Swinging the Compass
1) Locate two transit objects (objects in line) and determine their true bearing on the chart
(2) Secure the boat at anchor so that these transits are exactly aligned. Swing the boat's head until it is pointing due north.
(3) Read off the transit bearing on the compass and apply the variation.
(4) The difference between this result and the true transit bearing is the deviation on this heading. If the true bearing is greater, the deviation is named east; if it is the lesser, the deviation is named west.
(5) Repeat the procedure taking transit bearings on each of the cardinal points.
(6) From the results, make up a deviation card
Further errors are caused by boat heel, and the difficulty of reading when on a moving platform. An electronic compass can be set up to remove the effect of deviation, but it is still a magnetic compass.
Hand bearing compasses usually show very small deviation effects which can be ignored if you use them away from a magnetic area.
Deviation can also be East or West, but the same logic applies:
Sounds simple but try it once.
SD