Alfredo Contaldo · video

What this video covers

Verifying the accuracy of a magnetic compass and its deviation table requires comparing a compass bearing of a known coastal alignment against the true bearing plotted on a nautical chart. A compass is subject to two primary errors: magnetic declination, which varies by geographic location and time, and deviation, which results from onboard metallic masses like engines or diving tanks. When a vessel’s compass is replaced or shows inconsistent readings, a professional compass adjuster performs "compass swings" to calibrate the instrument using known coastal alignments. To verify the deviation table, a navigator identifies two conspicuous landmarks on a chart, such as a lighthouse and a church, and measures their alignment with the steering compass. By calculating the algebraic difference between the true bearing and the compass bearing, the navigator determines the magnetic variation. If the calculated deviation does not match the value listed in the vessel's deviation table for the current heading, the compass requires professional recalibration.

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