A flat map can show the shape of the land in 3-D with contour lines. Each contour line joins points that are the same height above sea level. Walk along one and you never go up or down.
- The contour interval is the change in height from one line to the next. It is printed on the map.
- Index contours are darker lines with their elevation written on them, often every fifth line.
- Lines close together mean a steep slope; lines far apart mean a gentle one.
- A set of closed loops is a hill; the smallest loop is the top. Loops with little ticks pointing inward mark a hollow.
- Where a stream crosses contour lines, the lines bend into V's that point upstream.
Key idea
Elevation of a point = the value of its contour line. Count lines between two points and multiply by the interval to find how much higher one is. Close lines mean steep ground.
Why it works
Picture slicing a hill with flat sheets of glass every 20 m and tracing each slice onto paper from above. Where the hill is steep, you climb 20 m in a short sideways step, so the tracings land close together. On a gentle slope you must walk far to gain 20 m, so they spread out.
Worked example: Reading heights
Index contours are labeled 300 m and 400 m, with 4 thinner lines between them. Point P is on the second line above the 300 m index. What is its elevation?
- From 300 to 400 there are 5 steps (4 thin lines make 5 gaps).
- Interval: $100 \div 5 = 20$ m.
- Two lines above 300 m: $300 + 2 \times 20 = 340$ m.
Answer: 340 m
Worked example: How high is the top?
The highest closed loop on a hill is 460 m and the interval is 20 m. What can you say about the summit?
- The summit is above the 460 m line, since that loop surrounds it.
- It is below 480 m, or there would be a 480 m loop.
- So it is at least 460 m and less than 480 m.
Answer: Between 460 m and 480 m
Common mistake
Reading closely spaced lines as flat or low ground. It is the opposite: crowded lines mean the land rises fast, like a cliff. Another slip is taking a contour line for a road or a trail. Check: every contour line has a single height, and two lines never cross.