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An Introduction To National Grid References

What are National Grid References?

Look at any Ordnance Survey map and you will see it is overprinted with horizontal and vertical lines forming a "Grid" of squares. This grid acts as an index system allowing you to locate any point on a map accurately with a single alphanumeric code (a code that contains letters and numbers) instead of a lengthy description or set of directions that can be ambiguous.

For example suppose you want to take part in a walk that is being organised from the Last Drop Village, how easy would it be for you to find the meeting point?

  • From a description of the location: When you reach the Last Drop Village, continue past the main car park (1) in front of the swimming pool and bear left at the end of the lane (2) shortly after the Demdyke Suite car park - possibly a bit confusing with several different car parks, but you could probably find it.
  • By keying the post code BL7 9QA into your Sat Nav - this would take you to a farm track off Hospital Road (3), the car park you want is at the end of Haydock Lane (2)
  • By using the National Grid Reference SD723140 - this will take you straight to the correct car park without the need for any extra details or directions.

OK, so these Grid References are an easy way of pinpointing a location accurately but only if you know what the letters and numbers of the code mean, so lets break the code down to its individual elements.

picture of first letter of national grid reference The area around the UK and its outlying islands is divided into 500km x 500km (310½ x 310½ mile) squares, each is allocated a different letter of the alphabet from A to Z (except I). This gives the large 2,500km x 2,500km (1,553½ x 1,553½ mile) box opposite. Our location is somewhere within the "S" square, the first letter of the Grid Reference for your walk start point. S
picture of second letter of national grid reference Each of these is further divided into 100km x 100km (62 x 62 mile) squares, again each is allocated a different letter of the alphabet from A to Z (except I) preceded by its parent "S" label above. The start point is now somewhere within the "SD" square, as shown opposite.
picture of 1km squares of national grid reference Each of these 100km x 100km (62 x 62 mile) squares is now split into 1km x 1km (0.6 x 0.6 mile) divisions. This time though each vertical dividing line is numbered from 0 to 99 going from left to right (eastwards), they are called EASTINGS. Each horizontal dividing line is also numbered from 0 to 99 going from bottom to top (northwards), these are called NORTHINGS. Each 1km x 1km square is labelled with the numbers of the two lines that meet in the bottom left hand corner of the box. So our example destination is somewhere within the 1km² covered by the box labelled SD7214 as shown opposite.
picture of 100m squares of national grid reference Each of these 1km x 1km (0.6 x 0.6 mile) squares is now further split into 10 vertical and horizontal divisions each measuring 100m x 100m (109 x 109 yards). So our example destination is somewhere within the area covered by the square labelled SD723140 putting us within sight of our walk start point. This is known as a 6 figure grid reference as it has 6 numbers.

The map itself only shows the 4 figure grid reference squares, this last split in our 6 figure reference is estimated visually or can be measured using a "romer" scale; which you will find printed on the baseplate of the military style and some other compasses or it can be bought separately.

It is essential that the EASTINGS numbered from left to right are read first followed by the NORTHINGS running from bottom to top. Get them the wrong way round and you will end up in totally the wrong location. - giving a grid reference of SD140723 instead of the correct SD723140 for our walk start point example would put you in the Irish Sea just off the Barrow-In-Furness sand banks.

One way to make sure you use the correct numbering may be to memorise the phrase "along the corridor then up the stairs"

The map section showing your walk meeting point is reproduced below with the grid numbering you will see on the 1:50,000 Landranger map emphasised. Notice the meeting point (2) is slightly above the 14 line - remember the grid reference tells you the square that your location is in NOT its exact location.

For general navigation and location finding this level of accuracy to within 100 metres (109 yards) is perfectly satisfactory, but for more remote pathless and/or large areas with few navigation features, a higher level of accuracy may be required and the OS Grid Reference system extends to 10 figures (the full Grid Reference) by two further stages, each dividing the squares into 10 further sub divisions to put you within 1 metre (3.3 feet) of your location.

The more figures you use the greater the accuracy as summarised in the table below for the OS Grid Reference of our example location.

Full 10 figure8 figure6 figure4 figure
Full Grid ReferenceSD 7236514032SD 72361403SD 723140 SD 7214
Accuracy1 metre10 metres100 metres1 km

A handy summary reference for plotting a 6 figure reference is usually provided within the legend of OS maps. It includes the 2 prefix letters which should always be included, as without them you could be referring to a location in any 100km square!