Compass Errors in Practice

On passage, you must combine variation and deviation to find total compass error. Take variation from the chart compass rose (updating for annual change), then take deviation from your boat's deviation card for the heading you are steering. The two are applied separately using CDMVT — they are NOT simply added together because deviation depends on heading, so changing one may change the other.

A common mistake is to apply variation and deviation the wrong way round, or to forget that deviation changes when you alter course. Another frequent error is using an outdated deviation card — the card must reflect the current magnetic environment of the boat. Always re-swing after fitting new electronics or moving heavy ferrous objects.

In calm conditions and at speeds where leeway is negligible, you can check your compass by comparing the compass heading with the GPS Course Over Ground (COG). If you are making good way through the water with no tidal stream abeam, COG should equal your true heading. Apply variation and deviation in reverse to see if the compass reading matches. Any consistent discrepancy suggests the deviation card needs updating.

Be aware of heeling error: when a sailing yacht heels, ferrous masses that were beside the compass are now partly below it, altering the vertical component of the magnetic field and introducing additional deviation. This error is greatest on north/south headings and can be compensated with a vertical heeling-error magnet inside the binnacle. Heeling error is not shown on a standard deviation card because the card is produced with the boat upright.

Fluxgate compasses measure the Earth's magnetic field electronically and feed heading data to instruments and autopilots. They still sense magnetic north and must be calibrated (the electronic equivalent of swinging), but they are less affected by heeling because they can be mounted low in the hull away from interference. Gyro compasses, common on commercial vessels, find true north using the Earth's rotation rather than magnetism, so they are unaffected by variation or deviation — but they are expensive and rarely found on yachts.