The magnetic compass and variation

From true to magnetic

In Module 1 every direction was true: measured from true north, which on a Mercator chart runs straight up the meridians, and written with a T, as in 045°T. That is how you measure and plot on the chart. But out on the water you steer by a magnetic compass, and you take bearings of landmarks with one too — and a magnetic compass does not point to true north.

This lesson explains why, how big the difference is, where to find it and how to convert between true and magnetic directions. Lesson 10 then adds the last piece: the small error of the boat’s own steering compass.

A compass points to magnetic north; the chart is drawn to true north. Variation is the bridge between them.

How a magnetic compass works

The Earth has a magnetic field, produced deep inside it, which behaves roughly like a giant bar magnet set slightly askew to the Earth’s axis. A magnet that is free to turn — a compass needle — lines up with that field, so its north-seeking end (usually marked N or coloured red) points to magnetic north.

A yacht’s steering compass works the same way, built for life at sea:

• Magnets fixed under a circular card, marked in degrees, keep the card pointing the same way while the boat turns around it.
• The card sits in liquid, which damps its swinging as the boat moves, and the compass is mounted so the card stays roughly level as the boat heels and pitches.
• A mark on the bowl, in line with the boat’s fore-and-aft line, is the lubber line. You read the heading on the card against it.

A hand-bearing compass is a small hand-held version for taking bearings of landmarks; you will use it in Lesson 11.

True north and magnetic north

True north is the direction of the geographic North Pole, the point on the Earth’s axis where all the meridians meet. For navigation it is fixed, and the chart is built around it.

A compass points instead to magnetic north. The north magnetic pole — where the Earth’s field points straight down — lies in the Arctic, hundreds of kilometres from the geographic pole, and it is not fixed. It wanders as the field deep in the Earth changes: in recent decades it has moved tens of kilometres a year, from the Canadian Arctic towards Siberia.

Strictly, a compass follows the local direction of the Earth’s field rather than aiming exactly at the magnetic pole, and the field is a little irregular. So you never work the difference out from a map of the poles: it is measured, and printed on the chart.

Variation

Variation is the angle between true north and magnetic north at a particular place. It is named after the side that magnetic north lies on:

• If magnetic north lies to the west of true north, the variation is west (W).
• If magnetic north lies to the east of true north, the variation is east (E).

Scientists, and many land maps, call the same angle magnetic declination. It changes in two ways:

• With place. Around the British Isles it is currently small — within a few degrees of zero — but in some parts of the world it is more than 20°. It can differ slightly even across one chart, which is why charts often have more than one compass rose.
• With time. As the Earth’s field slowly changes, the variation at any place changes too, typically by a few minutes of arc a year. Charts print this annual change so you can bring the figure up to date.

Reading variation from the chart

You met the compass rose in Lesson 4. Its outer ring is true — 000° under the star, lined up with the meridians — and its inner ring is magnetic: the same scale turned round by the variation, so its 000° points to magnetic north. On the practice chart the inner ring’s zero sits against 357 on the outer ring: 000°M is 357°T, because magnetic north is 3° west of true.

In the middle is the variation statement, VAR 3°00'W (2025) (8'E). It says the variation was 3°00'W — 3 degrees 0 minutes west — in 2025, and that it changes by 8 minutes of arc to the east every year. (There are 60 minutes in a degree.)

Bringing it up to date. Work out how many years have passed and multiply by the annual change. A change in the same direction as the variation makes it bigger; a change in the opposite direction makes it smaller.

Converting between true and magnetic

Measure a direction from magnetic north instead of true north and the figure changes by the variation. Which way? Think about where you start counting.

With westerly variation, magnetic north lies west of true north — anticlockwise from it. Counting clockwise from magnetic north you start sooner, so you have further to go: every magnetic direction is bigger than the true one, by the variation. With easterly variation magnetic north lies clockwise of true north, you start later, and every magnetic direction is smaller.

Variation West, Magnetic Best; Variation East, Magnetic Least.

“Best” means bigger: with westerly variation the magnetic figure is the bigger one. It is only a memory aid — if you are ever unsure, sketch the two norths and see which way the line turns. If an answer passes 360, take away 360; if it drops below 000, add 360.

The T V M D C ladder

Navigators set conversions out in a fixed order, from the chart at the top to the compass at the bottom: True – Variation – Magnetic – Deviation – Compass. Write the letters down the page and fill in what you know:

Coming down the ladder, from the chart towards the compass: add west, subtract east. Going up, from the compass towards the chart: add east, subtract west. Lesson 10 fills in the deviation step and adds a memory aid for working up the ladder, CADET: Compass ADd East → True.

A course from the chart, in °M

In practice you measure on the chart in true — with the plotter squared to the meridians, exactly as in Lesson 6 — then apply the up-to-date variation to get a magnetic course. You can also read magnetic directions straight off the rose’s inner ring, but that ring is drawn for the year printed on the rose, so on an old chart it can be a degree or more out. Measuring in true and applying the variation yourself works on any chart.

Where else to find variation

• Chart notes. A chart or harbour plan without a rose of its own may give the variation in a note — read the notes.
• Isogonic lines. Some small-scale charts show variation with lines joining places of equal variation.
• Pilot books and almanacs may quote the variation for an area.
• GPS sets and chart plotters can work the variation out for themselves, and can show bearings and courses in either °T or °M. Find the setting, know which yours is using, and make sure it matches the way you are working.

Common mistakes

• Applying it the wrong way. 045°T with 3°W is 048°M, not 042°M. Sense-check every answer: with westerly variation, magnetic is bigger.
• Forgetting the annual change. An old chart reading VAR 6°10'W (2003) (9'E) has 2°34'W by 2027. Using 6°W would put every course more than 3° out — more than 0.5 M off track for every 10 M run.
• Using a distant rose. Use the rose nearest your position.
• Losing the letter. A course without °T or °M can be read either way.
• Crossing north. 358 + 4 is 002, not 362; 002 − 3 is 359, not −1.

Measure in true, update the variation, apply it the right way, and label the answer.