After a magnetic compass is adjusted to reduce deviation the normal practice is to "swing ship" to record the deviation which remains after adjustment is complete. Swinging ship involves steadying the ship's course on a series of cardinal and intercardinal magnetic headings, and then comparing the magnetic compass reading with the gyro compass reading on each heading. After the gyro is corrected to true and variation is factored in, the difference remaining is residual deviation. The residual deviations are entered in what is called a deviation table which is used in converting magnetic compass heading or bearing to true or vice versa.
The USCG has a series of deviation table problems which may be found on license exams. They are easily solved as will be shown in the example problem below.
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