A contour line joins points of the same height above sea level. To read relief, find the contour interval, count the lines between two points, and compare how close together the lines are. Contour work appears in map-skills questions on relief, slope and landforms.
This lesson sits in scale, distance and relief and feeds directly into calculating a gradient.
What do the lines tell you?
Every line has one height. Moving from one line to the next always changes the height by the contour interval, which is printed in the key.
Lines close together mean a steep slope, because the ground rises a lot in a short distance. Lines far apart mean a gentle slope. Every point on a single line is at the same height, so walking along a contour does not go up or down.
How do you find relative height, step by step?
- Read the contour interval from the key.
- Find the labelled lines (often every fifth line is labelled, or spot heights are given) and work out each unlabelled line from them.
- Name the height of each point, using the line it sits on or an estimate between two lines.
- Subtract the lower from the higher to get the relative height.
Worked example
This map is invented for practice. The contour interval is 10 m.
Point A sits on the line labelled 70 m. Point B sits on the line two lines below the one labelled 60 m, so B is on 40 m. Point C lies halfway between the 50 m and 60 m lines.
Height of B: one line below 60 m is 50 m, two lines below is 40 m.
A compared with B: 70 − 40 = 30 m, so A is 30 m higher than B.
Height of C: halfway between 50 m and 60 m is about 55 m, as an estimate.
Slope: between A and the next line down, the lines are 0.5 cm apart. Between B and the line above it, they are 2 cm apart. The slope near A is steeper, because the same 10 m height change happens over a shorter map distance.
The mistake to watch for
Mistaken answer: the contour interval is 10 m, and the lines 70, 60, 50 and 40 make 4 lines, so A is 40 m higher.
The student counted the lines rather than the gaps.
If A is on 70 m and B is on 40 m, only the lines at 60 and 50 lie between them, and there are three gaps from 70 down to 40. The correct difference is 3 × 10 = 30 m.
Correct this by writing the height beside each line you pass, then subtracting the two end values.
Check yourself
1. The interval is 10 m. Point P is on the 120 m line and point Q is on the 90 m line. Which is higher and by how much?
Show answer
P is higher. 120 − 90 = 30 m.
2. Along one side of a hill the lines are 0.4 cm apart. On the other side they are 1.5 cm apart. Which side is steeper?
Show answer
The side with lines 0.4 cm apart is steeper. Closer lines mean the same height change over a shorter distance.
3. A point lies halfway between the 60 m and 70 m lines. Give its height.
Show answer
About 65 m, as an estimate.
Where this leads next
Now put numbers on slope in calculating a gradient with compatible units. The statistics and distribution explorer and the map scale, contour and gradient practice tool help you rehearse the reading.
If you keep miscounting lines or mixing up steep and gentle slopes, a teacher in online one-to-one Geography tuition can check your reading on real maps.