You’re standing at the car park, OS map in hand, and all you can see is a tangle of brown lines going nowhere in particular. You know they mean something. You’re just not sure what.
Learning how to read contour lines on an OS map is one of those skills that transforms the whole experience of walking. Once it clicks, you stop looking at a flat piece of paper and start seeing the actual shape of the land. Hills, valleys, ridges, steep drops. It’s all there, once you know what to look for.
This guide will walk you through exactly that. No jargon, no fuss.
You can also find a broader article if oyu are just getting started, before coming back to this lesson. The Ultimate Guide to Map and Compass Navigation: Core Skills
What Are Contour Lines?
Contour lines are the brown lines on an OS map that show the shape of the ground. Every line connects points that are at the same height above sea level.
The number printed on the line tells you the height in metres. If a contour line is labelled 300, every point on that line is exactly 300 metres above sea level.
On a 1:25,000 OS Explorer map (the orange ones, and the ones most walkers use), the contour interval is usually 10 metres in hilly or mountainous terrain. In flatter areas it can be 5 metres, so always check your map’s legend. On a 1:50,000 OS Landranger map, the contour interval is also 10 metres.
Every fifth line is a thicker brown line, called an index contour. These are the ones with the height numbers on them, and they make it much easier to count up or down the hillside.
Why does this matter? Because the lines alone tell you how high something is. The spacing between them tells you how steep it is.
How to Read Contour Lines on an OS Map: The Basics
Here’s the key rule: the closer together the contour lines, the steeper the ground.
Think of it this way. If you had to climb 10 metres in height over a very short horizontal distance, that’s a steep slope. The contour lines would be packed tightly together. If you’re climbing the same 10 metres but spread over a long, gentle slope, those lines would be spread far apart.
So tightly packed lines equal steep. Widely spaced lines equal gentle. It really is that simple.
A few practical things to know when reading OS map contours:
- Read the numbers. Contour lines have a high side and a low side. The numbers always read uphill, so if you tilt the map to read the number normally, you’re facing uphill.
- Count the lines. To work out how much height you’ll gain between two points, count the contour lines between them and multiply by the contour interval (usually 10 metres in hilly terrain).
- Look for the index contours. The thicker lines with numbers are your anchor points. Use them to orientate yourself on the hillside.
Reading contours is a seeing skill, and you only get it by looking at a lot of them. Our free contour lines quiz shows you a shape and asks you to name the landform, which is the fastest way to find out whether it has actually gone in.
This is the foundation of OS map reading. Once you’re confident here, everything else starts to fall into place.
What Different Contour Patterns Mean
The shape of the lines tells you the shape of the ground. Here are the main patterns and what they mean.
Use our Free Interactive tool to help visualise contour lines on a map. This tool will help you see differt types of terrain in 3D. If enought poeple like it we will continue to make to better for you.
Concentric circles or ovals indicate a hill or summit. The lines close in on each other toward the centre, where the highest point is. The tighter the circles, the steeper the hill.
A V-shape pointing uphill (into higher ground) indicates a valley or stream gully. Water runs downhill, so rivers and streams always flow away from the point of the V. This is a really useful one to spot when you’re trying to navigate in poor visibility.
A V-shape pointing downhill (into lower ground) indicates a spur or ridge running out from the hillside. These are the bits of ground that stick out between valleys.
Widely spaced, roughly parallel lines indicate a broad, gently sloping hillside. This is the easy walking ground.
Lines that barely exist or are very far apart indicate flat or nearly flat ground, like a plateau or a wide valley floor.
Lines that merge together or stack almost on top of each other indicate a cliff or very steep drop. You may also see rocky outcrop symbols alongside them on the map. This is one to pay close attention to when planning a route.
Lines spaced far apart near the top of a hill but getting progressively closer as you go down indicate a convex slope. This is important for safety, not just navigation. A convex slope hides the terrain below from your line of sight when you’re standing on it. You can’t see what’s ahead until you’re already committed to the descent. If you spot this pattern on the map, take extra care before you start dropping off a ridge.
Learning to read these patterns is the real skill. It’s not about memorising shapes, it’s about training your eye to match what’s on the map to what’s in front of you.
| Landform | What the contours do | What it looks like on the ground |
|---|---|---|
| Valley | V shapes pointing uphill, towards high ground | Ground falling away either side of you, often with a stream in the bottom |
| Spur or ridge | V or U shapes pointing downhill, away from high ground | A nose of ground pushing out, falling away on both sides |
| Summit | Closed rings, smallest in the middle | Ground falls away in every direction from you |
| Saddle or col | Two sets of rings with a waist between them | The low point on a ridge between two tops |
| Steep ground | Lines packed tightly together | A hard climb or a sharp drop |
| Gentle ground | Lines spaced well apart | An easy angle you barely notice |
| Cliff | Lines touching or merging | Not walkable. Plan around it |
Contours are the one navigation skill that only comes from looking at a lot of them. The free contour lines quiz shows you a shape and asks you to name it, and the interactive contour tool lets you turn real terrain over and watch the lines change with it.
