Most people know that contour lines show height on an OS map. But knowing that and actually being able to look at a map and picture the terrain in your head, that’s a different skill entirely. That’s the bit that takes practice.
This tool is designed to help you bridge that gap.
Hover over the contour map on the left and you’ll see the same ground in 3D on the right, along with an elevation cross-section below it. Switch between terrain types, hills, ridges, valleys, saddles and cliff edges, and watch how the contour pattern changes for each one.
It’s completely free to use. No login, no sign-up, nothing to download.
There is a free article on how to read contour lines to help you get started with contour lines
How to use this tool
Select a terrain type from the buttons above the map, then hover your cursor slowly across the contour map. The 3D view and cross-section update in real time so you can see exactly what that ground would look like on foot. Pay attention to how closely the lines are packed together. That’s your best clue to gradient. Tight lines mean steep ground. Wide spacing means easy walking.
Module 2 · Reading terrain
See the shape of the ground in the lines
The same piece of real hill, three ways at once. Move across the map and watch the height, the steepness and the shape all change together. Drag a line across the map to cut your own cross-section through it.
Start with the shapes, then move on to the real hills — and click through the buttons underneath to see each one.
The map
Drag to cut a section
The ground
Drag to turn
Cross-section A to B
Colour shows how steep each stretch really is
Height
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above sea level
Slope
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Underfoot
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Slope faces
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Grid reference
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Loading terrain…
One moment.
Elevation data: Ordnance Survey Terrain 50 via AWS Terrain Tiles. Contains OS data © Crown copyright and database right 2026, used under the Open Government Licence v3.0. The model is sampled at 50 m, so fine detail is generalised and summit heights shown on the map may read a few metres below the published height.
What the contour line patterns are telling you
Once you know what to look for, contour lines stop being a jumble of orange squiggles and start becoming a proper picture of the ground ahead. Here’s a quick reference for what you’ve just explored in the tool above.
Hills and summits. Concentric rings that close in toward a single high point. The tighter the rings, the steeper the climb to the top.
Twin hills and undulating ground. Two separate sets of rings, often with wider-spaced lines in the dip between them. This is the kind of ground you’ll cover a lot of in the UK hills.
A ridge. Parallel lines running along the length of the high ground. The lines on either side pack tightly as the slopes drop away.
A valley. V-shaped contour lines where the point of the V always aims uphill, toward the head of the valley. Valleys on an OS map look like a series of Vs nested inside each other.
A saddle or col. An hourglass shape where lines pinch inward between two high points. In the UK this is often where you’ll find a path crossing from one valley to the next.
A cliff or steep ground. Lines that are almost touching or appear to merge. If you see this on a map, give it a wide berth unless you know exactly what you’re doing.
Want to go deeper with your map reading?
This tool was originally built for students on the Bondringo Mountain Navigation Course. It’s one of several resources I’ve put together to help people get genuinely confident with a map and compass before they head into the hills.
The full course takes you through everything: reading terrain from contour lines, taking and following a bearing, locating yourself on a map, planning a route, and staying safe when visibility drops. All self-paced, all online, and built around UK OS maps.
If you found this tool useful, the course will take you a long way further.
View the Bondringo Mountain Navigation Course
A note on this tool, and how you can help shape it
I’ll be straight with you. This started as a resource for course members and I decided to make it publicly available because I think more people should have access to good, free navigation learning.
Right now it covers the six most important terrain types you’ll encounter on a UK OS map. If it gets a good reception, I’d love to put more time into it. More terrain examples, more detail on how to identify features when you’re already out on the hill, maybe some quiz-style challenges to test yourself.
If you’ve used the tool and have thoughts on what would make it better, what’s working, what’s confusing, what you’d like to see added, drop us an emailon our contact page. I’ll read every one.
Contour line FAQ
What are contour lines on an OS map?
Contour lines are the orange lines on an Ordnance Survey map that connect points of equal height above sea level. Each line represents a specific elevation. On a 1:25,000 OS map the lines are typically spaced 10 metres apart vertically.
How do you read contour lines?
The closer together the contour lines, the steeper the ground. Lines that are widely spaced indicate a gentle gradient. Closed rings indicate a summit or high point. V-shaped lines pointing uphill indicate a valley. Use the tool above to see exactly how each terrain type appears on a map.
What is the difference between index contours and normal contour lines on an OS map?
Every fifth contour line on a 1:25,000 OS map is drawn thicker and darker. This is called an index contour and is usually labelled with its height in metres. They make it much easier to count your way up or down the map without losing track.
Can you learn map reading online?
Yes. Many of the core skills, including reading contour lines, understanding grid references and planning a route, can be learned online before you head out into the hills. The Bondringo Mountain Navigation Course covers all of these in a structured, self-paced format built around UK OS maps.