How to read contour lines before you leave home

A good walk often starts at the kitchen table, with a paper map spread out. Half an hour spent reading the contour lines tells you where the route climbs, where it gets steep and where you could turn back. You need a topographic map at 1:25,000 or 1:50,000, a pencil and a little patience.

What a contour line is

A contour line joins points of the same height above sea level. Imagine slicing a hill horizontally every 10 metres and drawing each slice's outline from above.

Two numbers on the map help you read them:

• The contour interval is the height between one line and the next. It is printed in the map's legend or margin. Common intervals are 5, 10 or 20 metres, and it can differ between map series and even between sheets of the same series. Check it every time you use a new map.
• Index contours are drawn thicker, usually every fifth line, and carry a height label. The numbers are printed so that their tops face uphill, which is a quick way to tell which way the ground rises.

Spacing tells you steepness

The closer the lines, the steeper the ground. Lines bunched tight together mean a steep slope; lines spread wide apart mean gentle ground. Where they merge or a cliff symbol appears, expect crags.

Read the spacing along your route, not just at the summit. A path that crosses tightly packed lines head-on climbs hard; one that runs along them, between two lines, stays almost level.

Watch for spacing that changes on a slope. Lines close together at the top and spreading out lower down describe a concave slope, steep above and easing below, where you can usually see the ground ahead of you. Lines spread out at the top and bunching lower down describe a convex slope: it starts gently and steepens as you descend, and the steep part may be hidden from you until you are on it. In mist, that is the slope to treat with care.

Shapes tell you the landform

Contour lines make a handful of shapes again and again.

• Summit. Closed rings getting smaller, with a height or a spot height in the middle.
• Ridge. Long, narrow loops stretched along a line, with steeper sides falling away on each side.
• Spur. A tongue of high ground sticking out from a hill. The contours form a V or U that points downhill.
• Valley or gully. The opposite: contours form a V that points uphill, often with a stream drawn in the middle. A sharp V is a narrow gully; a broad U is an open valley.
• Col or saddle. The low point on a ridge between two higher points, where two sets of rings face each other like an hourglass.
• Flat ground. Very few lines over a wide area, often boggy in the hills.

The spur and the valley are easy to mix up. Use the height labels or the streams: water always runs down valleys, so a blue line in the middle of a V means the V points uphill.

Working out the climb

To estimate the height you will gain, follow the route with a pencil and count the contour lines you cross going uphill, then multiply by the interval. Index contours make this quicker: count the thick lines and the thin ones in between. Do it for each climb separately, because a route that goes up and down three times adds up to more ascent than the height of its highest point.

To turn that into time, walkers in Britain have long used Naismith's rule: allow about one hour for every 5 km of distance, and add about one hour for every 600 m of ascent. It is a starting point for a fit walker on good paths with a light pack. Add time for rough ground, a heavy pack, bad weather, stops and steep descents.

Planning the day with contours

Once you can read the ground, use it.

Find your handrails. A handrail is a long feature you can follow along your route: a wall, a stream, a ridge crest, the edge of a forest. In poor visibility they make navigating far simpler.

Find your catching features. Something that tells you that you have gone too far, such as a stream across your path or a road. Note one beyond each key turn.

Mark the steep and the uncertain. Circle the parts of the route where the lines bunch up, where the path fades on the map, or where a convex slope hides what is below.

Plan your ways off. Look for gentle spurs and valleys that lead down to roads or villages from different points of the route. If the weather turns, you already know where to go.

Look for a camp. If you are staying out, look for flat ground near water but above it, out of the bottom of narrow valleys where cold air settles at night, and sheltered from the direction the wind is forecast to come from.

Practise where it is easy

Take the map on a walk you already know. Stop now and then and match what you see to the lines: the spur you are standing on, the valley to your left, the col ahead. After a few walks, the lines start to look like hills.

Before any trip, check the forecast and your route before you go, tell someone your plan, and carry the map and compass as well as your phone.