NAVIGATION · Lesson 4 of 6
Direction: True, Magnetic and Compass
30 min · Beginner friendly
You’ll learn toDefine true, magnetic and compass direction · Apply variation and deviation using the correct sign conventions · Convert reliably between true, magnetic and compass courses
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Ask three instruments aboard which way is north and you may get three different answers: the chart is drawn to true north, the Earth's field pulls a needle towards magnetic north, and your steering compass — sitting in a boat full of iron and electronics — settles on its own private compass north. Navigation depends on converting between the three without error.
Take a guess
Decide before you read: your plotted course is 090° T and the chart's compass rose says variation 4° W. Ignoring deviation, will the course you ask the helm to steer be bigger or smaller than 090?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
The Three Norths
- True direction (°T)
- Direction measured from true (geographic) north — the reference used on charts.
- Magnetic direction (°M)
- Direction measured from magnetic north — what a perfect compass free of the boat would read.
- Compass direction (°C)
- Direction measured from your compass north — what your steering compass actually reads.
- Variation
- The angle between true and magnetic north at your location, named E or W. Found on the chart compass rose.
- Deviation
- The angle between magnetic north and your compass north, named E or W, varying with . Found on the deviation card.
Knowledge check
What is the key difference between variation and deviation?
Magnetic north currently sits in the Canadian Arctic and wanders year by year, and variation differs from place to place — 2° W in one cruising ground, 15° W in another. That is why the chart states the local figure rather than leaving you to remember one from home waters.
Finding Variation
Variation is printed inside the compass rose on your chart: a figure, a year, and an annual of change — for example variation 3° 30' W (2022), decreasing about 6' annually. Update it to the current year, then round to the nearest half or whole degree; nobody can steer finer than that anyway.
Worked example
Updating variation from the compass rose
The rose on the Westhaven chart reads: variation 3° 30' W (2022), decreasing about 6' annually. It is now 2026. What variation should you use?
- Years elapsed since the rose was calculated: 2026 − 2022 = 4 years.
- Total change: 4 × 6' = 24' decrease.
- Decreasing westerly variation means the westerly angle shrinks: 3° 30' W − 24' = 3° 06' W.
- Round to a practical figure for steering and plotting.
Use 3° W (strictly 3° 06' W) for this year's navigation.
Knowledge check
A rose on an older chart reads: variation 4° 30' W (2020), decreasing about 5' annually. It is 2026. What variation do you use?
Finding Deviation
Deviation comes from a deviation card, produced when a compass professional swings the compass — measuring its error on a series of headings. Enter the card with the boat's heading, interpolating between rows, never with the direction you happen to be sighting. On a well-installed yacht compass the numbers are small, a degree or two either way; a used away from the boat's ironwork is normally assumed to have no deviation.
| Compass heading | Deviation |
|---|---|
| 000° | 2° W |
| 045° | 1° W |
| 090° | 0° |
| 135° | 1° E |
| 180° | 2° E |
| 225° | 1° E |
| 270° | 0° |
| 315° | 1° W |
Knowledge check
Petrel is steady on a heading of 225° C when you want to convert a course. Reading the card above, what deviation do you apply?
The Ladder: True to Compass
Converting is climbing the ladder one rung at a time — True — Magnetic — Compass — applying one named error at each step. CADET gives the direction: Compass ADd East (to get) True; going the other way, add westerly. Then check the answer against the second rhyme: error west, compass best; error east, compass least — if your figure breaks the rhyme, you added the wrong way.
Worked example
True to compass: a course to steer
Your plotted course from Westhaven to Salt Island measures 090° T. Variation is 4° W, and the deviation card gives 2° W for a heading near east. What do you ask the helm to steer?
- Write the ladder: True → Magnetic → Compass. Travelling towards compass, ADD westerly errors.
- Apply variation: 090° T + 4° (W) = 094° M.
- Apply deviation: 094° M + 2° (W) = 096° C.
- Check with the rhyme: both errors west, so the compass figure should be the biggest — 096 is greater than 090.
Steer 096° C.
Knowledge check
A true course of 180° T with variation 6° W becomes which magnetic course?
Coming Back: Compass to True
Worked example
Compass to true: plotting a bearing
You sight a beacon at 210° C using the steering compass. Deviation on your current heading is 3° E; variation is 5° W. What do you plot on the chart?
- Travelling from compass towards true, use CADET: ADD easterly, subtract westerly.
- Apply deviation: 210° C + 3° (E) = 213° M.
- Apply variation: 213° M − 5° (W) = 208° T.
- Sanity check: net error is 2° W, so compass (210) should read higher than true (208) — error west, compass best. It does.
Plot the at 208° T.
Work it out
Two conversions from the yacht Petrel (deviation card above). (a) Your plotted course is 264° T; variation 3° W; take deviation for a heading near 270°. What compass course do you steer? (b) Later you take a steering-compass bearing of 118° C while heading 135° C; variation 3° W. What true bearing do you plot?
Within 1° is fine.
Within 1° is fine.
Key points
- Three norths: true (chart), magnetic (Earth's field), compass (your needle).
- Variation = true vs magnetic; from the compass rose; depends on place and year.
- Deviation = magnetic vs compass; from the deviation card; depends on the boat's heading.
- Convert one rung at a time: True — Magnetic — Compass.
- CADET: from compass towards true, add easterly errors. Going the other way, add westerly.
- Check every answer with: error west, compass best; error east, compass least.
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