It should be called magnetic VARIATION. Deviation is a term used to find out how the compass is affected by say a ship being made of lots of steel. Before a ship departed on a voyage (I did this on an Aircraft Carrier) the ship would head out to a quiet part of the harbour, and we would "swing the compass" using the services of a couple of tugs. meaning turning the ship round 360 degrees, and noting the magnetic "deviation" This information you would put into the deviation card which would be kept close to the compass. To find your true course, you would use the Magnetic variation (as published on every chart) and then you would consult your deviation card to see what other adjustments you had to make. Easy EH???
