So you’ve opened an OS map and it’s covered in numbers. Small blue ones round the edges, brown ones on the wiggly lines, black ones beside little dots. What are they all for?
Here’s the short answer. The blue numbers along the edges are grid numbers. The numbers on the vertical lines, which you read along the bottom of the map, are eastings. The numbers on the horizontal lines, which you read up the side, are northings. Put them together, with the two grid letters for that part of the country, and you have a grid reference, an exact address for any spot on the map. The brown numbers on the contour lines are heights in metres, and the black numbers beside dots are spot heights.
In this guide we’ll go through every kind of number on the map, then learn how to read a grid reference properly, four figures first and then six. It’s the one map skill that’s built for being found rather than finding your way, which is why it’s worth ten minutes now.
What do the numbers on the map represent?
There are four kinds of number on an OS map, and once you know which is which the map gets a lot less busy.
Blue numbers: the grid
The thin blue lines that cover the map in squares are the National Grid. Each line has a number, printed in blue in the margin and repeated across the map face. These are the numbers you use for a grid reference.
Each square is one kilometre across. On a 1:25,000 OS Explorer map that’s 4 cm. On a 1:50,000 OS Landranger it’s 2 cm.
Brown numbers: contour heights
The brown numbers sitting on the contour lines tell you the height of that line in metres above sea level. They usually sit on the thicker contour lines, which makes counting up and down a slope quicker, and on an Explorer map the contours are generally 5 or 10 metres apart in height depending on the area. A handy trick is that they’re printed so the top of the number faces uphill. If you can read it the right way up, you’re looking up the slope.
If contours still feel like a puzzle, how to read contour lines on an OS map takes you through them.
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Black numbers by a dot: spot heights
A black dot with a number beside it is a spot height, the height of that exact point in metres. Summits often have one. A blue triangle with a dot in it is a trig pillar, and the number beside it is its height.
Numbers on roads and paths
Road numbers like A591 or B5289 are exactly what they look like. They have nothing to do with where you are.
In short, if you want to say where something is, it’s the blue numbers you need.
What is a grid reference?
A grid reference is a way of describing any spot in Great Britain as a short string of letters and numbers. Something like NY 342 151, which happens to be the summit of Helvellyn.
The two letters come first. The whole of Great Britain is split into big 100 km squares, and each one has its own pair of letters. NY covers much of the Lake District, SH covers Snowdonia, SK the Peak District, NN the area around Ben Nevis. You’ll find the letters for your map printed in blue in the margin and explained in the key.
Then come the numbers, read from the blue grid lines. The more numbers you give, the smaller the box you’re describing.
Why do the letters matter so much? Because the numbers repeat. The same six figures turn up in every 100 km square in the country. Leave the letters off and you could be describing a hill in Wales or a field in Kent.
What are eastings and northings?
Eastings are the vertical grid lines. Their numbers get bigger as you go east, so you read them along the bottom or top of the map, left to right.
Northings are the horizontal grid lines. Their numbers get bigger as you go north, so you read them up the side of the map, bottom to top.
A grid reference always gives the easting first and the northing second. Always. That’s the whole rule, and it’s where most mistakes come from.
Along the corridor and up the stairs
The way everyone remembers it is along the corridor and up the stairs. You walk along the corridor before you can go up the stairs, so you read along the map (eastings) before you read up it (northings).
Say it a few times. It sounds daft and it works.
How to read a 4 figure grid reference
A four figure grid reference picks out one whole kilometre square. It’s good enough for “meet at the car park” and not much more.
Here’s how to do it.
- Find the square you want.
- Look at the vertical line on the left-hand side of that square. Read its number along the bottom of the map. That’s your easting, say 34.
- Look at the horizontal line along the bottom of that square. Read its number up the side. That’s your northing, say 15.
- Put them together, easting first, with the letters in front: NY 34 15.
The key point is that a grid reference always names the bottom-left corner of its square. The square runs up and to the right from there.
How to read a 6 figure grid reference
A six figure grid reference narrows that kilometre down to a box roughly 100 metres across. That’s the accuracy a rescue team can work with, and it’s the one worth learning properly.
You do exactly what you did for four figures, then add one more number to each half.
- Find the four figure square, NY 34 15 in our example.
