How do I read contour lines on a map?

Everything difficult about hill navigation becomes easier once contours read as shape rather than as squiggles. They are the only feature on the map that never changes, never gets removed and is always there in mist, and they are what experienced navigators use when the paths and walls have run out.

What you'll need

• An Ordnance Survey 1:25,000 or 1:50,000 map
• A pencil

Step by step

1. Find the contour interval in the legend.On British 1:50,000 mapping the interval is usually 10 metres; on 1:25,000 it is commonly 10 metres in the lowlands and 5 metres on some sheets. Get this wrong and you will misjudge every slope on the map.
2. Read the spacing as steepness.Lines close together mean a lot of height in a short distance, which is steep ground. Lines far apart mean gentle ground. Lines merging into a solid band, or the map showing a crag or cliff symbol, means it is not walkable.
3. Find the index contours and read the numbers.Every fifth contour is drawn thicker and carries a height figure. The number is printed so that it reads the right way up when you are looking uphill, which tells you which way the ground rises.
4. Recognise a valley or a stream line.The contours make a V, and the point of the V points uphill, upstream. Water runs down the middle of it, and the sharper the V the tighter the valley.
5. Recognise a spur or a ridge.The contours make a rounded U or a blunt V, and the point of it points downhill. A spur is a nose of high ground sticking out from a hillside — often the easiest ascent line and always the driest.
6. Recognise a summit and a col.Concentric closed rings, smallest at the top, is a summit or a knoll. Where two summits are joined by a low point, the contours pinch in from both sides like a waist — that is a col, bealach or saddle, and it is a key navigational feature because it is where routes cross ridges.
7. Recognise convex and concave slopes.Contours widely spaced at the top and tightening downhill is a convex slope, which is dangerous because you cannot see over the edge from above. Tight at the top and widening below is concave, and you can see all of it. This distinction genuinely matters in poor visibility.
8. Practise by matching map to ground in good weather.Stand somewhere with a view, orient the map, and identify by eye each spur, valley, col and top in front of you. Twenty minutes of this teaches more than any amount of reading.
9. Use contours to check where you are.Ask what the ground should be doing if you are where you think you are — rising to the left, dropping away ahead, a stream on the right. If the ground disagrees with the map, believe the ground.

Tips

• Contour counting is a real technique: crossing four contour lines on a 10-metre map means you have climbed or dropped 40 metres, which is measurable on a watch altimeter or a phone barometer.
• The gap between contours combined with the map scale gives you a slope angle. On a 1:25,000 map, contours a millimetre apart is very steep indeed.
• Streams shown on the map start further downhill than the ground actually gets wet. Wet ground above a mapped stream is normal.
• In fog, contours are your primary information. Learn them well enough that you can navigate a featureless plateau on shape and bearing alone.

Common mistakes

• Assuming the contour interval without looking — A route judged on a 10-metre assumption when the map uses 5-metre contours doubles the ascent you think you are doing, and vice versa.
• Reading every V as a valley — The direction of the point is the whole distinction. Point uphill is a valley; point downhill is a spur. Getting it backwards will lead you off the wrong side of a hill.
• Trusting a path over the contours — Paths get rerouted, eroded and abandoned. The shape of the ground is the only thing on the map that is guaranteed still to be there.

Questions people ask