Magnetic Compass Errors Trainer

Earth's magnetic lines run flat at the equator but dip toward the nearer pole at higher latitudes. The compass magnet follows that dip, which pulls the float's centre of gravity off the pivot toward the equatorward side. On east/west headings that offset mass lags or leads when you change speed, swinging the card.

The error

On east or west headings, accelerating and decelerating cause an apparent turn. On north or south headings the centre of gravity stays in line with travel, so speed changes produce no error.

Memory aids

Once speed stabilises, the magnet settles and the compass realigns. Shown swings are illustrative, not to scale.

A turn is also a form of acceleration. As the card swings through North or South, the displaced centre of gravity sits side-on to the turn, so the compass leads or lags. Through East and West the forces line up with the pivot, so there's no significant error.

Memory aids

How much

The correction grows the closer your roll-out heading sits to North or South — read it straight off the guide:

Level off at the corrected reading, then wait for the compass to settle onto your true heading.