What Are Contour Lines?
Contour lines are lines on a map that join all points at the same height above sea level. They are drawn at regular height intervals (called the contour interval) — commonly every 5 m, 10 m, or 50 m, depending on the map scale.
If you walked along a contour line in real life, you would neither climb nor descend — you would stay at the same height.
Reading Contour Lines: Key Rules
- Closely spaced contours → steep slope
- Widely spaced contours → gentle slope or flat land
- Contours that form a circle → hill or peak (the innermost ring is highest)
- V-shaped contours pointing uphill → valley (river flows down through the V)
- V-shaped contours pointing downhill → ridge or spur
Contour lines never cross each other because no two different heights can exist at the same point on a map. If they did cross, a single location would have two different heights — impossible.
Cross Sections
A cross section (or profile) is a side view showing the shape of the land along a line. To construct one:
- Draw a line between two points on the map
- Mark where each contour line crosses the profile line
- Plot these heights on a graph
- Join the points to show the shape of the land
Cross sections are used by engineers planning roads, railways, and tunnels, and by walkers planning hill routes.
Relating Contours to Landforms
| Landform | How it appears on a contour map |
|---|---|
| Hill | Concentric closed circles, higher in the middle |
| Valley | V-shapes pointing uphill (up-slope direction) |
| Ridge/spur | V-shapes pointing downhill |
| Cliff | Contours very close together or overlapping |
| Flat land / flood plain | Very few or no contour lines |
Ordnance Survey Maps
In the UK, Ordnance Survey (OS) maps use contour lines at 10 m intervals on 1:25,000 maps (where 1 cm = 250 m). Heights in metres are printed on contour lines, and spot heights (isolated height points marked with a dot and number) show the height of individual peaks or other features.
To estimate gradient between two points: find the height difference (number of contours × interval) and the horizontal distance from the map scale. Gradient = rise ÷ run. A slope going from 100 m to 300 m over 4 km has a gradient of 200/4000 = 1 in 20 (or 5%).
Real world: HS2 railway tunnel through the Chiltern Hills
HS2, the UK's high-speed rail project, must pass through the Chiltern Hills in central England. Engineers used detailed contour maps (1:10,000 and 1:25,000) to assess gradients. Rail networks require gentle gradients (usually less than 1%) — steep hills must be tunnelled or excavated. The Chilterns tunnel is 16 km long, one of the longest in the UK. Without accurate contour analysis, engineers cannot plan the depth of the tunnel or the angle of approach. Every metre of unnecessary depth adds millions of pounds in excavation costs.
Real world: Avalanche risk mapping in the Alps
Swiss, Austrian, and French authorities use detailed contour maps to identify avalanche-prone slopes. Avalanches typically start on slopes between 30° and 45° — gradients that are visible on contour maps where lines are closely spaced. The entire Austrian Alps has been mapped for avalanche risk, with thousands of paths identified. Villages in narrow valleys (where contour V-shapes indicate river valleys surrounded by steep slopes) are most at risk. Protective barriers, deflection walls, and exclusion zones are all planned using contour analysis.
Real world: Cross sections and the Channel Tunnel
The Channel Tunnel (Eurotunnel), connecting England and France, required extensive geological cross-section analysis. Engineers studied cross-section profiles of the seabed between Dover and Calais to find the right rock layer (chalk marl) for tunnelling. The tunnel runs at depths of 40–75 m below the seabed. Geologists used boreholes, seismic data, and cross-section mapping to ensure the tunnel would follow a consistent depth through stable rock. The 51 km tunnel — the world's longest undersea tunnel — opened in 1994.
Where do you see this in real life? Next time you drive over a motorway bridge or through a hill tunnel, think about how engineers used contour maps to decide whether to cut through the hill or go over it. Road building follows the path of least resistance — the gentle gradients and flat valleys that appear on topographic maps as widely-spaced contour lines and flat flood plains.