Contour lines explained

Start with the key

Every line has a height, and every map chooses an interval.

Imagine a hillside reached by still water at a series of fixed levels. Each waterline would join ground at one height. Draw those lines from above and the result behaves like contours on a map.

The contour interval is the vertical difference from one line to the next, not the distance across the ground. Ordnance Survey states that many OS maps use a 10 metre interval, with 5 metres used in some flatter areas. The map key is authoritative for the sheet or digital style being read.

Slope from spacing

Close lines mean the same height change occurs over less distance.

When contours are close, the ground gains or loses each interval quickly, so the slope is steep. When they are spread out, the same vertical change takes place over more horizontal distance. Even spacing suggests a fairly constant slope. Spacing that tightens uphill suggests the slope steepens; spacing that opens uphill suggests it eases.

• Broad spacingGentler ground, though the surface may still be rough, wet or obstructed.
• Tight spacingSteep ground that may slow travel and increase the consequence of a slip.
• Lines almost mergingVery steep ground where another map symbol or route description should be checked for cliffs or rock.
• Uneven spacingA changing slope, with steeper and gentler bands along the same hillside.

Map scale matters. Measure the horizontal distance on the ground before turning height change into a gradient. A screen zoom does not change the source scale or add new terrain detail. How contour spacing reveals slope develops that calculation without treating line density as a complete terrain assessment.

Read the pattern

The direction of the V separates a valley from a spur.

Closed rings normally describe a hill when their values rise towards the centre. A valley cuts into higher ground and often carries a stream. Where contours form a V, Ordnance Survey's practical rule is direct: a V pointing uphill marks a valley, while a V pointing downhill marks a spur.

A ridge is an extended line of high ground. A col or saddle is the lower crossing between two higher areas, shown by contours pinching into an hourglass-like pattern. These shapes remain useful in poor visibility because they describe the ground rather than a temporary sign or building. The pattern is examined in more detail in how contours show ridges and valleys.

Put the line on the land

Count crossings and notice the angle of approach.

1. Trace

   Follow the route across the contours.

   A route crossing many lines climbs or descends. A route running between them stays closer to one height.

2. Count

   Use the interval to estimate height change.

   Check the start and finish heights as well as the number of crossings, especially where the line doubles back.

3. Picture

   Name the landforms along the route.

   Identify valleys, spurs, ridges, bowls and cols so the line becomes a sequence of terrain rather than a flat trace.

4. Verify

   Compare the profile with the map.

   The elevation profile shows height over distance. The contours explain which side of a valley or ridge produces that profile.

What remains unseen

Contours simplify the surface between intervals.

A small cliff, bank or hole can sit between two lines and still block progress. Contours do not state whether the ground is tarmac, bog, heather, loose rock, woodland or a maintained trail. They may describe average slope while hiding a short steep step.

Use contours with paths, access information, vegetation and rock symbols, current route notes and what is visible on the ground. A topographic map brings those selected layers together, but the straightest line across it may not be the easiest or safest route.

Contours show the shape of the land at the map's level of detail. They do not remove the need to read the rest of the map.