- Imagine that square split into ten equal strips from left to right. Count how many tenths across from the left-hand line your point sits. Say it’s two tenths across. Your easting becomes 342.
- Now imagine the square split into ten strips from bottom to top. Count how many tenths up from the bottom line your point sits. Say it’s one tenth up. Your northing becomes 151.
- Put them together, letters first: NY 342 151.
On a real map there are no lines marking the tenths, so you judge them by eye. Many baseplate compasses have a small scale on the edge called a romer, marked in tenths of a grid square for 1:25,000 and 1:50,000, which takes the guesswork out of it. Put the corner of the right scale on your point and read off how many tenths it sits from the grid line to the left and the grid line below.
A 6 figure grid reference example
Let’s put it together with Helvellyn. The summit sits in the square whose bottom-left corner is easting 34, northing 15. It’s about two tenths across that square and a tenth up from the bottom. So the six figure grid reference is NY 342 151.
The best way to make this stick is to do it a few times. Our free grid reference practice tool gives you a point on a map, asks for the reference and tells you straight away if you’ve got it. Ten minutes on it is worth more than reading this twice.
How to find a grid reference on the hill
On paper, it’s the method above. Find where you are, find the square, read along the corridor and up the stairs, and judge the tenths.
On your phone, it’s quicker. The OS Maps app shows the grid reference of your position, and our free Grid Reference Finder gives you yours in the browser, with a button to send it by message. The position itself comes from GPS, which needs a view of the sky rather than phone signal, but a web page has to load first, so open it while you still have signal. Deep gullies and thick woodland can slow the fix down.
So why learn it the hard way? Two reasons. Phones run out of battery and fail in the cold, usually at the worst moment. And if you understand what the numbers mean, you can check a phone’s answer against the map instead of trusting it blind.
Where do people go wrong with grid references?
These are the mistakes that come up again and again.
- Northings first. The big one. NY 151 342 and NY 342 151 are both real places, about 27 kilometres apart. Along the corridor, then up the stairs.
- Using the wrong line. The number for a square comes from its left-hand and bottom lines, not the right or top.
- Leaving off the letters. Without them the numbers could be anywhere in Britain.
- Rounding up. If your point is seven and a bit tenths across, the figure is 7, not 8. You give the tenth it’s sitting in.
- Guessing the tenths on a Landranger. The squares are half the size of an Explorer’s, so the tenths are tiny. Use the romer.
- Reading it out too fast. Under stress, numbers get swapped. Write it down before you call anyone, then give the letters first and each digit on its own: “N Y, three four two, one five one”.
Frequently Asked Questions
What are eastings and northings?
Eastings are the vertical grid lines on an OS map, numbered from west to east, so you read them along the bottom of the map. Northings are the horizontal lines, numbered from south to north, so you read them up the side. A grid reference always gives the easting first and the northing second. The way to remember it is along the corridor and up the stairs.
What is the difference between a 4 figure and 6 figure grid reference?
A four figure grid reference, like NY 34 15, identifies a one kilometre square. A six figure grid reference, like NY 342 151, splits that square into tenths each way and identifies a box about 100 metres across. Six figures is the one to give the emergency services, because it narrows the search to a small patch of hillside.
What are 8 and 10 figure grid references?
They add more precision. Eight figures describes a box about 10 metres across and ten figures a box about one metre across. Phones and GPS units often give ten figures. On a paper map, six figures is the practical limit for most walkers, and it’s the standard you’ll be asked for.
Do I need the letters in a grid reference?
Yes. The two letters name the 100 km square you’re in, and the numbers repeat in every one of those squares across Great Britain. Without the letters, a grid reference could point to several places hundreds of kilometres apart.
What is grid north?
Grid north is the direction the vertical grid lines point on an OS map. It’s very close to true north but not exactly the same, and magnetic north, where your compass points, is different again. For reading a grid reference you don’t need to worry about it. It matters when you take a bearing with a compass.
Give it ten minutes
Right, over to you. Open the free grid reference practice tool and work through the levels until along the corridor, up the stairs is automatic.
If you’d like to set your phone up to give you a grid reference reliably in an emergency, that’s exactly what our free Digital Safety Net course covers. It’s free, and you can start today.
Ask any questions in the comments below.
