one quarter of the area indicated by the rest of the Grid Reference.

For example, within the square SK:

The details of the construction of the National Grid from latitude and longitude is mathematically complex.

In summary, the process is that the irregular shape of the Earth is approximated by an **ellipsoid** or **spheroid**. This spheroid is then transformed onto a flat surface using a mathematical transformation known as a **projection**. The **National Grid** is then a cartesian coordinate system calculated on the resulting flat surface.

Different **spheroids** and **projections** are used in different countries and regions of the world. For Great Britain, the technical details are as follows:

**Spheroid**

**Name:**Airy Spheroid (1830)**Semi-major axis:**6,377,563.396m**Semi-minor axis:**6,356,256.910m

**Projection**

**Type:**Transverse Mercator (also known as Gauss Kruger)**True Origin:**49° North, 2° West**False Origin:**400km West and 100km North of True Origin**Scale factor on central meridian:**0.9996012717

Ordnance Survey have a number of resources that explain the National Grid in detail, the common ones can be accessed using the links below:

- OS Explanation of British National Grid [opens in new window]
- OS British National Grid FAQs [opens in new window]
- Edina National Grid Reference Converter