The UK Ordnance Survey grid is a grid of 100 kilometre squares with a true origin at 49° North and 2° 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 two letter grid structure Ordnance Survey map references for locations in Great Britain are usually given as:
[2 Grid letters], [3 figures(easting)],
But can also be given as:
[6 figures(easting)], [6 figures(northing)]
Each 500km square is then subdivided into 25 squares, 100km by 100km. Each of these 100km squares is allocated a reference letter (A to Z, omitting I, starting with A in the north-west corner of the parent 500km square).
In this way each 100km square can be referred to by a unique two letter reference, with the first letter referring to the parent 500km square, and the second letter referring to a particular 100km square within it.
The approximate extent of the resultant 100km grid is illustrated in the map of the UK displayed on the right.
You can estimate or measure more exact locations right down to a single square metre.
|SS||S||100 km sqr|
|SS||4||8||SS48||10 km sqr|
|SS||40||87||SS4087||1 km sqr|
|SS||403||875||SS403875||100 m sqr|