NWP105A: Use maps, plans and drawings

Contour lines

What are they?

Contour lines on a map link points of equal height above sea level. Numbers are used to indicate the height. The numbers on the contour lines are at set intervals.

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  • Contour lines show us the shape of the land surface (topography) on a map. These pictures show the same island. On the left is a view from the side (cross profile view). On the right is a view from above or how you would see it on a map (map view).

    On the Contour lines are drawn as rings around areas of land that are the same height.

  • You can see the shape of the whole island in the map view by looking at the island's shoreline, but this map view does not show the height of the land. Click through the next few frames and watch the map view change. We can work out the height of parts of the island by seeing the changes to the map view if the sea was to start rising. Click next to continue.

  • Here, the sea level has risen a little bit and there is already a change in the shape of the island in the map view because parts of the land are now covered in water. Click next to continue.

  • Here the sea is a little higher again. The island actually looks like two islands now because some parts of the land are completely underwater. Click next to continue.

  • There is an even bigger change now as all but the highest point of the island can not be seen. Click next button.

  • If we replace the water levels with lines, you can see how contour lines work. Contour lines on a map will always go up and down by the same amount. On this map the lines go up by 20 metres each time. Click on the hotspots for more information.

    • These measurements show the height of the land above sea level. The lowest number here shows the height to be 20 metres above sea level.
    • The spot halfway between the contour lines will probably have a height halfway between the numbers eg 50 metres shown here.
  • Use your mouse to explore the contour lines on the map view on the right. Roll over the different contour lines and watch the figure on the left change height to reflect the changing contours of the island. The side view or cross section can give you an idea of the gradient or steepness of the slopes on a topographic map.

Please download the Adobe Flash Player from www.adobe.com to view this interaction, or use the text alternative instead.

Contour intervals

A contour interval is the difference in height between contour lines.

When contour lines are close together, it indicates a steep slope. When they are further apart it indicates a gentle slope.

A location that is found halfway between two contour lines will have a height that is halfway between the heights shown on the contour lines.

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Contour lines close together indicates a steep slope.

Contour lines far apart indicate a gentle slope.

The height halfway between contour lines is halfway between the heights shown on the nearest contour lines.

Please download the Adobe Flash Player from www.adobe.com to view this interaction, or use the text alternative instead.


Cross section of contour map showing the shape of the landform

Cross sections

You can use contour values to plot the shape of a landform on a graph. This is called a cross section.


Contour gradients

A gradient is the amount of a slope there is on the side of a valley, railway or road. It shows the ratio between the increase in height and the horizontal distance between two points and is worked out by dividing the vertical distance by the horizontal distance. A gradient can be written as a percentage or a ratio. On topographic maps it is usually written in the form of a ratio.

a triangle with a height of 10 meters and a length of 100 meters

In the diagram above, the contour gradient is 1 in 10 or 1:10.

Here is the same gradient in percentage form:

Gradient sign showing 10%