3rd Billericay Scout Troop

Magnetic Variation

To use a compass accurately you need to know about magnetic variation. In order to understand magnetic variation, you need to know all about the three norths.

The Three Norths

North is north. Right? - Well, not exactly. There are three different 'norths' we need to learn about for map reading. They are:

• True North

  True North is the 'top of the world', the point on the Earth's surface around which the world rotates. The lines of longitude on the globe all meet up at the point known as True North.

• Magnetic North

  Magnetic north is the point on the Earth's surface that a magnetic compass will point towards. It is not quite at the North pole, in fact it is located in northern Canada. To make matters worse, it moves around. The precise location of Magnetic North changes ever so slightly over the years.

• Grid North

  The maps we use have a grid of squares drawn over them. These are very important for giving map references. The direction the lines run in is called 'grid north.' Since these lines are parallel, it is impossible for them all to point north. If one of the grid lines goes straight through the north pole, then the grid line 1 km to the right must miss the north pole by 1 km, so it cannot be pointing to true north.

This is the British National Grid.
Note how the vertical 'North' grid lines are parallel. The red line does point directly to true north. Those to the right point a little to the east of true north. Those to the left point a little west of true north.

Note how the lines of longitude meet at the North Pole. They are not parallel. Note how they get closer and closer together the further north you go.

Compare these lines of longitude with the British National Grid. The longitude lines all point to True North. They get closer together further North.

For map reading, we really only care about Grid north (the north on our map) and magnetic north (the north on our compass). We want to be able to determine bearings using our map, and then follow them with the compass. Or we want to take a bearing using our compass and then plot it on the map. In either case, we need to be able to translate from Grid to Magnetic North, and vice versa.

All we need to know is the angular difference in degrees between the two Norths. This is called the Magnetic Variation. Once we know the variation for the current year and the local area it will suit us for the whole of our expedition. (We will not be walking far enough, or long enough for the variation to change significantly!)

There are a few ways to find the magnetic variation in your area. The easiest is to use this service at the British Geological Survey website. For our local area around Billericay, Magnetic north is approximately 3 degrees west of grid north.

A second option is to check your map. Most Ordnance Survey maps will show the magnetic variation. You need to read the information correctly, because it is only correct for the year the map was made. The map will give instructions to adjust to the present year.

The Third option, is to figure it out yourself using a landmark. If you can be certain of your current position, and can clearly make out a distinct landmark, you can work out the variation.
You simply take a bearing of the landmark, and compare with the bearing according to the map. The difference between the two is the magnetic variation. Simple!

Using the Magnetic variation

If we determined a bearing from our map, and then followed that exact bearing according to our compass we would be walking slightly in the wrong direction. In order to be precise we must compensate for the magnetic variation. We need to add or subtract the magnetic variation to our bearings.
We must remember when to add and when to subtract the magnetic variation. For our local area near Billericay, the variation is 3 degrees west. Magnetic north is 3 degrees west of Grid north.
Whenever we calculate a bearing from our map, we must ADD the three degrees to use it with the compass.
Whenever we take a bearing using the compass, we must SUBTRACT the three degrees to use the bearing on the map.

If we wanted to walk directly north according to our map, i.e. grid north. We would need to ADD three degrees. This illustration shows a compass set to travel north according to the map (grid north). Magnetic north is indicated by the green line. The bearing is set to three degrees. See how the magnetic bearing is three degrees to the west (left) of the bearing indicator which is now pointing (grid) north.