top of page

The Grid System and Grid References: Introduction to maps 1.3:

Updated: Jan 23, 2021

Harvey Maps grid lines and numbers

More in this series:

Below is the answer sheet to the questions in the last part:

Answer Sheet - 1.2
Download PDF • 478KB

Part 3 in my Introduction to Maps will look at the UK Grid System, Grid References and why they are important to navigation.

Grid References

UK maps are overlay-ed with blue lines running vertically (North - South) known as Eastings as they are spaced going across the map, and horizontally (East - West) known as Northings as they are spaced going up the map.

These lines create squares known as GRID SQUARES. A single Grid Square is 1km x 1km and roughly 1.4km across the diagonal. They are useful for the following points:

  • Plotting positions on the map using Grid References

  • Measuring distances (This will be covered in a later article)

  • Taking bearings with a compass (This will be covered in a later article)

Mountain training illustration showing eastings and northings
Mountain Training Navigation in the Mountains Publication Image

Using this grid, we can identify and pin point large and small features on the map. These are known as Grid References. As walkers and climbers, we focus on two types of grid references:

  • 4-figure grid references (known as 4- fig)

  • 6-figure grid references (known as 6-fig)

4-figure Grid References

Eastings and Northings are numbered and these are used to create the Grid Reference. The numbers refer to the square to the right (Eastings) and above (Northings). A 4-figure Grid Reference is used to identify a single 1km x 1km Grid Square. Easting are read first, followed by the Northings or the saying "Along the Corridor and Up the Stairs" (see image above).

Using the image above, the 4-figure grid reference of the red square would be:


See the below image for another example. Working with the saying "along the corridor and up the stairs", the 4-figure grid reference for the Red X is:


6-figure Grid References

4-figure grid references are perfectly adequate to locate features on a map, for example saying the Trig Point is located in grid square 84,59. However, this may be too vague to pin-point small features on a busy map or if you want to give your exact location to rescue services.

For a more precise location, we can further divide a 1km x 1km grid square into 100m x 100m squares.

Mountain Training illustration showing 4 figure grid references and 6 figure grid references
Mountain Training Navigation in the Mountains Publication Image

We can do this visually by eye or by using navigational aids such as a compass or a roamer (more on these navigational aids further down).

By subdividing a grid square into 10 more Eastings and Northings we use the same concept as before to gain our 6-figure grid reference. See the above and below image for an example:

Mountain Training Hill Walking illustration showing 6 figure grid references
Mountain Training Navigation in the Mountains Publication Image

To find the 6-figure grid reference for the red square, we go "along the corridor" to get to "64". We then divide the square into 10 and continue along the corridor. In this example the Red square is "6" (because it's to the right of 6). We write this down as 646,___. Continue "up the stairs" and we get to "50". Divide the square into 10 again and "go up the stairs". In this example we reach "4" (because it's above the 4). We now have our complete 6-figure grid reference which we write down as 646,504

See the below picture for another example:

Using a romer to help plot 6 figure grid references

The location of the black dot is in grid square 21,31. This is an accuracy of 1km x 1km. To further pinpoint it's position to a greater accuracy of 100m x 100m we give the 6-figure grid reference which is 213,317

The UK Grid System

As mentioned, the entire UK is divided up into a grid based system of squares. These smaller squares are part of the bigger national grid system with squares measuring 100km x 100km. Each of these squares are defined by two-letters, for example SH, NG etc. There are about 50 of these 100km x 100km squares that cover the mainland of the UK, each with their own two-letters to identify them. These letters are particularly important when it comes to giving Grid References, to Mountain Rescue in an emergency for example.

All of these 100km x 100km squares are broken down into the smaller 1km x 1km grid squares and labelled 00 - 99. This means that the same Grid Reference can relate to ALL of the roughly 50 100km x 100km squares covering the UK mainland. By using the two-letter prefix, we can narrow down that grid reference to a particular area. (see below image)

Mountain Training Hill Walking illustration showing the UK grid system
Mountain Training Navigation in the Mountains Publication Image

Let's use the Trig Point on the summit of Snowdon as an example:

The 6-figure grid reference is 609,543 (The red dot in the image)

OS Maps app showing the summit of Snowdon
OS Maps app

However, all these places below (red dot) share the exact same grid reference 609,543 :

The four corners of the below square are the locations above, all sharing the same grid reference.

OS Maps App showing the same grid references
OS Maps app

The only way we can narrow the grid reference down to the summit of Snowdon and make it unique is by using the two-number prefix at the start of your grid reference.

The correct grid reference for Snowdon's summit is:

SH 609,543

The two-letter prefix is really important when giving your location to the emergency services as mentioned. All maps will have the two-number prefix printed on them, either in the corners or printed in the legend.

However, some maps may sit between TWO two-letter prefix areas so it is really important to know whether your map does this or not and find out where to find the prefix.

Take the South-Western area of the Lake District for example.

The prefix changes between The Old Man of Coniston in the South, to Wetherlam in the North. See the image below:

OS Maps 1:25,000 scale showing the Old Man of Coniston in the Lake District

Notice how it changes from NY in the north to SD in the south. It is clearly defined at the edge of the map.

In a scenario where you get injured off the summit of the Old Man of Coniston at Grid Reference SD 272,978 but you give the emergency services NY 272,978, the location you have given them is 100km to the North! it puts you at roughly 20km North East of Lockerbie, in Scotland!

Ensure you know what prefix to add to your grid reference in case you need to use it to pin point you position in an emergency.

Using Romers

I mentioned briefly about using romers to aid you in 6-figure grid references. Romers add a visual aid to help break down your 1km x 1km grid square into the 100m x 100m squares used for 6-figure grid references. See below for some examples of romers and how to use them:

  • Image 1: Compass romers (Use the corner of the romer in relation to scale of the map; 1:25k, 1:50k, 1:40k)

  • Image 2: Harvey Maps pocket romer (use the corner in relation to scale of map)

  • Image 3: Shaven Raspberry pocket romer (Place it over the whole grid square)

Image examples of how to use the different romers to find the grid reference of the Trig Point on the summit of Moel Siabod in grid square 70,54:

Answer: 705,546

Below is the question sheet to this article.

Question Sheet - 1.3
Download PDF • 2.12MB

Answers to these questions are in the next Part of this series.


This brings us to a close on this part of my Introduction to Maps series on the UK grid system and grid references. To put these skills into practice and to learn more, take a look at my Navigation Course I offer. All confirmed bookings will receive a 10% discount code to use on Harvey Maps products from their site.


Next Up:

The next article in this series will be Part 4: Resources and Care of Maps

Stay tuned for updates by signing up to my newsletter!

See you At The Edge!

310 views0 comments


bottom of page