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.

 

Inside a compass is a small magnet attached to a needle. Given that magnets attract each other, 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. (N.B. Since a compass is affected by magnetic fields, it needs to be used away from iron and steel objects, e.g. pocket knives and railway tracks)

 

Parts of a Compass

 

There are several kinds of compasses, but the one shown is the base plate type, which is the most suited walking or hiking.

• Compass Needle. This always points to the magnetic North Pole.

• Compass Housing. This can be turned separately 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 on the housing (but not shown on the drawing) will be numbers on a scale from 0 to 360, representing 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 it enables you to 'set' a map.

• Magnifying glass. Hope you know how to use that.

• Direction of travel arrow. If you set a bearing, then once aligned this arrow tells you 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.

How to take a bearing (and use it)

 

The first questions should be “why do I want to use a compass? and, 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 that we can see. (later on, we will use the compass with a map, which is a bit more useful). It also enables us to reach our destination even when the land is not flat or we cannot see the end point throughout the entire journey.

• 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 make this mistake at first.

• Point the compass base (direction of travel arrow) towards the object you wish to go to.

• Keep the compass pointing in the correct direction and turn the housing until the orienting arrow is 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 travelling, which would then mean the compass you set up so beautifully is now pointing to the wrong place!

• Follow 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

 

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.

Which North ?

 

Believe it or not there are three North Poles. True North, Magnetic North and Grid North. Grid North is only applicable to Great Britain (Note: Northern Ireland has a different grid north).

• 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 the Ordnance Survey’s maps of Great Britain point to.

• Magnetic North comes from the earth’s magnetic field and wanders 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 rule 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."

Alternatively an easier rhyme is

 

"Add for Mag, Get rid for Grid"

 

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.

The difference (declination) between magnetic North and Grid North is indicated on OS maps. Therefore, when taking a bearing from the outside world to use on a map, you should take this declination from the bearing you read on the compass and vice versa.

 

Magnetic North is estimated at 4o West of Grid North for 2004, decreasing by about ½o every Four Years. (i.e. in 2036 True North will be the same as Grid North)

 

Did you know?

• Every few tens of thousands of years the Earth’s 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!!

• Orienteering maps are actually drawn slightly differently to the OS Landranger and Explorer maps. The orienteering maps are drawn so that the grid lines are actually aligned with magnetic north, so that there is no need to adjust the compass. This is worth remembering when the Group does its orienteering days.

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