Contour lines are imaginary lines drawn on Ordnance Survey maps connecting points of equal elevation and are used to illustrate relief on a map. Contour lines show height of the ground above mean sea level (MSL) in metres. If you were to walk the path of a contour line in real life, you would remain at the same elevation the whole time, never travelling up and down. Thus a contour line with a 60 metres marking indicates that any place along that contour line is 60 metres above sea level. If the numbers associated with specific contour lines are increasing, the elevation of the terrain is also increasing. If on the other hand the numbers associated with the contour lines are decreasing, there is a decrease in elevation.

Image 1 shows how cartographers (map makers) managed to devise an ingenious way to show three-dimensional topography on a two dimensional flat surface with the use of contour lines.

Topographical maps and contour lines can present a major challenge to understand. These videos will help us understand what contour lines represent and how useful they are to interpret relief of the land.

Besides height above sea level contours also show the shape of the terrain. In fact these lines are very useful because they illustrate the shape of the land surface or its topography on the map. Such maps with contour lines on them are called topographical maps.

The change in elevation from one contour line to the next is always the same within the same map. We call the spacing between contour lines the contour interval. For example, if your map uses a 10 metre contour interval, you will see contour lines for every 10 metres of elevation – lines at 0, 10, 20, 30, 40 and so on. Different maps use different intervals depending on the topography. However contour lines are usually drawn at 10 metre intervals on a 1:50,000 scale map and at 5 metre interval on a 1:25,000 scale map.

To make topographical maps easier to read, every fifth contour line is darker and wider in colour. This is known as the index contour. To avoid the map being cluttered with numbers only index contours are labelled with height values.

To determine elevations, pay attention to the amount of space in between lines. Where contour lines are close together, elevation is changing rapidly in a short distance and the terrain is steep. Where contours are further apart elevation is changing slowly, indicating a gentler slope and when far apart they indicate flat terrain.

Contour lines give us a lot of information and show us the shape of the terrain. For example:

A rounded hill: may be showed as closed concentric contours.
A Cliff: if you see two or more contour lines converge until they appear as a single line, this represents a cliff.
Valleys and stream flow: Contour lines on the map also show how water will travel across the land. Contours that form a V can tell you which direction water will flow. The bottom of the V points in the upstream direction. You can also use contour lines to determine the direction the river is flowing as water always flows downhill.

There are other ways how height can be shown on maps besides the use of contours. These are:

Spot Heights: a spot height shows exact heights by a black dot with a number next to it.
Layer Shading: maps are sometimes shaded to show the height of land.
Triangulation pillars: are often found on hill tops. The height is marked on the map by a blue triangle with a dot in the middle. The height is marked alongside.

As we have seen by looking at contour lines we will be able to visualise what a hill or a valley will actually look like.

As we have seen in the video the pattern made by contours tells us a lot about the relief of the land. Relief means the height and shape of the land. The handout entitled Relief and Contour Lines shows some of these patterns. Now your task is to work out the worksheet entitled Matching Activity. The patterns shown by the contours on the worksheet represent different shapes of hills. Try to match each shape (cross-section) with the corresponding contour lines pattern.

Now you can work out the activities found in the worksheet entitled Contours WS available in the resources section.