National Grid References - an introduction

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.

For more information on how Grid References are applied to the different Ordnance Survey products available through Digimap please look at the Help pages on Data tile structure. If you want to convert Grid References into Coordinates look at the National Grid Reference Converter pages. In summary, Grid References are used for tile names as follows:

Note:OS MasterMap data is a seamless dataset and therefore not available in tiles. For this reason, it is not discussed on this page.

Land-Line.Plus tiles:(Note that Land-Line.Plus data was withdrawn from Digimap in September 2008)

• Urban tile names (500m x 500m) are of the form SK 0472 SE;
  i.e. 2 letters, 4 digits, quadrant name
• Rural tile names (1km x 1km) are of the form SK 0474;
  i.e. 2 letters, 4 digits
• Moorland tile names (5km x 5km) are of the form SK 07 SW;
  i.e. 2 letters, 2 digits, quadrant name

Meridian, Land-Form PANORAMA and 1:50,000 Colour Raster tiles:

• Tiles are 20km x 20km and their names are of the form SK 04
  i.e. 2 letters, 2 even digits

Strategi tiles:

• There are two large tiles covering the whole of Great Britain. These are called GBNorth and GBSouth

1:25,000 Colour Raster tiles:

• Tiles are 10km x 10km and their names are of the form SK NW
  i.e. 2 letters, 2 even digits

1:10,000 Colour Raster tiles:

• Tiles are 5km x 5km and their names are of the form SK 04 NW
  i.e. 2 letters, 2 even digits, quadrant name

The First Two Letters

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

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

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.

The quadrant name

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:

Technical background

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