Grid references are used all over the world to specify exact locations on the Earth's surface, and are invaluable for a wealth of outdoor actitives; everything from rambling to military exercises. In the United Kingdom, any location (apart from a few outlying islands) can be uniquely defined up to an accuracy of 100m by a code consisting of two letters and six numbers.

The letters come first and are the most significant part of the reference. The country is divided into 100km squares, each with a two letter identifier. The full set of codes is shown below.
```
HP
HT HU
HY HZ
NA NB NC ND
NF NG NH NJ NK
NL NM NN NO
NR NS NT NU
NW NX NY NZ
SC SD SE TA
SH SJ SK TF TG
SM SN SO SP TL TM
SR SS ST SU TQ TR
SV SW SX SY SZ TV
```
Each one of these 100km squares is subdivided into 1km squares, labelled 00 to 99 horizontally and 00 to 99 vertically. These squares are clearly marked on most British maps. Find the square which the location is in, and quote the horizontal figure, then the vertical figure. The will give four digits, say 31 15.

The final two digits pinpoint a 100m square within the 1km square. Imagine that the 1km square is divided into 100m squares, labelled 0 to 9 horizontally and 0 to 9 vertically. Find which square the location is in, and quote the horizontal figure (say 4), then the vertical figure (say 9).

The six figures are quoted with the three horizontal figures first, then the three vertical ones. Hence is this example, the reference would be quoted as 314159.

Often, when it is clear what 100km square is being referred to, the letters are omitted. Occasionally, when only 1km accuracy is required, a four figure reference is given, where the third and sixth digits have been left out. On the other hand, when 10m accuracy is required, a further two figures can be added.

Some examples of grid references are