A deviation card is produced after your onboard compass, usually located at the wheel binnacle, has been swung. Ideally this is done annually.
The compass deviation card marks deviation, as the name suggests, for different headings. Readings are usually taken at 30 degree intervals from 000 and 360 degrees.
As we know, variation is different to deviation. Variation is location specific. Magnetic variation is the same for all vessels in a specific area, no matter what heading they are on. Deviation is vessel specific and also varies dependent on the vessel’s heading. This is because deviation is affected by the location of ferrous metals and electronic equipment on the vessel and the effect will differ depending on the heading of the vessel.
To swing a compass and produce a deviation card the best thing to do is find transits marked on a chart. Drive down that transit, pointing the heading down the transit line. Allow for Variation to obtain magnetic heading and then the difference between the compass heading and the transit’s position line, allowing for variation to obtain a magnetic bearing, is deviation on that heading. Mark this on the card and then repeat for transits, ideally every 30 degrees, in order to complete a deviation card.
Once you have an up-to-date card, date it and affix it in your navigation station. It is then used to reduce True bearings to Compass bearings and vice versa using the acronyms, True Virgins Make Dull Company and C ade T.
In other words, when converting True to compass, first apply variation to obtain magnetic course and then apply deviation for that magnetic course to obtain a compass course. Apply westerly and easterly variation using C adet T. Compass to True - Add East and deduct West.