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Back to basics: Map reading - WillCycle
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Back to basics: Map reading

But  which  map  is  best?

Get together a random group of cyclists, then ask them which is best – paper maps, or digital maps – then sit back and watch the at times surprisingly heated discussion that follows. Cyclists are often reliant on mapping, or some form of navigation, and it seems everyone has their favourite. 

Map  and  compass

This post is not about which is best, but rather a deeper dive into paper mapping, and specifically, the dying art of using a map and compass. The first thing you need to know is that magnetic north (which is what a compass points at) and true north (north on a map) are not the same at all. The difference between those two, measured in degrees, is called magnetic declination. To complicate matters, magnetic north isn’t stationary point, but slowly wanders around over time. A good paper map will therefore tell you not only the magnetic declination for the area it covers, but also the date it was printed, and by how much the magnetic declination changes per year.

This means that, if you were navigating using a paper map printed in 2000, and a compass, you will need to find the magnetic declination the map lists for the area it covers, and add to that the annual change in magnetic declination, until you get to the current year. But that’s still not the end of it. Now you know the magnetic declination for where you are, you will either need to add it to the compass bearing, or deduct it from the compass bearing. Magnetic declination will either be negative, or positive.

The  lock  ring

If negative, use the phrase “Rid to grid” to help you remember to deduct the total magnetic declination, adjusted for the current year, from the compass reading. Better compasses have a rotatable, marked, lock ring, and you’ll find experienced navigators lock that ring to the local magnetic declination. Once they’ve done that, they’ve effectively calibrated the compass for the local conditions, and they can then simply read the calibrated bearings off the lock ring.A prismatic compass, with lockable ring, allowing the compass to be calibrated to local magnetic declination

In the UK, magnetic declination varies from around -1° 17′ in Cornwall, to around -1° 55′ in the north of Scotland. In Cape Town, South Africa, it’s -25° 34′, while in San Francisco, USA, it’s +13° 12′. 

Incidentally, there’s some fascinating research into the ability human beings have to tell direction. Effectively, we have a built-in compass, though some people seem to have forgotten how to use theirs.

Degrees, Grads  &  Mills

In case you didn’t know it, on most (but not all) compasses, direction is measured in degrees (for which we use the ° symbol)  and seconds (for which we use the ‘ symbol) away from magnetic north. Why don’t all compasses use degrees? There are other units of measure – these are mils and grads. Grads is a metric measure, which divides a compass face into 4 quarters, with 100 grads per quarter, instead of the 360 degrees of an imperial compass. 

Mills (short for mil-radians) are more complex, and is rarely used outside military forces. Basically, it’s based on Pi (3.1416, shortened). On the face of a compass, there are two Pi radians. Each radian is divided into 1000, giving a total of 6 283 mils, but in practice that’s usually rounded down to 6 200 mils. To to complicate matters further, some mil-graded compasses round it down to 6 000. Why use such a seemingly overly complex system? A mils-graded compass has additional benefits: it can help you determine the physical size of an object in the distance, and also be used to determine how far from something you are, but that’s well beyond what I want to cover here.

Ferrous  metals

Once you understand how to use a map and compass, there’s one other thing you need to remember: stay away from ferrous metals when taking a compass reading. If you require an accurate reading, you will want to be at least five metres away from any ferrous metal. Yes, that does mean removing your wrist-watch, if you’re wearing one. It is for this same reason that you should never waste your money on a “survival knife”, Leatherman copy, or similar, if it has a built-in compass. Any such items that are of good quality absolutely will not contain a compass.

The  map

Now you know the basics of measuring direction, and operating a compass, let’s look at the map itself. Paper maps are (obviously) of a fixed scale, and for hiking or cycling, you want to have at least a 1:25 000 scale map. That simply means every unit of measure on the map is 25 000 of those units of measure out in the world, regardless what unit of measure you used.

If full-blown off-road navigation though the wilderness isn’t quite your thing, why not do something far more enjoyable, and cycle the Somerset Circle? Simply click here to read all about this wonderful, almost 100 mile long weekend cycling getaway.


As you unfold your map, pay attention to how it has been folded – there’s a reason to the method used, as it allows you to be able to use the map without fully unfolding it (though paper maps do work best when fully unfolded). Next, you need to orient the map. This involves physically turning the map, so it matches visible landmarks around you. If that means the maps ends up being upside down, then either you move so it’s the right way up, or you use the map upside down.

Orient  the  map

Remember, often, you cannot use a map correctly if it isn’t aligned with the landscape, and with the compass. The top of a map is always north, so if you plan to be moving south, you will end up using the map upside down, as you want to be able to look up from the map at the landscape you’ll be travelling through.

Contour  lines

Maps have contour lines. Think of these as similar to water marks you might see around a lake, or reservoir, during a drought. Each contour line represents an increase in elevation. When contour lines are far apart, we’re looking at a gentle incline, when close together, a steep incline, and when on top of each other, we’re looking at a cliff. Once you understand how contour lines work, you can start figuring out the best route to get from where you are, to where you want to go, avoiding steep hills, cliffs, and similar obstacles.

For  the  rest  of  us

Having strong map-reading skills can be essential in some circumstances. For example, to Search & Rescue teams, it can mean the difference between life and death. For the rest of us, while a really useful skill to have, it isn’t essential. My advice is simple: learn how to navigate using a map and compass, but mostly rely on digital navigation. Get a decent GPS unit, or do what I did – switch to using your phone for navigation, with a hefty power bank as back-up, and with the power bank being charged by the power from the hub dynamo.

 

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