The Compass

The needle of a compass is magnetized and will point to anything metallic or magnetic.

The magnetic force is supplied by the molten iron in the earth's core. The red colored end of the needle (often also with letter N on it) will point towards the north pole of the earth's magnetic field.

To get the basic directions all that is needed is to line up the North on the compass dial with the north end of the compass needle and all of the other directions will line up as well.

degrees and bearings

The direction you have to travel to get to your destination is usually referred to as the bearing. Realizing that there are 360 degrees in a circle, compass bearings are referenced by the number of degrees they are away from north, in a clockwise direction.

For example, due East would be 1/4 the way around the circle, or 090 degrees. In turn, due South would be halfway around a circle or 180 degrees, due West equals 270 degrees, and due North would be 360 degrees or back to 000 degrees.

The numbers on the compass dial (bezel) are the bearings referred to above.

using the compass to good effect in the Lake district in white out conditions

finding your way to a destination (walking on a bearing)

In order to navigate to a destination, which is usually referred to as a bearing when using a compass (or a waypoint on a GPS)

you must know 2 things; A) Where am I now? and B) Where am I going?

Once you have established these, you place the long edge of the compass over the top of map so that the side of the compass is exactly following the route you wish to take. You must do this using the side of the compass that is parallel to the "direction of travel" line on the compass and is pointing the same way.

Once this is lined up you move the bezel (round housing containing the compass needle) so that the lines in the base of the bezel point to the grid north on the map. Whilst you do this you must take care not to move the compass base plate.

When this is done you can pick up your compass and after altering the magnetic variation (see below) you have the bearing readily to walk on.

This is often called the Silva - 1 - 2 - 3 system

magnetic variation and declination

Unfortunately, the north pole that the compass needle points towards is not the same one that Santa lives in, and in 1998, it was about 725 miles away.

To add to the confusion it is constantly moving! Depending on where you live on earth, this may result in a compass error (called magnetic variation)
[this is called declination in America and some parts of the world]

this magnetic variation can vary from a few degrees up to 30 degrees depending on where in the world you are.

The amount of error (magnetic variation) in your area is indicated by a series of arrows located in the margin of your map.

three "North's"

In the figure below, the three "north's" are shown.

In Britain, as a "VERY ROUGH" guide magnetic variation is 4 or 5 degrees to the West of true Grid North

since magnetic variation is constantly changing, this reading should not be relied upon, especially for older maps.

look at your map to establish the current variation.

here is an example of error caused by not using the magnetic variation

Lets assume your are in an area where the magnetic north is 5 degrees different to north, and you walk on a bearing without first compensating this for difference.

If you walk for one mile (1.61 km) and you do not compensate for this variation,
you will end up nearly 500 feet (152 meters) away from the target.

As you can realise, this amount of error should be compensated for.

We do this by adding the magnetic variation to the bearings taken from the map before we use them.

see more on errors caused by walking on a wrong bearing below

adding and taking the magnetic variation

If you take a bearing from the map and intend to walk on it using your compass
you must first add the magnetic variation.

If you take a bearing in a "field" situation using a compass and then want to use a map to identify or check the feature,
you must first subtract the magnetic variation.

there is a commonly used rhyme that some people use to assist in the memory of this
it is
"from grid to mag add
from mag to grid get rid"

either way you must get this the right way around or you will double your error caused by the original variation.