This week we thought we'd take you through getting to know your compass.

Let's go through each part ...

1. This is the base plate - this is what the compass is mounted on and has rulers and scales on it to help you measure the distance. You will notice a black line running the length of the base plate - when taking your bearing this can be used to line up where you are and where you want to go to.
2. This is the compass housing - within the rotating bezel there is a magnetic needle (see point 3 for details). Around the bezel are marked the 360 degrees of a circle - these will be used for taking your bearing.
3. This is the compass needle. It is floating in a liquid, usually alcohol, so that it can freely rotate. The red end always points to magnetic north.
4. These are the orienting lines - they are fixed within the compass housing and can be aligned with the easting lines on your Ordnance Survey map to ensure accuracy when aligning the compass with grid north.
5. This is the orienting arrow. This is also fixed within the compass housing. When you come to take your bearing you will align the compass needle with this arrow.
6. This is the index line - on this compass it is a small black line within the compass housing bezel but on some compasses it may be a small arrow. This is where you will read your compass bearing from.
7. This is the direction of travel arrow - it does what it says on the box - this points in the direction that you want to travel in and what you will follow after you have taken your bearing.
8. These are the compass scales or romer scales. They can be used to help measure distance and accurately work out your six figure grid references.

Next week we'll move on to have a look at how you use a compass - but first we need to also understand the north points. If you have a look at your Ordnance Survey map - on the legend you will see a section called North Points. When it comes to compass navigation - this is an important section on your map.

There are three different norths. The first is true north - this is where the north pole is and for the purposes of navigation can be ignored. The second is grid north - this is the top of your map. It's the same for all Ordnance Survey maps which means that rather than being one single point it is a long line, as if the world were flat and all along the top is north. The third and final north is magnetic north. This is where your compass points to and is governed by the magnetic field of the earth which is always moving. When taking a bearing we need to allow for the difference between grid north and magnetic north and this is called magnetic variation or declination.

This is where you need the map legend. Under the north points section that we found earlier it will tell you how many degrees to the east or west magnetic north was at the time the map was printed and what the annual rate of change is. Something along the lines of:

Magnetic north is estimated at 2'22 and 2'14 west of grid north respectively for Jul 11. Annual change is approximately 09' east.

Rounding this up/down to the nearest whole degree - it means that magnetic north is 2 degrees to the west of grid north on this occasion. When we come to take our compass bearing next week - we will need to add these two degress onto our compass bearing so that we can align grid north and magnetic north.

So now you know the parts of your compass and the three different norths.