How to define a Grid Reference

The Ordnance Survey Grid Reference System enables the accurate pinpointing of any UK location through the use of a map reference conforming to the following convention:

[2 Grid letters] [3 figures(easting)] [3 figures(northing)]

To arrive at this map reference, the United Kingdom is first divided into a series of 100km squares.

Each 100km square is then given a unique two letter code such as NT, SU, TQ, etc.

Map of Wales showing 100km OS grid structure

Wales, for instance, is covered by eight 100km squares.
The approximate extent of each of these 100km squares is illustrated on the left.

Therefore the grid reference for any location in Wales always starts with one of the following:

This indicates within which particular 100km square the location is to be found.

Much greater accuracy is of course required if the grid reference system is to be of any practical value. Each two letter 100km square is therefore subdivided by vertical and horizontal grid lines, at 10km intervals, to produce one hundred 10km squares.

Map showing subdivision of 100km Square SN
The vertical line forming the left hand side of the 100km square is labelled zero. Each successive vertical grid line within the 100km square is labelled in kilometers.

The horizontal line forming the bottom of the 100km square is also labelled zero. Each successive horizontal grid line within the 100km square is labelled in kilometers.
Any intersection of a vertical and horizontal line within square SN is therefore so many kilometers east of that square‘s zero point (the easting), and so many kilometers north of that same zero point (the northing).

The centre point of square SN could therefore be said to have a grid reference of SN 50 50.

Each 10km square within the SN square is then further subdivided into a hundred 1km squares.

Map showing subdivision of a 10km Square within 100km square SN The diagram on the left is an enlarged view of the four 10km squares in the bottom left hand corner of the SN square shown above.

The first 10km square is subdivided by vertical and horizontal grid lines at 1km intervals.

The centre point of this 10km square could be said to have a grid reference of SN 05 05.

To accurately establish the grid reference of the adjacent black dot, a further 10 x 10 grid must be superimposed on the 1km square within which the dot is located.

The dot can then be seen to be 4.5km east of zero (easting), and 5.5km north of zero (northing) within grid SN.

So its grid reference would be SN 045 055.

Ordnance Survey Maps

Explorer maps are amongst the most useful, for walking, Ordnance Survey maps published. These use a scale of 1:25 000 (4cm to 1km), and follow the general principal of the grid system discussed above.

If only one 100km square is covered by a map, the two letter code for that square can usually be found in each corner of the map. If more than one 100km square is covered, the relevant two letter codes are usually clearly printed where the 100km squares meet.

Each 10km square shown on a map is further subdivided into one hundred 1 km squares. The squares are formed from blue grid lines, spaced 1 km apart.

The eastings are marked along the top and bottom edges of each map, and the northings down the sides of each map.

When submitting defining a location, like the start of a walk, use the following convention for all OS Grid References:
[2 Grid letters] [3 figures(easting)] [3 figures(northing)]

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