Class to represent an Ordnance Survey of Great Britain (OSGB) grid reference.

British National Grid

• Projection: Transverse Mercator
• Reference ellipsoid: Airy 1830
• Units: metres
• Origin: 49°N, 2°W
• False co-ordinates of origin: 400000m east, -100000m north

A full reference includes a two-character code identifying a particular 100,000m grid square. The table below shows how the two-character 100,000m grid squares are identified. The bottom left corner is at the false origin of the grid. Squares without values fall outside the boundaries of the British National Grid.

Within each 100,000m square, the grid is further subdivided into 1000m squares. These 1km squares are shown on Ordnance Survey 1:25000 and 1:50000 mapping as the main grid. To reference a 1km square, give the easting and then the northing, e.g. TR2266. In this example, TR represents the 100,000m square, 22 represents the easting (in km) and 66 represents the northing (in km). This is commonly called a four-figure grid reference.

It is possible to extend the four-figure grid reference for more accuracy. For example, a six-figure grid reference would be accurate to 100m and an eight-figure grid reference would be accurate to 10m.

When providing local references, the 2 characters representing the 100,000m square are often omitted.