Contour Lines and Elevation
Contour lines are how a flat sheet of paper tells you about a mountain. Once they click, a map stops being a diagram and starts being a landscape.
What a contour line is
A contour line joins points that are all at the same height above sea level. Every point along the 200 metre contour is exactly 200 metres up. Imagine slicing a hill horizontally, like cutting a loaf sideways, and drawing round the edge of each slice: that outline, seen from above, is a contour.
Because they are level, contour lines never cross each other, and a single line never splits. If two appear to merge, the ground there is vertical, which is drawn as a cliff symbol instead.
The contour interval
The contour interval is the height difference between one line and the next, and it is printed near the legend. Ten metres is common on walking maps; a map of flat farmland might use five, and a map of the Alps twenty or more.
You cannot read a map's steepness without knowing its interval. Contours packed two millimetres apart mean something very different on a five metre interval than on a fifty metre one. Every fifth line is usually drawn thicker and labelled with its height, which is called an index contour and exists so you can count heights quickly without following every line.
Spacing is steepness
This is the single most useful thing contours tell you. If lines are close together, the ground rises quickly over a short horizontal distance, so the slope is steep. If they are far apart, the same height change is spread out, so the slope is gentle. Evenly spaced lines mean a uniform slope; lines that bunch towards the top mean a slope that gets steeper as you climb.
This is why measuring distance on a map is only half of route planning. Two routes of identical length can differ by an hour if one crosses tightly packed contours and the other follows a valley floor.
Telling a valley from a ridge
Both make V or U shapes in the contours, and confusing them is the classic contour-reading error. The rule that settles it: in a valley, the V points uphill, towards higher ground. On a ridge, the V points downhill.
There is an easier check when a stream is drawn. Water runs down the middle of valleys, never along the top of ridges, so a blue line inside the V tells you immediately which one you are looking at. This is sometimes called the rule of Vs.
Worked example
On a map with a 10 m contour interval, you cross eight contour lines going uphill over 400 m of horizontal distance. How steep is the slope?
- 8 lines × 10 m = 80 m of ascent
- 80 m rise over 400 m distance
- 80 ÷ 400 = 0.2
A gradient of 0.2, which is 20 per cent, or about 1 in 5. Steep enough to be hard work, but walkable.
Reading contours at a glance
| What you see | What it means |
|---|---|
| Lines very close together | Steep ground, possibly too steep to walk |
| Lines far apart | Gentle or nearly flat ground |
| Closed rings getting smaller | A summit or high point |
| Closed rings with inward ticks | A depression, hollow or quarry |
| V pointing uphill | A valley, often with a stream in it |
| V pointing downhill | A ridge or spur |
| Two summits with a low gap between | A saddle or col, usually the easiest crossing |
| Lines touching or merging | A cliff or vertical face |
Check what you learned
Five questions on contours and elevation. Pass at 70% or better to complete the lesson.
Practice with Games
Ready to test what you learned? Jump into a game and put it into practice.
Contour lines, in one paragraph
A contour line joins points of equal height above sea level, so every point along the 200 metre contour is exactly 200 metres up. The height difference between one line and the next is the contour interval, usually printed near the legend and commonly 10 metres on walking maps. Because the interval is constant, the horizontal spacing of the lines tells you the gradient: lines close together mean steep ground, lines far apart mean gentle ground. Contour lines never cross, and where they appear to merge the ground is vertical and a cliff symbol is used instead. Closed rings mean a summit, or a depression if small ticks called hachures point inwards. A V shape pointing uphill is a valley, usually with a stream in it, while a V pointing downhill is a ridge, and the low gap between two summits is a saddle.
Common questions about contour lines
What do contour lines tell you?
They tell you the height of the ground and, through their spacing, how steep it is. Each line joins points of equal elevation, so following one keeps you at a constant height. Crossing many lines in a short distance means a steep climb.
Why do contour lines never cross?
Because a single point on the ground can only be at one height. Two contours crossing would mean that point is at two different elevations at once. Where the ground is genuinely vertical, the lines would coincide, so mapmakers replace them with a cliff symbol.
What is a contour interval?
The fixed height difference between one contour line and the next, stated near the map legend. A 10 metre interval means each line is 10 metres higher than the one below it. You cannot judge steepness from spacing without knowing the interval.
How do I tell a valley from a ridge on a map?
Look at which way the V in the contour lines points. In a valley the V points uphill, towards higher ground. On a ridge it points downhill. If a blue stream line runs through the middle of the V, it is definitely a valley, because water does not flow along ridge tops.
What is an index contour?
Every fifth contour line is usually drawn thicker and labelled with its height. That thicker labelled line is the index contour, and it exists so you can read heights quickly without counting every line from a known point.