Reading and understanding a contour map.

1. The Basics.

It was proved beyond reasonable doubt some time ago that the World is round. Strangely, road maps still show the World as if it were completely flat - few, if any, indications are given as to the rise and fall of the ground which we are all aware of in the natural course of events.

This is because road maps are what are termed 'Planimetric Maps', showing the ground as if the onlooker were directly overhead. These maps are fine for driving, as all you need to know is which road to follow - gradients, valleys and other geographical features being largely irrelevant.

On foot it is quite a different story. You may be crossing ground which undulates gently, only to be suddenly confronted with a vertical ascent which is beyond your ability to surmount - and - though unlikely - it could run for miles in either direction! It follows that some method of knowing what sort of terrain lies in front of you is is beneficial - and in some cases essential.

Very often the closed ends of two 'U's' can be seen pointing toward each other. This is a sure indication of a pass between two areas of higher ground and, when thought about, makes perfect sense. If you started on either section of higher ground and walked downhill in the direction that the closed end of the 'U's' were pointing, you would be bound to end up at the pass (See Fig. 5).

It's interesting to note that some features cannot be accurately represented on a contour map. The classic is the Inaccessible Pinnacle (The InPin) in the Skye Cuillins range - being wider at the top than at the bottom, map-makers are stumped when it comes to indicating a contour! Similarly, severely overhanging cliffs are a cartographer's nightmare - but we'll avoid those anyway, eh?

2. Scale - what is it and which one is best?

In the UK, the Ordnance Survey is the primary source of maps, though others are available. One piece of terminology that needs to be understood from the start is small scale versus large scale. On a small-scale map, landscape features are shown relatively small, the map itself covering a much larger geographical area than a large-scale map where features are, of course, shown much larger.

For example a 1:250,000 scale map such as the OS Routemaster series, shows a tract of land 1 kilometre long covering a mere 4 mm. on the page. On an OS Pathfinder series 1:25,000 scale map, the same tract of land covers 4 cm. (For those of you who, like myself, grew up on miles and yards that means 2½ inches = 1 Mile.) It can be seen from this that the small-scale map isn't going to be of much use to walkers - it simply doesn't show enough detail. An easy way to remember what scale a map represents is - the bigger the number, the smaller the scale.

Ridge contours can be confused with valley contours as they, too, form 'U' shapes - the difference is that the closed end of the 'U' points to lower ground. (See Fig.3)

Streams, indicated by blue lines (as in Fig. 2), are a sure sign of a valley - though not all valleys have them! By the way, it can be a good idea to follow a stream down a broad, open valley if you're not sure where you are and wish to reach lower ground but even this can get you mired in boggy sections. Never, ever try this on steep slopes, or those heavily wooded. You can quickly find yourself trying to negotiate very steep, wet ground - water will always find the quickest way downhill!

To be really certain if a group of contour lines indicate a ridge or a valley, you need to look for the heights marked along the contour lines in order to determine in which direction the land slopes. Index contours (mentioned above) have the heights above mean (average) sea level printed on them, every fifth line being an index contour.

If the two nearest index contours are located nearest any given spot, then traced along until you come to the height (altitude), it will readily be seen which direction the land slopes at that point and the 'V' or 'U' can be identified as a ridge or valley (See Fig.4).

Once you have grasped the above principle, you can move on to the next one - how contour lines relate to valleys. Simply put, contour lines in valleys form 'V' shapes (typically narrowing and steep valleys), which can spread to wide 'U' shapes (typically broad, shallow valleys) - but the effect is the same - the closed end points to higher ground (see Fig. 2).

Contour maps show the ups and downs of the terrain by means of contour lines. A contour line represents the same height (or altitude) above sea level throughout its length. If one were drawn around a lake, it would exactly match the shoreline - water finds its own level, therefore the shore is always at the same altitude.

One of the easiest ways to understand contour lines is to imagine a perfectly conical hill - say like a child's drawing of a mountain. Seen from the side it looks like a perfect triangle, yet seen from the top it's a perfect circle. (See Fig.1, complete with cute little arrows.)

Each of the lines running around it are the contour lines and are at the same height above sea level. The height change from one line to the other is called the vertical interval or more commonly the contour interval. On any given map it is of equal value, usually 10 metres on the 1:50,000 and 1:25,000 scale maps used by most walkers.

That means that the gain (or loss) in height between any contour line on these maps is 10 metres. It follows from this that if lines are widely spaced, the gradient (how steep the hill is!) will be gentle - close together and it will be steep. Very closely gathered contour lines indicate that a cliff is present and are probably best avoided!

Lines that are multiples of 50 metres above sea level (called index contours) are slightly heavier and usually numbered (150, 200, 250 etc.), which helps you to figure out which way the gradient is going - up or down.

This is really the basic and first principle of understanding contour maps - the fact that concentric, unbroken contour lines form complete paths, more often than not irregular in shape, that describe the shape of hills and mountains at the height at which the contour is mapped.

Given that most mountains are anything but regular, the contour lines can be of the most tortuous shapes imaginable - yet a little practise will enable you to begin to 'see' in your mind the physical shape described by this ingenious system.

The OS maps most useful to walkers are the 1:50,000 scale Landranger and 1:25,000 Pathfinder (now being replaced with the Outdoor Leisure series). I find the Pathfinder most useful for the kind of walking I tend to do - i.e. no great distance! - but, if you plan to do lengthy linear walks, such as featured here, the Landranger, with its greater geographical coverage, may prove more useful, though at the expense of slightly less detail.

It should be noted that on these maps all contour lines are in red and there is also a wealth of other information present in the form of map symbols. These symbols represent many features found on the ground such as camp sites, public telephones, parking sites, mountain rescue posts(!) and, of course, footpaths.

The explanation ('legend') for these symbols can be found on every OS map, usually on the back cover and these help to bring the map to life. With a little practise you will be able to identify features that will aid your navigation - lines of electricity pylons and church spires always being a good bet - and the whole process of getting from A to B will become much easier and enjoyable.

One thing must be remembered about maps, though - they are only as accurate as when they were drawn or revised. Although major features - mountains, lakes and the like - don't tend to move around too much, things like hedges, minor roads and field boundaries can and do. Small trees become thick woods, ponds silt up - I think you get the picture.

Reading and understanding a contour map is the first and perhaps most essential part of the skill of navigation across country. The ability to interpret what is on the map and translate it into physical features around you - or vice versa - could be all you need to get around an afternoon's walk in the Cotswolds - but to identify where things are with precision you need to be able to understand grid references, which is what I'll go into next.

Grid references - how they work and how to read them.

How to choose a compass

Using map and compass together.