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44
Source excerpts
Bowditch Ch. 6 §601
Bowditch Ch. 6 §601 — The Magnetic Compass — Variation and Deviation Compass error has two parts. Variation is the angle between true north and magnetic north at a place; it is printed inside the chart's compass rose and changes with position and slowly over time. Deviation is the angle between magnetic north and the vessel's own compass north, caused by the vessel's magnetism; it changes with the vessel's heading and is read from the deviation card. Each is labelled east or west.
Bowditch Ch. 6 §602
Bowditch Ch. 6 §602 — The Magnetic Compass — Variation and Deviation Converting a course between true and compass follows the sequence True - Variation - Magnetic - Deviation - Compass (T-V-M-D-C). When correcting from compass toward true, add easterly errors and subtract westerly ("correcting, add east"); when uncorrecting from true toward compass, reverse it — add westerly and subtract easterly. Variation moves between true and magnetic; deviation moves between magnetic and compass.
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Under 33 CFR §164.78, a person directing a towing vessel is required to know and apply variation and deviation under which condition?
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