Magnetic variation
As we stated earlier the compass is a tool for converting directional information from the earth to the map and vise-versa. However, with this conversion there is an error. Below is an explanation of that error, information on how to find out what that error is and how to adjust for it.
Before we start there are 2 things you have to understand;
1) The grid lines on the map are NOT lines of Longitude. Lines of longitude converge at the North Pole. These grid lines are exactly 1km x 1km square. Each north-south grid line therefore points to a different place and not to the North Pole.
2) The magnet on the compass needle doesn’t point to the North Pole either but somewhere north of the Hudson Bay in Canada.
As our system of grid lines does not match up to the earth’s magnetic field, to convert from the bearing gained from the map to one that you follow on the ground you have to make an adjustment. This is the magnetic variation.
It gets more complicated. As each grid line points to a ‘different north’ but the needle on the compass always points to the same place, the variation is different depending on where in the UK you are. Figure 2 shows (in a vastly exaggerated form) that if you are in the South West the magnetic variation is smaller than if you are in the North East. Angle B is less than Angle A.
It gets worse still. Magnetic north is moving. Magnetic North is moving to the East and therefore the difference is declining. Back in 2004 it was 5° in Cumbria. Now in 2010 it is 2°.
The rule in most of the UK, at present, is that if you are taking a bearing from the map and going to follow that bearing on the ground you add the magnetic variation and if you are working from the ground to the map you subtract it. The easy way to remember this is that the map is a small thing and the earth is a big thing so you have to add (make the bearing bigger) when going from the small thing to the big thing. At present the variation in the South West of the UK is 0 and In time I expect this to be the reverse as Magnetic North passes across our grid lines.
So after all that explanation, In the UK at the moment the magnetic variation is so small that it is really not worth bothering with and there is the danger that you will actually make you bearing less accurate by allowing for it.
To find out what the magnetic variation is where you are you have to look at the notes on a map. It will say something like. Magnetic north (make sure you are reading the bit about Magnetic North and not True North) is estimated at 2°24' West of grid north at the centre of the sheet for July 2009. Annual change is approximately 10' East.
So if it is July 2009 you know that you have to add 2.5° and it is moving a degree every 6 years (there are 60' in a degree) so in 2015 you will add 1.5°. In reality you never really add half a degree as our compasses only have increments in 2°.