Compass to True Heading (Variation and Deviation)

Three norths, and they rarely agree. TRUE north is where the meridians on the chart point, at the axis of rotation. MAGNETIC north is where the earth's field points, currently well off in the Canadian Arctic and moving briskly. COMPASS north is where the needle in front of you actually points, which is magnetic north disturbed by the iron and the electrics of your own vessel or aircraft. The angle from true to magnetic is VARIATION, a property of the place and the year. The angle from magnetic to compass is DEVIATION, a property of your vessel and of the heading it is on.

The chain is remembered as TVMDC — True, Variation, Magnetic, Deviation, Compass — and the mnemonic for the order is "True Virgin Makes Dull Company". The mnemonic for the SIGNS is "east is least, west is best", and that one is the source of more errors than everything else on this page combined, because it does not say which direction it applies in. Here is the rule written out, in both directions, with no rhyme involved.

Going FROM COMPASS TO TRUE, which is what this page computes: add easterly corrections, subtract westerly ones. Compass 250° with 3° west deviation and 15° east variation gives $250 - 3 + 15 = 262^{\circ}$ true. Going FROM TRUE TO COMPASS, the other way down the chain: subtract easterly, add westerly. That second direction is the one "east is least, west is best" describes. Apply it in the wrong direction and you do not merely fail to correct the error — you double it, so a 20° west variation becomes a 40° mistake, which on an ocean passage is a different continent.

Two practical notes. Variation changes with time and is printed on the chart's compass rose with an annual rate of change beside it; a chart twenty years old can be a degree or more out, and NOAA and NGA's World Magnetic Model is what a modern plotter uses instead. Deviation changes with heading, which is why it lives on a card of its own rather than as a single number, and why the card has to be redone after any significant change to the vessel — new electronics, a steel cargo, or in small craft a phone left on the binnacle. Deviation past about 5° means the compass wants swinging and adjusting, not a larger correction applied to a bad instrument.