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What are Contours?

They are brown lines drawn on Ordnance Survey, Harveys and other similar maps to represent points of equal height at regular vertical intervals to give a 2 dimensional picture of what you see on the ground based on aerial photographs.

profile of a hill and its representation on a 1:25,000 Outdoor Leisure map
diagram 1
This shows the profile of a hill represented on a 1:25,000 Outdoor Leisure map using contours. Notice the different shapes (solid and dashed black lines on the profile), both with the same contour pattern. You need to bear this in mind when on a walk as a peak nearly the same height as a 4-storey building (10mtrs, 33ft) being less than 1 contour interval may not show on the map. The same applies for one nearly as high as a 2-storey house on a 1:25,000 Explorer map with 5mtr contour intervals.

This particular hill has 2 peaks with a total climb of ranging from just over 20mtrs (66ft) to 45mtrs (147ft) if you follow the route along the line A - B in the diagram.
alternatve profile of a hill and its representation on a 1:25,000 Outdoor Leisure map
diagram 2
But now look at diagram 2 with identical contours but with a very different profile, your route from A to B now starts with a descent rather than a a climb and the total height climbed to reach B could be up to just over 40 mtrs (131 ft). Using the analogy of building height that is 4 storeys or floors less of the building to climb.

If you’re not sure whether the slope of a hill is up or down look for the height figures as they will always be the right way up for going up hill, as shown by the fact that they read upside down in the diagram.

Having determined how much climbing there is likely to be on your route you also need to note how steep your climbs are. This is indicated by the spacing of the contour lines, the closer they are together the steeper the slope. So for example referring to diagram 2, if you wanted to head north from point C you would have a much steeper and hence more energetic climb than continuing on the original route to point B.
representation of hill features on a 1:25,000 Outdoor Leisure map
diagram 3
What else can we determine from the contours?
Taking a look at diagram 3 notice that the added detail of the contours in diagram 1 now includes:
  • A greyed area above point C, this indicates an escarpment with a steep side and rocky outcrops - so referring back to the previous paragraph above, this route is not only much steeper, it is also very likely to include a sheer rock face so probably best avoided as a walking route.
  • The indents along the line from D down to E tells us it is a small valley or gully (the protrusions of a small ridge or spur in diagram 2).
  • Conversely the protrusions along the line F - G indicate a small ridge (the indents of a small gully in diagram 2). Whilst not the most prominent ridge in diagram 3 it has been highlighted to emphasise the potential confusion between a ridge or spur and a valley or gully. The most prominent one being just to the left of point A.
  • Point H is a ‘col’ or ‘saddle’ - a small ridge between 2 high points or peaks.

Care should be taken when interpreting the various contour features as some of them can be easily confused as shown by the valley and ridge examples above. This is by no means an exhaustive list of the features and aspects that can be read by interpreting the contours on a map and competence in this navigational skill will only come with practice. Ideally this would, at least initially be with the help of a friend or walking colleague who is already proficient in these skills.