True, magnetic and compass: variation and deviation

Offshore Theory, written by a superyacht bosun with a commercial Yachtmaster Offshore · 30 October 2026

There are three norths on a boat: true north, where the chart's meridians point; magnetic north, where a perfect compass points; and compass north, where your own compass actually points. Every course and bearing has to be moved between them, and the only hard part is the sign.

Variation, from the chart

Variation is the difference between true and magnetic north. The compass rose on the chart prints it for a given year, with how much it changes each year. For example:

4°10′W 2015 (8′E)

That reads: in 2015 the variation was 4°10′ west, and it changes by 8′ east each year. Easterly change makes a westerly variation smaller. In 2026, eleven years later: 11 × 8′ = 88′ = 1°28′. 4°10′ − 1°28′ = 2°42′W, which is 3°W to the nearest degree.

Deviation, from the boat

Deviation is your compass's own error, caused by the iron and electrics around it. It is different on different headings, which is why it comes as a table, the deviation card, entered with the boat's heading. It belongs to the steering compass; a hand-bearing compass has its own, which you keep small by standing clear of steel and electronics.

The rule for the sign

Going from compass towards true (C → M → T): add east, subtract west. The usual memory aid is CADET: Compass ADd East, True.

Going from true towards compass (T → M → C): the opposite, add west, subtract east. Another aid: "error west, compass best; error east, compass least".

Worked example 1: a course for the helmsman

The course to steer off the chart is 120°T. Variation is 3°W, and the deviation card gives 2°W for that heading.

• True to magnetic, westerly: add. 120° + 3° = 123°M.
• Magnetic to compass, westerly: add. 123° + 2° = 125°C.

Check it backwards with CADET: 125°C, subtract 2°W, 123°M; subtract 3°W, 120°T.

Worked example 2: a bearing with the steering compass

On that heading of 125°C, you sight a lighthouse over the steering compass at 310°C. The deviation is the one for the boat's heading, 2°W, not anything to do with the bearing.

• Compass to magnetic, westerly: subtract. 310° − 2° = 308°M.
• Magnetic to true, westerly: subtract. 308° − 3° = 305°T. That is the position line: draw it from the lighthouse on its reciprocal, 125°T, as for any bearing in a fix.

Total error

Variation and deviation can be combined into one figure, the compass error, by adding them with their signs: 3°W and 2°W make 5°W; 3°W and 2°E make 1°W. One conversion with the compass error gives the same answer as two steps.

The common slips

• Applying the rule the wrong way round. Write the direction of travel (T to C, or C to T) before you start.
• Using last decade's variation. Update it from the rose every time.
• Entering the deviation card with the bearing instead of the heading.
• Putting deviation on a chart bearing. The chart has no deviation; only your compass does.

Where this is in the course

True, magnetic and compass: variation and deviation →

Free reading notes: true, magnetic and compass →

Chart work: the plotter and the compass rose →

Questions

What is variation?

The angle between true north and magnetic north at your position. It is printed on the chart's compass rose for a given year, with its annual change, and it is the same for every boat in that place.

What is deviation?

The error of your own compass, caused by the boat's steel, engine and electronics. It changes with the boat's heading and is given on the boat's deviation card.

Which way round do I apply them?

From compass to magnetic to true, add easterly errors and subtract westerly ones. From true to magnetic to compass, do the opposite: add westerly, subtract easterly.

Does deviation depend on the bearing I am taking?

No. Deviation of the steering compass depends on the boat's heading. A bearing taken with the steering compass is corrected with the deviation for the heading she is on at the time.

How do I update the variation on an old chart?

Multiply the annual change by the number of years since the year printed on the rose, and add or subtract it in the direction the change says. Round the result to the nearest degree for chartwork.

Related guides

• Course to steer and the tidal triangleThe one-hour vector triangle drawn step by step, with leeway, and down to a compass course.
• The cocked hat and crossing bearingsChoosing marks, the angle of cut, the order you take them in, and what the cocked hat tells you.
• The running fixOne mark, two bearings and the run between them: how to transfer the line, with and without a stream.
• Planning a passage, step by stepFrom the question of whether to go, through the tide at each end and the stream, to the plan on one page.