Close contour lines mean a steep slope, and wide spacing means a gentle slope. To put a number on it, turn the spacing into ground distance and divide the height gain by that distance.
This lesson belongs to contours, height and cross-sections. Later lessons use this skill, for example drawing a cross-section.
How does spacing show steepness?
Between two neighbouring contours the height changes by the contour interval. If they are close, that change happens over a short distance, which is steep.
If they are far apart, the same change is spread over a long distance, which is gentle. The heights on the lines tell you how high the land is, not how steep.
Worked example: two slopes compared
This is a worked example. A map has a scale of 1:25 000 and a contour interval of 20 m. On slope X, neighbouring contours are 0.8 cm apart. On slope Y they are 3.2 cm apart.
Step 1: convert map distance to ground distance.
- X: 0.8 cm × 25 000 = 20 000 cm = 200 m.
- Y: 3.2 cm × 25 000 = 80 000 cm = 800 m.
Step 2: find the gradient (rise ÷ run).
- X: 20 ÷ 200 = 0.1, which is 1 in 10.
- Y: 20 ÷ 800 = 0.025, which is 1 in 40.
Step 3: write the comparison. “Slope X is steeper than slope Y, because it rises 20 m in 200 m, whereas Y takes 800 m to rise the same 20 m. X is four times steeper (800 ÷ 200 = 4).”
The mistake: close lines mean high land
A common belief is: “These contours are close together, so it is high.” This mixes two things.
| Clue | What it tells you | Where to read it |
|---|---|---|
| Numbers on the lines | Height | The labels |
| Spacing between lines | Steepness | The distance between lines |
A low hillside can have tight lines if it is steep, and a high plateau can have wide spacing if it is flat. The fix is to read the numbers for height and the spacing for slope.
Check yourself
On a map with scale 1:50 000 and a contour interval of 10 m, two neighbouring contours are 1.5 cm apart. Calculate the gradient.
Answer
Ground distance: 1.5 × 50 000 = 75 000 cm = 750 m.
Gradient: 10 ÷ 750 = 0.0133, which is about 1 in 75.
Because the distance is large compared with the rise, this is a gentle slope.
What to study next
Next, learn what contour shapes tell you in distinguishing a valley from a ridge. Test the whole cluster with the contours practice set.
The contour profile and slope reasoning trainer gives extra drills. For a teacher to watch your measuring, see online one-to-one Geography tuition.