Géographie

Contour Lines: Reading Height and Steepness From a Flat Map

A contour line joins points of the same height. Close lines mean a steep slope, wide ones a gentle one, and here is how to estimate a height between two.

Contour line

A contour line is a line on a map that joins all the points which are at the same height above sea level.

Où les élèves le rencontrent: Grade 6 geography, and again in every map-skills paper, where candidates are asked to estimate a height, describe a slope, or match a cross-section to a set of contours.

La réponse en bref
A contour line is a line on a map joining all the points that are the same height above sea level. Because every point on one line is at the same altitude, contour lines never cross. Their spacing carries the information: lines drawn close together mean a steep slope, lines far apart a gentle one.

Un exemple

Contour interval 10 m. A point halfway between the 150 m and 160 m lines is about 155 m.

The only honest answer is a range with a sensible estimate inside it: the point is above 150 m and below 160 m, and if it sits roughly midway across the gap, 155 m is the figure to give. Nothing on the map states its height exactly, and nothing needs to. What the question is testing is whether you found the interval in the key, identified which two lines you were between, and interpolated rather than guessed.

Find the interval before you do anything else

The contour interval is the vertical distance between one line and the next, and it is stated in the key. It might be 5 metres on a detailed walking map, 10 metres on a general one, or 100 metres in an atlas — and every height you read depends on knowing which. Two maps of the same hill drawn at different intervals look like two different landscapes.

Every fifth line is usually drawn thicker and carries a printed number; these are index contours, and they let you count in fives rather than following each line individually. To work out which way the ground rises, find two numbers and see which direction they increase in, or look for the spot heights and summit markers printed nearby.

What the patterns mean

Contours describe shape as well as height, and a handful of patterns cover most of what a map-skills question will show you. The key ones are worth learning as pictures rather than as rules.

The most useful of them is the V. Where a contour crosses a stream valley it bends into a V, and the point of that V faces upstream — uphill, towards the source. Read a few of them and you can tell which way a river flows without any arrow being printed on the map.

Concentric closed loops: a hill, with the smallest innermost loop at the summit
Closed loops with short ticks pointing inwards: a hollow or depression rather than a hill
V-shapes pointing uphill: a valley, with the V pointing upstream
Rounded noses pointing downhill: a spur of high ground reaching out from a hillside
Evenly spaced lines: a uniform slope. Widely spaced at the top, tight at the bottom: a concave slope
Lines merging into one: a cliff, where the ground is effectively vertical

Spacing is gradient, and gradient decides what you can build

Because the vertical gap between lines is fixed, the horizontal gap between them is a direct measure of steepness. Lines packed tightly together mean a large rise over a short distance. Lines spread widely apart mean the same rise takes a long walk to achieve.

That is why contours answer questions that have nothing to do with height. Where would a road be cheapest to build? Where is a site flat enough for an airfield or a reservoir? Which route up this hill is the gentle one? All of them are read off the spacing, not off the numbers.

Questions fréquentes

Can contour lines cross each other?

No. A single point of ground has one height, so it cannot lie on two different contours at once. The only case that comes close is a vertical cliff or an overhang, where the lines are drawn touching or merged into a single thick line, and a cliff symbol is normally added as well.

What does it mean when contour lines are very close together?

The ground is steep. The height gain between the two lines is fixed by the contour interval, so squeezing that gain into a short horizontal distance is the definition of a steep slope. Lines that almost touch usually indicate a crag, and it is the pattern walkers look for first.

How do I find the highest point on a map?

Look for the smallest closed loop in a set of concentric rings, then read the highest labelled contour around it. The true summit is somewhere above that line but below the next one that would have been drawn — unless a spot height or a triangulation pillar is printed there, which gives the exact figure.

What is the difference between a contour line and a spot height?

A contour is a line joining many points of equal height; a spot height is a single dot with a number beside it, giving the surveyed height of that one place. Spot heights fill the gaps between contours, particularly on summits and along roads where the contour lines alone would leave the height uncertain.

Dernière mise à jour 21 août 2026