The UK Ordnance Survey Grid

Follow the Adit guide to maps, mapping and cartography

The UK Ordnance Survey grid is a grid of 100 kilometre squares with a true origin at 49 degrees North and 2 degrees West. To avoid difficulties with negative co-ordinates a distance of 400 kilometres is added to the easting and 100 kilometres subtracted from the northing. This gives an apparent point of origin near to the Scilly Isles off Lands End. Each 100 kilometre grid square is assigned a two letter sheet code.

The UK Ordnance Survey sheets shown on a UK map

The UK OS Grid is a transverse mercator projection but is not aligned with the worldwide UTM grid system.

Ordnance Survey maps further divide the 100 kilometre squares into smaller squares with sides of ten kilometres.

These squares are numbered 0 to 9 from the south west (lower left hand) corner in both an easterly and northerly direction.

You can use these grid lines and numbers to identify a 10 kilometre square. You square coloured red (left) is at location SH46. After the sheet letters we have the number 4 for the easting and then 6 for the northing.

You can estimate or measure more exact locations right down to a single square metre.

Sheet	Easting	Northing
SH			a 100 kilometre square
SH	4	6	a 10 kilometre square
SH	44	65	a 1 kilometre square
SH	445	657	a 100 metre square
SH	44574	65743	A metre square - the limit of the co-ordinate system as it is currently used. this would normally be written as SH4457465743

UK Ordnance Survey grid references should always start with the sheet reference, this is followed by the Easting and then the Northing. Some people like to remember the order for the Easting and Northing value by using a simple mnemonic. "You have to go along the corridor (across the map) before going up the stairs (up the map)." OK, not strictly a mnemonic and it does rather depend upon a rather outdated, though still common, UK house layout but it works for a great number of people.

Other formats for describing OS grid references have been devised (often as a form of shorthand). These include BRC Tetrads (e.g. TF70U) which describe a 2 Km square, Quadrants (e.g. TF70NW) which describe 5 Km squares as well as the now outdated all numeric grid references originally used by the Ordnance Survey.

The system of sheet letters can be thought of as extending to include additional, notional sheets from AA through to ZZ. This extended alphanumeric range makes it possible to include locations outside of the OS survey area. Some of our AditSite users have found this very helpful, particularly when they have wanted to record and plot marine observations.

Our own AditMap and AditSite programs can accept OS grid references in all of these formats and can accept UTM grid references, Latitude and Longitude co-ordinates and Irish OS grid references.


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