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Home > Reference and Guides > Maps & Directions > Understanding Maps > Grid References

Glossary of Map Abbreviations

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British National Grid References

There is a set of geographic grid references commonly used in Great Britain, devised by the Ordnance Survey (OS) and commonly quoted in maps, guide books and other publications. This system is known as the British National Grid Reference System and is used across Great Britain and its outlying islands and to indicate the position of any particular point or area. A similar system, used throughout Ireland (including Northern Ireland), is known as the Irish National Grid Reference System.

Grid Letters

The whole of Great Britain has been divided into a number of squares, 100 km by 100 km. Each of these squares is denoted by a unique set of 2 letters; for example:

Lerwick (Shetland Islands) is in square HU
Glasgow is in square NS
York is in square SE
Birmingham is in square SP
London is in square TQ
Penzance is in square SW

Grid Digits

Each of the 100 km by 100 km lettered squares are further subdivided into 100 squares - each of them 10 km by 10 km. The gridlines created from these squares are marked from 0-9 along the horizontal and vertical-hand axis, with 0,0 describing a position at the bottom left hand corner.

Measurements along the horizontal axis (running west-east) are called 'eastings' (as they run towards the east), whilst measurements along the vertical axis (from south-north) are called 'northings' (as they run towards the north).

For example, Edinburgh has a grid reference of NT 27; this means that it is situated in a square whose origin (its south-west corner) is 2 squares east of and 7 squares north of the origin of the NT square, i.e. 20 km east and 70 km north from the bottom left hand corner.

A grid reference is made up by a two letter code followed by an even number of digits. The first half of the digits indicates the first coordinate (the eastings) and the second half denotes the other (the northings).

4-Figure Grid References

Each of the 10 km x 10 km squares can be further subdivided to create 100 equal squares of 1 km x 1 km. This means that the major lettered square (e.g. square NT) would now contain 1,000 smaller squares; 100 both along the bottom and up the side of the grid. Each of these squares is labelled from 00-99; these are the numbers you may find printed along the bottom and sides of an Ordnance Survey (OS) map.

As described earlier, the names given to any square within this grid is a reference to the origin of that square (the bottom left-hand corner), reading firstly from west to east, and then from south to north. For example, Edinburgh Castle's 4-digit grid reference is NT 25 73; i.e. it can be found in the 1 km x 1km square whose south-west corner is 25 km east and 73 km north from the south-west corner of square NT.

6- Figure Grid References

By dividing each of the 1 km x 1 km squares into 100 equal squares of 100 m x 100 m, a 6-figure grid reference can be created. This allows a specific location to be pinpointed fairly accurately, narrowing it down to anywhere within a particular 100 m x 100 m square. The additional gridlines formed by this subdivision of the 1 km squares do not usually appear on maps as their inclusion would make it very difficult to see any other details. Consequently, the map user must judge for themselves how many tenths along and up the square any particular point is.

For example, Edinburgh Castle has a 6-figure grid reference of NT 253 736. Looking at a map of the NT square, the user would find the gridlines marked 25 along the bottom of the map and 73 up the side. Where these lines intersect would be the origin (the south-west corner) of the square that contained the Castle. Looking at this square, the user would then be able to locate the Castle 3 tenths along the bottom of the square and then 6 tenths up. This means that the Castle is located 25.3 km east and 73.6 km north from the bottom-left corner of square NT.

Accuracy of Grid References

In general, the more figures a grid reference has, the more accurate it is. The most commonly used references are as follows:

Grid Reference	Accuracy
XX	Anywhere within a 100 km x 100 km square
XX 1 1	Anywhere within a 10 km x 10 km square
XX 12 12	Anywhere within a 1 km x 1 km square
XX 123 123	Anywhere within a 100 m x 100 m square
XX 1234 1234	Anywhere within a 10 m x 10 m square
XX 12345 12345	Anywhere within a 1 m x 1 m square

Note: The 8-figure and 10-figure grid references described in the last 2 rows of the above table are only obtainable with the use of a Global Positioning System unit (GPS).

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