Three norths
- True (°T): the direction of the geographic North Pole. Charts are drawn in true.
- Magnetic (°M): where a compass would point with no local interference. The angle between true and magnetic north is variation. It depends on where you are and changes slowly each year.
- Compass (°C): what your steering compass actually reads. The angle between magnetic and compass north is deviation, caused by the boat's own engine, electronics and steel. It changes with the boat's heading.
Variation from the compass rose
The compass rose on the chart gives variation for a given year and its annual change. Say it reads 3°15′W 2016 (8′E), meaning 3°15′ west in 2016, decreasing by 8 minutes a year (the change is eastwards, so westerly variation gets smaller).
By 2026 that's 10 years × 8′ = 80′ = 1°20′ of change. 3°15′ − 1°20′ = 1°55′W, which you'd round to 2°W.
Deviation from the deviation card
A deviation card lists the deviation for different headings of your boat. It's looked up by the boat's heading, not by the bearing of whatever you're looking at. Part of a card might read:
| Ship's head (°C) | Deviation |
|---|---|
| 045 | 3°W |
| 090 | 3°E |
| 135 | 1°E |
The sign rule
Going from true towards compass (chart to helm):
- add westerly variation and deviation,
- subtract easterly variation and deviation.
Going from compass towards true (helm or hand-bearing compass to chart), do the opposite.
Worked example: true to compass
You've worked out a course of 095°T. Variation is 2°W.
- True to magnetic: westerly, so add. 095 + 2 = 097°M.
- Look up deviation for a heading of about 097: the card gives 3°E near 090.
- Magnetic to compass: easterly, so subtract. 097 − 3 = 094°C. That's what you give the helm.
Worked example: compass to true
You take a bearing of a headland with a hand-bearing compass: 250°C. A hand-bearing compass used well away from the boat's magnetic influences is normally taken to have no deviation, so 250°C is 250°M.
Magnetic to true: going towards true you reverse the rule, so subtract the westerly variation. 250 − 2 = 248°T. That's the line you plot.
Checking deviation with a transit
Line up two charted objects, such as a beacon and a church spire, so you're running along the transit. Measure the transit's bearing on the chart, say 040°T, which is 042°M with 2°W variation. If the steering compass reads 045°C on that heading, the compass reads 3° higher than magnetic, so deviation on that heading is 3°W. Repeat on different headings to check the card.
Common mistakes
- Applying the rule in the wrong direction: add west going towards compass, subtract west going towards true.
- Looking up deviation for a bearing instead of the boat's heading.
- Forgetting to update the chart's variation for the current year.
- Applying the steering compass's deviation to a hand-bearing compass.
Converting your answer to compass is the last step of every course to steer.