Choosing and using compass.

You may have seen in school science experiments the old trick of floating a magnetised needle on a bowl of water - slowly it swings around, supported by surface tension, until it points North/South. This is the simplest form of compass, aligning itself with the Earth's magnetic field. The next step up, still fairly primitive, is the type where the needle is suspended on a pivot, which allows it to swing freely, and is usually housed in a circular case. This is often marked only with the Cardinal Points - North, South, East and West. This kind of compass will let you determine directions only very roughly - for more precise navigation a more sophisticated device is needed.

The first thing your compass should enable you to do is to sight a landmark and take its bearing. A bearing is defined as 'the angle between a line heading North from your position and a line heading towards the landmark.'  (The North line can be defined as either 'True', 'Grid' or 'Magnetic' North - more of this later!)

There are 360 degrees in a complete circle, with zero degrees being conventionally placed at the 12 o' clock position (North).  When counting degrees you always start at this zero point and count clockwise around the dial. Therefore North = 0 degrees, East = 90 degrees, South = 180 degrees and West = 270 degrees. To get a visual idea of this and what a bearing signifies, see the three drawings below.

The second thing that your compass should enable you to do is to measure angles on a map. Again, the angle that you require will be the one between a North-running line and a line which runs through your position and a mapped landmark. This angle is usually known as a course, since it will normally be used to pick the course that you will travel.

In order to measure this angle, what's really needed is a protractor - a device for measuring angles. Fortunately, compass makers have handily combined the compass and protractor into the (surprise surprise!) protractor compass, more commonly called a baseplate compass.

This type of compass is fundamental to most navigation concerning map and compass across country. It generally has a clear rectangular baseplate, with the compass housing - almost always circular - at one end. This housing, known as the capsule, can be rotated in relation to the baseplate.

The outer rim of the capsule is marked with North, South, East and West and also with degrees - which increase as you move around the capsule clockwise. The capsule is usually marked with a series of parallel lines, called North/South lines, which lie parallel to a line running through the North and South points on the capsule.

Most baseplate compasses also have an arrow, usually called the direction of travel arrow, marked on the base. It's called this because it shows the way you want to go when the capsule is set to a course you've taken off the map.

If you want to measure a bearing in the field, this is the arrow you point at the landmark you've chosen. In line with this arrow is the index mark, scribed next to the capsule.

This line makes it easier to read off the number of degrees to which the capsule is set - the capsule is turned to indicate the correct number of degrees required.

This all sounds horribly complicated. Take a look at the sketch to the right to see the main components of the baseplate compass - things will become a bit clearer, hopefully!

When the compass is held level, with the needle free to rotate, said needle will come to rest with the North end (usually red) pointing to Magnetic North. A lot of map and compass operations include rotating the capsule until the North end of the needle aligns with the North mark on the capsule.

To make this lining-up easier - and more accurate - the capsule has an orienting arrow inscribed, again usually in red, on its bottom surface. Another name for this arrow is the gate. For the sake of my typing-numbed fingers, from now on I'll say 'put the needle in the gate' instead of 'align the North end of the compass needle with the orienting arrow, making sure that red lines with red.' OK? Right!

When used together, the three working parts - baseplate, capsule and needle - enable you to take the bearing of a landmark when out in the field and measure courses on a map - more of that later, though. For now, a few more points regarding compasses.

A refinement of the baseplate compass has a folding mirror attached to the baseplate. This allows you to sight both the landmark you're using to take a bearing and the capsule at the same time. This enables you to take a more accurate reading as well as startling yourself with the reflection of the wind-blown apparition staring back at you! Although they are initially a little tricky to use, if you need to take really accurate bearings, this is the kind of compass for you.

Now for the three kinds of North. Bet you thought there was only one North, eh? Wrong. There are three, namely True, Magnetic and Grid North. Wait! Don't run away in horror! All will be explained.

You see, compass needles hardly ever point to True North - they point to Magnetic North, the difference being known in the UK as the variation. In Britain this can be between two and five degrees, though it is only absolutely accurate for the year of publication on OS maps, which usually show a diagrammatic sketch of the three Norths in the margin and also give the increase (or decrease) in variation over the next few years so a calculation can readily be made of the variation as of the present moment. (Have a look at the sketch to see what I mean.)

It follows that map and compass work will involve some simple addition or subtraction when using a standard baseplate compass.

Why? Well, when you take a bearing from a landmark, you have measured an angle between Magnetic North, using the compass needle, and that landmark. A course taken from a map, on the other hand, measures the angle between Grid North and your landmark - hence the need for the arithmetic. More of that later!

If you can't be bothered with all that, you can buy a 'set-and-forget' compass. These have a gate that can be moved in relation to the North mark on the capsule. You simply set the gate at an angle to Grid North which is equal to the variation and that's it until you travel to a region where the variation, er, varies from where you now are.

It's much simpler than it sounds! When you have set this type of compass, the needle, when in the gate, points to Magnetic North whilst the North mark on the capsule indicates Grid North. No sums needed! This may sound gimmicky, but believe me, get out across open country on a cold day and the mind can become a bit sluggish without you realising it. It may not be life or death but you could face a much longer walk home than you'd intended if you forget to correct for variation......!

Do ensure that, whatever model compass you buy, it is fluid-damped. This means that the capsule is filled with fluid which damps the needle's movement, without which the needle spins and wobbles like a nervous breakdancer. Decent compasses will operate through a range of temperatures which most of you will be out in - if you are out in weather below -40 degrees or above +40 degrees centigrade, which is the range I mention, you really don't need my help - you need professional help, and quickly! (Or your name is Fiennes).

