Navigational Skills 4 All about Contours
By Dave Roberts 
Navigational Skills 4 All about Contours
What is a contour line?
Contour lines denote the height of the land at any point along that line. So if you followed a 100m contour line around the hill, your altitude at every point would remain at 100m. That’s why traversing a hillside without loss or gain of height is known as contouring.
Depending on the maps you use, they can be at 5,10 or even 15m intervals. 5m contours are usually on maps that cover lower level areas, though the Clwydian Range explorer map is at 5m intervals. 10m is the norm on OS maps, while the strange 15m is found on Harvey maps. You’ll also have a thicker index contour at 50m on OS maps, or 75m on Harvey’s. They’re also numbered, though this can be a hit and miss affair with the numbering not always being where you need it where other have all the 10m contours numbered where others may have every other at 20, 30 and 10m intervals within 100m!
The example above is from Open Street map – but is rendered to resemble the contours on a 1:25,000 OS Explorer map.
What else are Contours good for?
Slope steepness.
Depending on how close the contours are to one another, you can determine the steepness of the slope.
The steep slope below Snowdon is exceptionally steep with the arrow distance dropping 350m (read – Cliff!) while the other arrow only falls 50m in the same distance.
Exceptionally steep ground may also feature crags on the map as well as contours being too close together to tell them apart.
Slope Type.
You can also cateogorise the slopes into three different types:
Convex Slopes. These start off shallow or gently, but steepen as you descend. The Killer Convex on the Llanberis Path of Snowdon is a perfect example.
The Llanberis path crosses a reasonably shallow area of slope, with the path often totally obscured in winter conditions. A slip here often proves fatal, as those unable to arrest their fall will continue to speed down the slope, which steepens and eventually deposits the poor soul over Clogwyn y Garnedd.
Concave Slopes conversely start off steep but peter out as you descend.
Some Slopes are just boring and uniform. Though a uniform slope can also be a shallow / gentle slope or steep.
Distinguishing High and Low ground?
Valleys and re-entrants easier to find as there’s often a stream on these and not running along top of a ridge! For the record, a re-entrant is simply an area of low ground, a valley on a smaller scale but tends to perpendicular to the ridge as opposed to the valley that’s parallel.
You can also determine the high ground from the direction that scree is drawn on the map – the large boulders are at the base of the slope, with debris drawn behind it. Crags are also drawn in such a way that once you see it, it’s obviously high ground.
Small Contour features such as ring contours.
Ring contours are usually small individual contour rings that indicate an area of elevated ground. They usually consist of no more than one ring, and the ones below show high points along a ridge.
The photo below shows the actual location – Bwlch Main on Yr Wyddfa, and what is one of the ring contours above.
Finally, spurs and re entrants can be tiny on the map, but much more obvious on the ground. A Spur is to a ridge as a re-entrant is to a valley (no, this isn’t a MENSA test) – or defined :
A spur is a lateral ridge or tongue of land descending from a hill, mountain or main crest of a ridge (Wikipedia).
Looking at the map below- from the col (which is a term for the low bit of land between two higher ones, or a pass) – you can see there’s a spur reaching down into the valley.
You can clearly see the spur in the image below as well as the valley to the right.
We could go on, but there are so many features that we could mention that there’s just no the space here. We could mention hanging valleys, aretes and so on, but it would soon turn into a GCSE Geography lesson. For an exhaustive list – Wikipedia as usual has a list of landforms.
Dave Roberts
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