Variation is the angle, at any given place and time, between true north, the direction of the geographic pole and of the meridians on a chart, and magnetic north, the direction in which a compass needle points when free of any local disturbance. It is expressed in degrees east or west and it changes both with position and slowly with time. A beginner needs to know that a compass does not point at true north and that the difference must be applied before laying a compass course on a chart, or a chart course onto the compass. An experienced navigator adds that variation is a property of the Earth and not of the boat, which is exactly what separates it from deviation, a term the two are often confused with. In older texts you will also see it called magnetic declination, the geophysicist’s word for the same thing.
Why the compass does not point true
The Earth’s magnetic field is generated by circulating molten iron in the outer core, and its poles are not at the geographic poles. The north magnetic pole currently lies in the Arctic Ocean well away from the geographic pole and is drifting at tens of kilometres a year. Because a compass needle aligns with the local field lines, which curve across the globe, the angle between magnetic and true north varies enormously from place to place. Around the British Isles it is currently a small westerly figure, a degree or two, having been more than ten degrees west a century ago; on the west coast of North America it is more than fifteen degrees east; in parts of the far north and south it exceeds thirty degrees. The pattern shifts every year, which is why the annual change is printed on the chart.
Finding and applying variation
Every paper chart carries one or more compass roses. The outer ring is true, the inner ring is magnetic, and the offset between them is the variation for that area, printed with the year of the survey and the annual rate of change, for example “3° 15’ W 2020 (8’ E)”. Multiply the annual change by the years elapsed and apply it to bring the figure up to date, remembering that an easterly annual change reduces a westerly variation. Isogonic lines, which join points of equal variation, are printed on small-scale ocean charts and in pilot books. Electronic chartplotters apply variation automatically from a built-in model, but every navigator should know how to do it by hand, because a chartplotter set to display true bearings alongside a magnetic steering compass is a source of endless confusion on a dark night.
The rules for applying it are the ones drummed into every navigation student.
- From true to magnetic: add westerly variation, subtract easterly. “West is best, east is least” when going from true to compass.
- From magnetic to true: subtract westerly variation, add easterly. The rhyme reverses.
- Order of corrections: true course, apply variation to get magnetic, apply deviation to get compass. Going the other way, compass to magnetic to true. Many sailors remember “True Virgins Make Dull Companions” for True, Variation, Magnetic, Deviation, Compass.
Variation and deviation are not the same
This is the confusion the glossary most needs to clear up. Variation is caused by the Earth and is the same for every compass in the same place. Deviation is caused by the boat: the engine, the keel bolts, the loudspeaker in the cockpit, the steel tools in the locker under the compass. Deviation differs from one boat to the next and changes with heading, which is why a boat has a deviation card listing the correction on each of eight or sixteen headings, drawn up by swinging the compass. Variation changes with where you are; deviation changes with which way you are pointing. Apply both, in the right order, and a chart course becomes a compass course you can actually steer.
Frequently Asked Questions about Variation
Is variation the same as deviation?
No. Variation is the difference between true and magnetic north caused by the Earth’s field and depends only on where you are. Deviation is the additional error caused by magnetic influences on your own boat and depends on the heading.
Does GPS use variation?
A GPS measures course over ground relative to true north and applies a built-in variation model if you set it to display magnetic. Make sure the plotter and the crew are working in the same reference before you steer to its numbers.
Why is variation east in some places and west in others?
Because the magnetic pole is offset from the geographic pole and the field lines curve. From places where magnetic north lies to the right of true north the variation is east; where it lies to the left it is west.
Variation is the first correction a navigator learns and the one it is easiest to become casual about in the age of chartplotters. Read it off the rose, bring it up to date, apply it in the right direction and keep everyone on board working in the same reference, and your compass course will take you where the chart says it will, which is the whole point of navigation.