How to read Contours?
Contours are the lines on the walking maps joining points on the ground of equal height. They form one of the most important elements of understanding and reading a map when walking or trekking in the hills or mountains. Whether you intend to walk in the UK mountains or the French Alps, this article will help you to understand what the contours are telling you. This in turn will help you to locate your position and choose the correct walking path.This article will be of assitance for those walkers going on our self-guided walking holidays.
Why are they useful when walking?
Because, as a general rule, contours are features which change little over-time, they are therefore very reliable for navigation during walking, as opposed to paths, field boundaries or even buildings which can change. By studying them before you set out you'll also be able to ensure that you pick a walk that is not too steep or includes too much altitude gain for your level of fitness.
What do they show?
As we have already stated, a contour line is a line on the map joining points of equal height. The exact height of these lines are written on them. Spot heights often provide an added level of detail at specific points such as summits, cols or river crossings. The vertical interval is the height between each contour. The Ordnance Survey 1:50,000 and 1:25,000 maps have a 10 metre vertical interval as do the French IGN 1:25,000 walking maps which cover the mountain and upland areas such as the Alps, Pyrenees, Massif Central.. Other countries have different intervals.
How to recognise features in the landscape from the contours?
There are a number of features in the landscape which can be detected with the contours. Here are a few of the basic features:
Hills or mountains form closed circular, triangular, square (corresponding to the shape of the mountain) rings which decrease in size as you move towards the summit.
The steepness of a slope can be determined by the closeness of the contours. The closer they are, the steeper the slope. In terms of walking paths, a path is reasonably steep once it climbs more than 100m in 1km (ie there are more than 10 lines in a 1km square.) Of course, once the terrain becomes this steep, so the path may go back and forth to go up the slope more gradually.
Valleys are represented by V shaped lines with the point of the V sloping up stream.There exact form can vary enormously, as can those for ridges. Ridges in general take the opposite form to valleys with the V pointing downslope.
Cols or passes are low points between ridges and hills and form a point with no lines between two features. Other flat features will also have an absence of contours.
Reading contours is all about relating these contour features on the map to the real features on the ground (and vice versa). This can be done by evaluating the ground where you are walking, defining what type of feature it forms and then identifying it on the map. In good visibility you can also look at features beyond your immediate location and then identify them on the map. You can also do the inverse by reading the map before you set out to discover what landscape forms you'll be walking through. Once out walking or trekking you can asses how well you interpreted the map. The more you practice, the better you'll be able to read your walking maps.