A note on how to work out grid references

UK National Grid image

Ordnance survey maps (not road maps) should be used to calculate grid references.

The United Kingdom National Grid is first divided up into 100km squares. Each of these is given code letters, e.g. TL. See illustration.

Each 100km grid square is divided into 10km grid squares. The grid reference of a 10 km square is comprised of the 100 km square code (e.g. NN) followed by two numbers that identify the bottom left corner of the square. The first number refers to the horizontal scale and the second to the vertical scale.

The following diagram shows how the coloured 10km square is referred to as "TL63".
Remember to read eastings (across), and then northings (up).

Grid reference illustration © CEH Monks Wood

Each 10km square is divided into 1km squares, shown with light blue lines on the 1:50,000 OS maps. The grid reference of a 1 km square is comprised of the 100 km square code (e.g. NN) followed by four numbers that identify the bottom left corner of the square. The first two numbers are the eastings and the second two the northings.

A six-figure reference, which identifies a 100m square, can be derived by dividing the 1 km square into 10 from the bottom left corner. In this case the first three numbers are the eastings and the second three the northings.

Take care shortening coordinates from GPS

A GPS may provide a reference of up to 10 digits, identifying a 1m square. The first half of the digits are eastings and the second half are northings. To shorten this to the 6-figure reference we want, take the first 3 digits of the eastings and the first 3 of the northings. E.g. TL 69154 37872 becomes TL691378.

PLEASE NOTE - on the Incidental Records form, you may use a local postcode or site name instead of a grid reference.

If you aren't sure about how to work out a grid reference for a site, the Ordnance Survey provide a good, quick tutorial on how to do it. You can also find out more about the UK national grid from Wikipedia.