Contours and ground shape

The art of map-making lies in detailing as many features and information of the ground as possible whilst remaining legible at its given scale representation of the world.

On both the 1:25,000 and 1:50,000 scales of ordnance survey map there can seem a bewildering amount of symbols and lines to interpret when starting to get familiar with navigation outdoors. The thin brown lines sprawling all over your map are a hugely important feature to appreciate not only giving you your height above sea level in metres but enabling you to see in your minds eye what the shape of the land will be ahead of arriving there.

A contour line is a lot like an isobar on a weather map but instead of joining points of equal pressure a contour line joins points of equal height above sea level. Always check the map you are using to ascertain how your contours are calibrated, typically appearing every 10m on the Ordnance Survey mapping system.

The humble contour lines of your map are ever present and ready to convey masses of information to you once you get the knack of interpreting them. If a given ordnance survey map had no contour lines on it then you would assume that all the land it mapped was at the same height i.e the ground did not go higher or lower than 10m anywhere within the mapped location. This can be seen on large areas of map around some of our flatter coastal areas – the Norfolk Fenlands for example.

If on the other hand, your map shows many contours all squashed in close together you can assume that this particular area is anything but flat! One of the basics of contour interpretation is observing how many contour lines you cross over along your given journey leg AND how close together they are.

If you were able to follow a contour line from your map physically on the ground you would by definition be ‘contouring’ neither gaining or losing height and something that sheep do so regularly in hill country they actually wear trails into the hillside! Going around a steep hill in mountainous terrain may be far more efficient than going up and over it and so contouring is a useful tool for the hill walker to know about.

More often however these little brown lines will directly cross your intended route at 90 degrees meaning you are either gaining or losing height (10m for every line you pass remember). Look closely at your map and find the closest numbers in brown to your location. You should very easily be able to see whether the next contour you come to is of a higher or lower number and therefore tell you if it is up or down hill.

With practice and the more time you spend looking at maps you will soon get the hang of interpreting up or down hill – looking at any close by water feature will help as all water runs downhill and ends in a larger water feature so by ascertaining where you are along a stream, for example, can tell you whether you will be then travelling uphill to its source or downhill to a lake.

Short of actually getting outdoors with a map and starting to relate them to the landscape there aren’t many ways to improve your understanding of ground shape. Have a look at the following series of diagrams and work out what would be happening to the ground as you followed the route shown by the arrows. Be as specific as possible with your descriptions, one of the keys to accurate navigation is

Be as specific as possible with your descriptions, one of the keys to accurate navigation is careful and detailed observation of contour lines – their proximity to one another, their directions and the regularity they appear over a given distance on the map.

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Adam Logan.

 

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