Do avoid buying a compass if you see an air bubble present in the capsule - it probably has a leak. Extremely high temperatures - the car's parcel shelf is a classic - can cause the fluid to expand to a degree where it bursts the capsule. Take heed from one who knows......

Also a good idea is to buy a compass in the region of the World you intend to use it. Why? Well, North of the Equator needles dip to the North, south of it they dip South. The manufacturers compensate for this by weighting the needle so that it can move freely when held flat. So if your great-aunt from Sydney sends you a compass to use here, chances are that it will bind and not work properly. You can always move Down Under, but that does seem a bit drastic!

Compasses are affected by other magnetic fields than the Earth's. Don't lean on a metal gate when using one, or stand under high-voltage power lines. Even small items - penknives, buckles etc. - can throw the needle out, so keep them well out of the way when using it.

Lastly, once you have learned the art of reading a compass, taking bearings and plotting courses and are confident of the absence of magnetic interference - trust it. It may only appear a simple tool - and so it is - but it is also a highly accurate precision instrument that you can - and some day you may have to - trust with your life.

Taking a bearing.

I mentioned earlier about 'taking a bearing' from a landmark. So what use is it? Well, a bearing enables you to do a very important thing: walk in a straight line! This may sound funny but, if you're out in open country, such as moorland, it's very easy to - yes! - lose your bearings.

Go on long enough and you might lose your marbles as well! Seriously, this is where that well-known phrase originates. It is a fact that trying to walk a straight line over any distance outdoors is well-nigh impossible without the aid of a compass. It's no good saying 'well it's midday, the sun's in the south so I'll take my bearing from that.'

As the day goes on, the sun's apparent movement will fool you but good. You'll end up on a remote heath and be devoured by bears. (OK so I'm given to exaggeration here.) Most people do tend to wander in circles when unguided, though. Whether this is due to one leg being  a bit stronger than the other, the terrain, or the phases of the moon doesn't matter - that's what happens. So by sighting a landmark, taking a bearing and walking the bearing, you will avoid all this grief and frustration. Here's how to do it -

Suppose you possess a standard baseplate compass. You're out on a flat, open heath and have decided from studying the map that you need to head West to get where you want to be. The map shows a hill with a lone tree on top over to the West. (This is a really good map, by the way!).

First, find which way North lies. Hold the compass flat with the orienting arrow turned so that it is in line with the direction of travel arrow. then turn yourself around slowly until the North (red) end of the needle is in the gate. Presto! You are now facing magnetic North! You will notice that West lies to your left. Have a look - can you see the tree? Yes!

Now to take the bearing.Turn to face the tree, this time ensuring that the direction of travel arrow (the big broad one etched into the top of the baseplate) is pointing at your landmark (the tree). The North end of the needle is still pointing North, but because you've turned is no longer in the gate. Turn the capsule until the needle rests in the gate, whilst still pointing the direction of travel arrow at the landmark.

You have now taken your bearing, which, if the tree is exactly to the West, is 270 degrees Magnetic. Remember, degrees are read clockwise from North, so West is 270, OK? The 'Magnetic' bit merely means that you have oriented your compass initially to Magnetic North, not Grid or True North. As you are only taking a reading from the compass, without relating to any bearings on a map, this causes no problem.

To walk the bearing, set off West and keep walking the way the direction of travel arrow is pointing when the needle is in the gate - you'll have to check more often than you think! 'Why?' I hear you say 'I can see the landmark anyway!' OK - suppose a mist rolls in, or it gets too dark to see very far, or the land dips or rises, or the Forestry Commission race up and chop the tree down. Then where are you? LOST! However, as long as you follow the bearing, you will reach the hill, pass the tree and gain the pub on the other side. Easy, isn't it? Well, maybe not quite that easy - try it on your local park a few times before venturing out on the Scottish plateaux in a Winter blizzard..... The sketches below will give you a better idea, I hope!

You will soon discover that in the real world you will need to take regular bearings in order to stay in a straight line - the land usually rises and dips too much for you to sight your final destination. The principle is the same, though - take sightings on landmarks that you can see are on your way of travel, and set and walk the bearing.

Here is where knowing how to read and interpret a contour map comes in - you can identify landmarks along the way from the map and then take your sightings from them. Walking in a straight line is fine - but what if there is something in the way?  If you can see across it, simply walk around and then follow your original bearing. If you can't, a different approach is needed. It could be a lake, quarry or just an impenetrable thicket.

Well, first thing to decide is which way you wish to go to skirt the obstacle. Let's say it's a lake, with a horrid quaking bog to the left. Don't know about you but I have a deep and abiding horror of quaking bogs - comes from walking on Dartmoor where they lie in wait for the unwary! So we'll go right, over the pleasant flowery meadow.

Make a 90 degree turn right (yes you can use your compass for this!) and start walking as straight a line as you can until you have gone far enough to pass the obstacle. Whilst you do this, count the number of double paces you take (every time your right foot hits the ground is one double pace). When you've gone far enough to clear the lake, turn 90 degrees to the left and walk a line until you have again cleared the obstacle (on this leg there is no need to count paces). Now turn 90 degrees left again and walk the same number of double paces as you did on the outward leg. If you now turn right, you should see your landmark again in front of you. Off you go! The sketch below will hopefully make this a bit clearer.

Reading and understanding a contour map.

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

How to use the map and compass together.

Now for the final stage - using map and compass together. Just click the link below to get there!

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