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Common Mistakes Beginners Make Reading Contour Lines
I’ve been on the hills with a lot of walkers over the years, and the same mistakes come up again and again for beginners.
Ignoring the spacing. People see contour lines and think about height, but forget about steepness. If the lines are tightly packed, that slope is going to be hard work. Always check spacing before committing to a route.
Not checking the interval. On a 1:25,000 Explorer map, the contour interval can be 5 metres in flatter areas and 10 metres in hilly terrain. Always check the legend before you start counting lines. If you assume the wrong interval, your sense of the gradient will be completely off.
Getting V-shapes backwards. It’s easy to confuse a valley with a spur if you’re tired or rushing. The rule to remember: V-shapes pointing uphill are valleys. The point always aims toward higher ground because that’s where the stream starts.
Not cross-referencing with the terrain. The best navigation skill you can build is the habit of looking at your map and then looking at the land around you to match them up. Don’t just look at the paper. Look up.
These aren’t criticisms. Everyone gets these wrong at first. The good news is, they’re easy to fix once you know what to watch for.
Putting It Into Practice: Reading the Landscape Before You Walk
Here’s how I’d recommend using contour lines before every walk.
Before you set off, spend five minutes with your OS map and trace the route with your finger. As you do, ask yourself: where is the ground rising? Where does it flatten out? Are there any steep sections I need to plan for?
Look for the V-shapes. Work out which ones are valleys and which are spurs. Identify the high points and low points along the route.
Then, when you’re actually out there, stop regularly and match the map to the ground. Find a distinctive contour feature, whether it’s a valley below you, a ridge to your left, or a steep rise ahead, and confirm you can see it on the map.
This habit is what separates walkers who can navigate from walkers who get lost. It’s not about being clever. It’s about staying aware.
Contour lines are one piece of the navigation puzzle. Once you can read them confidently, the next step is learning to combine them with compass work and grid references to navigate accurately in any conditions. That’s exactly what I cover in the Bondringo Mountain Navigation Course, a practical, step-by-step course built for walkers who want to feel genuinely confident on the hills.
If reading contour lines has felt like a mystery until now, it won’t for much longer.
For the full picture of walking navigation, from setting the map to moving in poor visibility, start there and come back to contours whenever you need them.
What is the contour interval on an OS map?
On a 1:25,000 OS Explorer map, the contour interval is usually 10 metres in hilly or mountainous terrain, and 5 metres in flatter areas. On a 1:50,000 OS Landranger map, it’s 10 metres throughout. Always check the legend on your specific map to confirm. It’s printed in the key.
How do I tell which way is uphill on a contour line?
Look at the numbers. They’re always printed so the top of the digit faces uphill. Tilt the map until you can read a contour number normally, and you’re facing uphill. Index contours (the thicker lines) usually carry the height labels, so those are the easiest ones to use.
What does it mean when contour lines are very close together?
It means the slope is steep. The lines represent fixed intervals of height, so if those intervals are squashed into a short horizontal distance, the ground rises sharply. Very tight lines that appear to merge indicate a cliff or near-vertical drop. If you see that ahead on your route, plan an alternative way around.
Can I use contour lines to find water on a map?
Yes. V-shapes that point uphill indicate valleys, and streams flow along valley floors. Look for the V-shape, follow the direction the point aims, and you’ll find the high ground where the water source starts. Blue lines on the map show rivers and streams directly, but contour V-shapes will often show you gullies and drainage lines before the stream becomes large enough to be marked.
What is a convex slope and why does it matter?
A convex slope curves outward, like the top of a dome. On a map, it shows as contour lines that are widely spaced near the hilltop but get progressively closer lower down. The danger is that when you’re standing on a convex slope, the steeper ground below is hidden from view. You can’t see what you’re walking into until you’re already on it. Always check the contour pattern before committing to a descent.
Are contour lines enough to navigate with?
They’re a key part of navigation, but not the whole picture. Contour lines tell you the shape of the land. You still need to combine them with grid references to pinpoint your location, compass bearings to set a direction, and map-to-ground observation to confirm you’re where you think you are. Used together, these skills give you reliable navigation in any conditions.
Once contours click, the next thing is putting them together with everything else the map is telling you. How to read an OS map covers the whole sheet, and how to use a compass on a map is what you reach for when the ground stops giving you clues.
