Definition
Admiralty charts print an outer ring graduated 000 to 359 degrees true and an inner ring showing magnetic north, annotated with the variation at a stated epoch and its annual change, for example 2 degrees 15 minutes West 2020, decreasing about 8 minutes annually. Add the accumulated change before using the magnetic ring. Best practice is to plot everything in degrees true from the outer ring and convert compass headings separately, avoiding the errors that come from mixing references.
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