The Earth's magnetic field is generated within the core and extends beyond the surface of the planet.
The Earths' field interacts with the native magnetic field of a bar magnet to set up forces; these forces will cause the bar magnet to swing and finally line up North to South along the magnetic meridian.
The difference between true North and magnetic North is the angle of variation. This angle changes with position on the surface of the Earth and also varies with time. The compass rose on a chart gives information about the value of magnetic variation when the chart was set up and also the rate at which this value changes. If the chart is a few years old then you can calculate the new value of the variation.
The diagrams show how magnetic variation in the Bristol Channel has changed over four decades.
The 2004 value is quoted as 03 45' W (9'E) The value in brackets is the annual rate of change. By 2009 the variation will have changed by 5* 9'E = 45' E. The variation would then become 03 45'W - 00 45'E = 03 00' W
In order to get a true course (T) from the compass, subtract the west magnetic variation from the compass heading (M).
True course = Magnetic course - variation West
If variation is East, than add to the magnetic heading to get true.
In order to get a magnetic course (M) from the chart, add the west magnetic variation to the true heading (T)
Magnetic course = true course + variation West
The magnetic field of the boat itself may need to be taken into account by 'swinging the compass'.The result is the deviation table. It gives the correction for deviation which applies to each compass heading.
To get a True heading from the compass
(i) Convert from compass course (C) to magnetic (M) by using the deviation table: this is achieved by subtraction of deviation West or addition of deviation East
(ii) Convert from magnetic (M) to true (T) by subtraction of variation West.
To get a compass course from True
(i) Convert from true (T) to magnetic (M) by addition of variation West
(ii) Using the magnetic course (M) as a reasonable indicator of the ship heading, look up the deviation value. Add deviation West or subtract deviation East to get the compass course (C)