How to Read an OS Map Before Your Next Ramble

An Ordnance Survey map is one of the most useful things you can put in your rucksack, and also one of the most intimidating when you first unfold it. Dense lines, tiny symbols and a grid of blue squares can feel like a foreign language. Yet knowing how to read an OS map is a skill you can pick up in an afternoon, and it will change the way you walk. You stop following other people’s routes and start making your own.

This guide covers the basics: choosing the right map, understanding scale, decoding symbols, reading contour lines and using grid references.

Choose the Right Map for Walking

Ordnance Survey publishes several series, but two matter most to ramblers.

• OS Explorer (1:25,000): The walker’s favourite. At this scale, 4 cm on the map equals 1 km on the ground, which is why it shows field boundaries, individual paths, stiles and small features.
• OS Landranger (1:50,000): At this scale, 2 cm equals 1 km. It covers larger areas with less detail, which is handy for planning longer trips or driving to a start point.

For most walks, Explorer is the better choice. Check the sheet number for your area, and note that popular regions such as the Lake District or Peak District are sometimes printed on both sides of weatherproof paper. Digital versions are also available through the OS Maps app, but the principles are identical.

Start with the Legend

Every OS map has a key, usually printed on the side or the back. Do not try to memorise it all. Instead, learn the symbols you will meet most often on a ramble, and look up the rest as needed.

Pay particular attention to:

• Rights of way: Public footpaths are shown as green dashes, bridleways as longer green dashes, and byways as green dots or dashes with a different pattern.
• Open access land: Shown with a yellow tint and an orange border on Explorer maps.
• Woodland: Green shading, with different symbols for coniferous and deciduous trees.
• Water: Blue lines and areas for streams, rivers, lakes and the sea.
• Buildings and landmarks: Pubs (PH), churches, farms, car parks and viewpoints use small symbols and abbreviations.

Take a few minutes to read the key before every new area. You will start noticing patterns very quickly.

Understand the Scale

Scale tells you how the map relates to reality. On a 1:25,000 map, one unit on paper equals 25,000 of the same unit on the ground. That sounds abstract, so lean on the practical shortcut: 4 cm equals 1 km.

To measure a route, lay a piece of string along the path on the map, straighten it and compare it with the scale bar. A small ruler or a measuring tool on a compass baseplate also works. Remember that the real distance you walk is a little more than the map distance, because paths wiggle and hills add extra steps.

Use the Blue Grid

The blue lines forming squares across the map are the National Grid. Each square is 1 km by 1 km on Explorer and Landranger maps. The grid helps you count distances quickly and pinpoint your position.

Each line is numbered along the edges. Lines running up the map are called eastings, and lines running across are northings. The golden rule: read along the corridor and then up the stairs. In other words, read the easting (left to right) first, then the northing (bottom to top).

Giving and Finding a Grid Reference

A grid reference is a short code that identifies a location. A four-figure reference tells you which square you are in. A six-figure reference narrows it down to a 100 metre area.

Here is how to work out a six-figure reference:

1. Find the easting number on the left side of the square containing your feature, for example 25.
2. Imagine the square divided into ten equal strips from left to right. Estimate how many tenths across your feature lies, for example 4. That gives you 254.
3. Find the northing number at the bottom of the square, for example 71.
4. Estimate how many tenths up the feature lies, for example 6. That gives you 716.
5. Combine them: 254716.

Practise by picking a church, a trig point or a pub on your map and writing down its reference, then check your answer by using the OS Maps app or a plotter.

Reading Contour Lines

Contour lines are the thin orange or brown lines that join points of equal height. They are the key to understanding how hilly a route will be.

• Lines close together mean steep ground.
• Lines far apart mean gentle slopes.
• Closed rings mark hilltops or hollows. Spot heights and numbers show elevation.
• V shapes pointing uphill usually show a stream valley, and V shapes pointing downhill suggest a ridge.

On Explorer maps, contours are typically drawn every 10 metres. Count how many lines your route crosses to get a rough idea of the climb. A route that crosses six lines gains about 60 metres, which is a modest hill, while twenty lines is a proper effort.

Before you set out, trace your route and notice where it climbs, where it descends and whether the steepest sections come early or late.

Identify Paths and Access Rights

Many new walkers worry about whether they are allowed to go somewhere. The map is your friend here. Look for green dashed lines marking public rights of way, and read the key to tell footpaths from bridleways. Remember that the map does not show every permissive path or private track, and rights of way can be diverted or closed temporarily.

If you are walking across farmland, keep to the line of the path, even if it crosses a crop. Your map helps you do that by showing the line, and our guide to public footpaths explains your responsibilities.

How to Walk Safely on Public Footpaths and Farmland

Orient the Map

To use a map in the field, you need to match it with the landscape.

1. Hold it flat and turn it so that features on the map line up with features around you, such as a road, a lake or a hill.
2. Use a compass if you have one. Place the compass on the map, line up the north arrow with the grid lines, and rotate the map until the needle agrees.
3. Identify two or three landmarks and see whether the map shows them in the right directions.

If you cannot match anything, do not guess. Walk back to the last place you were certain of, and start again. The most common navigation errors come from convincing yourself you are somewhere you are not.

Plan a Route on the Map

Planning on paper before you leave is how confidence is built. Try this process:

1. Choose a start point such as a car park or station.
2. Join up footpaths to form a loop, avoiding long road sections.
3. Note key features along the way, such as stiles, bridges and junctions, as handrails or checkpoints.
4. Measure the distance and count the contours to estimate time.
5. Mark places to bail out early if the weather or energy fails.

For a worked example of building a loop with a stop at the pub, see our guide to circular walks.

How to Plan a Circular Walk with a Pub Lunch

Back Up Paper with Apps

Many walkers combine paper and digital. An app with downloaded OS maps shows your live position, which is very helpful, but batteries die, screens crack and signal fails. Treat the app as a backup and the paper map as primary, or vice versa, but carry both. Practise with each, so you are not working things out for the first time when it begins to rain.

Common Mistakes to Avoid

• Ignoring the scale and underestimating distance.
• Confusing rights of way with permissive paths.
• Reading the northing before the easting.
• Not updating your position as you walk.
• Relying on a path that may have been changed since the map was printed.

Check the publication date on the map, as the landscape and footpath network do change.

Frequently Asked Questions

Which OS map is best for walking?
The Explorer 1:25,000 series is the best choice for most walks because it shows individual field boundaries and paths.

How do I give a six-figure grid reference?
Read the easting first, estimate tenths across the square, then read the northing and tenths up. Combine the two sets of three numbers.

Do contour lines show how steep a path is?
Yes. Lines packed closely together show steep ground, while widely spaced lines show gentle slopes.

Can I just use my phone?
You can, but carry a paper map as backup because batteries and signal are not reliable outdoors.

Conclusion

Reading an OS map becomes straightforward once you break it into parts: pick the right scale, learn the key, use the grid, interpret contours and check your position regularly. Start on familiar ground, practise a little each time, and soon the map will feel like a trusted friend.