The number of degrees east or west that Magnetic North is from the True (Geographic) North.
Variation changes according to your position relative to the North Magnetic Pole. It also changes gradually in any one position due to the movement of the magnetic poles. Variation is shown on Mercator charts at the centre of the compass rose. Also shown is the rate of annual changes (increase or decrease), from the stated year.
Deviation is the number of degrees East or West that the compass North is from Magnetic North.
On board ship the magnetic compass is deviated by magnetic fields due to the proximity of steel and iron and electrical equipment.
The ship’s magnetic field can be subdivided into two basic components as follows:
- a) Permanent Magnetic Field
- b) Induced Magnetic Field
Compass to true conversion
For all navigation calculations, it is better to adopt a pattern and then stick to it. For example, when it is necessary to convert a compass to a true course, the following layout should be used.
The calculation from compass to true course and vice versa is as follows:
- a) Compass to True Add East Subtract West
- b) True to Compass Add West Subtract East
An easy way to remember the above is CADET.
The difference between the true course and compass course is the compass error. If the compass course is larger than the true course the error is ‘West’. If the compass course is smaller than the true course the error is ‘East’.
This can be remembered by the aide memoir:
Compass Best Error West. Compass Least Error East.