This information is aimed at helping the competitors on Chasetrek to obtain a basic understanding of how a compass works and how to use it. As a consequence it uses the type of compass that is most suited to the event. This document can be freely copied by any none profit organisation, providing that the source "Tony Field of Chasetrek" is quoted.
This tutorial forms part of a set of information, which includes, some basics of map reading, how to use a map and compass together, some tips on how to avoid getting lost and finally some online questions.
How does a compass work?
Inside the earth is a big ball of Iron, which produces a magnetic field. The lines of the magnetic field all point in one direction, as shown on the diagram.
Inside a compass is a small magnet attached to a needle. I am sure you remember that magnets attract each other and in the case of the compass, the magnet of the needle will be attracted to the Magnet produced by the Earth.
Since the magnetic lines always point in one direction, the compass needle will also point in one direction. Thus we can know where the North pole is.
Since a compass is affected by magnetic fields, it needs to be used away from such items as iron and steel objects. These include: pocket knives and railway tracks
Other reference material: Magnetism
Parts of a Compass
There are several kinds of campasses, however I will only show the parts on one type. The type shown has a base plate and this is most suitable for orienteering work of the type used on Chasetrek.
Compass Needle: This always point to the magnetic North Pole.
Compass Housing: This can be turned from the base of the unit. On the housing will be marked the letters N, S, E and W for North, South, East and West. Also but not shown on the drawing will be numbers. This is a scale from 0 to 360 and represents the degrees around a circle.
Orienting Arrow: This is really part of the housing and turns with the housing. Along with the lines of the base of the compass they enable you to to 'set' a map, but more of that later.
Magnifying glass: Hope you know how to use that.
Direction of travel arrow: If you set a bearing, then once aligned this arrow says which way to walk.
Scales: In the above picture the scales are shown on the front of the compass, however there are often more scales down the sides and these enable you to take measurements from maps of the distance between two points. As you will see when we get to using a map and compass together, we can measure the distance between to places on the map and using the scale, work out how far between those points.
How to take a bearing and use it.
The first question should be why do I want to use a compass? What can it do for me?
Having understood that the compass needle always points in the same direction, we can use this to help us travel to any point which we can see. (later on, we will use the compass with a map, which is a bit more useful). We could use this technique when the land is not flat and we would therefore loose sight of the end object when we go down the valleys.
Firstly always hold the compass flat, otherwise the needle will touch the bottom or top of the housing and then it won't always point to the North. Seems silly to mention this, but you would be surprised at how many people when they are first using a compass don't do it.
Point the compass base (direction of travel arrow) to the object you wish to go to.
Keeping the compass pointing in the correct direction and flat turn the housing until the orienting arrow in underneath the RED end of the Needle
Look at the number on the compass housing where it touches the direction of travel arrow. Remember the number, which is the bearing to your destination. You need to remember the number since you could knock the compass housing while traveling which would then mean the compass you set up so beautifully is now pointing to the wrong place!
Following the direction of travel arrow, keeping the RED end of the needle over the orienting arrow. If you look at the compass say every 50 - 100 meters, it should get you to your target. Don't watch your compass all the while, if you don't want to fall over an obstacle.
How to set a bearing (or go in a particular direction)
Suppose you want to go West from your present position. Looking at the compass housing we can see the 'W' symbol.
We now turn the compass housing until the 'W' is over the top of the direction arrow.
Turn the compass base (keeping it flat) until the RED end of the needle is over the top of the orienting arrow.
As above, keeping the compass flat, follow the direction arrow.
Believe it or not there are three North Poles. True North, Magnetic North and Grid North. (I think that this latter one is only applicable to the UK). True North is the Geographical North Pole i.e. the one at the 'top of the world'. Grid North is where all the grid lines of a UK map point to. Magnetic North comes from the earths magnetic field and wonders around a bit.
When using a compass it will indicate magnetic North. When you need to transfer a bearing from the compass to the map, you need to alter the bearing (number of degrees) to compensate for the difference. The little rule when in the UK to remember how to alter the number is
"When going from small to large, add the difference. When going from large to small subtract i.e. make the number smaller." Therefore when going from a map (which is small) to the outside world (which is large) add on the difference to make the bearing bigger. When going from the outside world to the map, subtract the difference.
However after all that, the difference between magnetic North and Grid North (declination) on Cannock Chase is three degrees. Therefore, when taking a bearing from the outside world to use on a map, you should take away 3 degrees from the bearing you read on the compass and vice versa.
Other reference material: FAQ on Magnetic Declination. This is a detailed explanation for those who feel they need it!
Did you know?
The current North Pole of the Earth is actually a South pole. Every few tens of thousands of years the Earths magnetic field flips. However since the compass you are using has been made more recently than the last flip, you can confidently use you compass knowing that the Red end of the needle points to the North Pole of the Earth!!