Definition
The vessel is steadied on a series of headings, usually in calm water with no tidal stream, while the bearing of a distant transit, a shore object of known bearing, or the Sun's calculated azimuth is compared with the compass reading. The difference on each heading, less the known variation, is the deviation. A professional adjuster will then fit corrector magnets and Flinders bar to reduce it before recording the residual values on a new card.
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