How to Read an OS Map: Contours, Symbols and Grid References

A good paper map is one of the great pleasures of walking. It shows you what lies over the hill, where the streams run and how the land will feel under your feet. The Ordnance Survey, Britain's national mapping agency, produces maps that many walkers use. Here is how to get comfortable with them.

Understanding scale

Two scales are popular with walkers.

• Explorer maps, 1:25,000. Four centimetres on the map equal one kilometre on the ground. They show a great deal of detail, including field boundaries and footpaths, and are popular for walking.
• Landranger maps, 1:50,000. Two centimetres equal one kilometre. They cover a wider area with less detail, and suit planning longer trips and driving routes.

The grid of blue squares on the map is made of one-kilometre squares, which makes it easy to estimate distance.

Contour lines

Contour lines are the thin, usually brown lines that join points of equal height. They look like rings on a hill. Learning to read them is perhaps the most useful map skill of all.

• Lines close together mean a steep slope.
• Lines far apart mean gentle ground.
• Rings getting smaller usually mean a hill top.
• V-shapes pointing uphill usually show a valley or stream.

On Explorer maps the height difference between lines is typically five metres, and on Landranger maps it is generally ten metres. Check the key, since this can vary.

Common symbols

The key at the edge of the map explains everything, and it is worth reading it once at home. A few useful ones:

• Footpaths are shown with green dashes on Explorer maps, and other rights of way have their own symbols, such as bridleways and byways.
• Roads appear in various colours and thicknesses, depending on size.
• Woodland is usually green, and open access land has its own shading and boundary on the map.
• Blue symbols show water, from streams to lakes and the sea.
• Small symbols mark features such as churches, car parks, viewpoints, campsites and pubs.

Grid references

A grid reference describes a place using the numbers along the edges of the map. The rule is easy to remember: along the corridor, then up the stairs. Read the number along the bottom first (eastings), then the number up the side (northings).

A four-figure reference tells you which one-kilometre square a place is in. A six-figure reference narrows it down to a hundred-metre square, by imagining each square divided into tenths. This is helpful when describing your position to others, for instance in an emergency.

Orienting the map

Turn the map so that what you see on it matches the land in front of you. Find a feature such as a road, a hill or a stream and line it up. With a compass, you can align the map north with magnetic north, remembering that the difference between the two changes slightly over time and is noted in the map key.

Practise close to home

The best way to learn is to try it. Take an Explorer map to a local park or a short footpath and match what you see with the symbols. Follow your progress along the route, ticking off features as you pass. After a few walks, it will become second nature. For planning the route in the first place, see our guide on how to plan a day walk, and remember to pack the map using our kit list for British hills.

Frequently asked questions

Is a phone app as good as a paper map? Apps are useful but rely on battery and signal. A paper map is a dependable backup, and many walkers carry both.

Which scale is best for a beginner? The 1:25,000 Explorer is the usual choice for walking because of its detail on paths and field boundaries.

How do I judge how steep a slope is? Look at how tightly the contour lines are packed. The closer they are, the steeper the slope.