4.5 Magnetic Compass: Variation & Deviation

Why the Compass Matters on Master 100

Module Q162 — Navigation General Near Coastal (50 questions, 70%) and Q163 Chart Plot (10 questions, 90%) both assume you can convert between true, magnetic, and compass directions without hesitation. On a small passenger vessel under a COI, you may run a GPS chartplotter all day and still need a magnetic compass for backup, for Coast Guard exam problems, and for the classic piloting skills that keep you out of the rocks when electronics fail.

This section locks the definitions. The next section turns them into TVMDC arithmetic. Do not skip the vocabulary: most wrong answers on compass questions come from mixing up where an error comes from, not from bad addition.

Three Norths, Three Headings

Every direction you care about on the water can be labeled three ways:

• A true course of 090°T means you intend to travel due east on the chart grid.
• A magnetic course of 090°M means the direction is 090° relative to magnetic north.
• A compass course of 090°C means the lubber’s line sits on 090 on the compass card as installed on this vessel.

Magnetic north is not true north almost everywhere on Earth. The compass needle (absent ship effects) points toward the magnetic pole region, not the geographic pole. Compass north is not magnetic north on a steel or electromagnetically noisy vessel, because the hull, engines, electronics, and cargo bend the local field.

Mental model

Think of a chain:

True → (variation) → Magnetic → (deviation) → Compass

Going down the chain (true toward compass) is often called uncorrecting or converting “from true to compass.” Going up the chain (compass toward true) is correcting. Section 4.6 formalizes the memory devices; first own the physical meanings of the two errors.

Variation: Earth’s Magnetism (From the Chart)

Variation is the angular difference between true north and magnetic north at a geographic location. It does not depend on which boat you are on. Two vessels at the same spot have the same variation.

Where you get it

• The compass rose on a nautical chart (inner magnetic rose, outer true rose).
• The rose states something like Variation 14°W (2020) with an annual change such as 0°6' E.
• For exam and practical work, apply the annual change from the rose’s base year to the current year unless the problem gives variation outright.

East and West labels

• Easterly variation: magnetic north is east of true north (from the true meridian, magnetic north lies to the right/east).
• Westerly variation: magnetic north is west of true north.

If variation is 14°W, magnetic directions are 14° greater than true when you go true → magnetic (you add west when uncorrecting—covered fully in 4.6). The critical conceptual point: variation is a property of place and time, printed on the chart, independent of heading and independent of your vessel’s steel.

Worked variation update

Chart rose: Var 12°30' W (2018), annual change 6' E. Year of voyage: 2026.

Years elapsed = 2026 − 2018 = 8. Change = 8 × 6' E = 48' E = 0°48' E. Starting from 12°30' W, applying 0°48' easterly change reduces westerly variation: 12°30' W − 0°48' = 11°42' W (about 12°W if the problem rounds).

Exam problems often hand you a clean whole degree so you can focus on TVMDC; still know how roses work for Chart Plot and real piloting.

Deviation: Ship’s Magnetism (From the Deviation Table)

Deviation is the angular difference between magnetic north and compass north caused by magnetic influences on board. Soft iron, hard iron, generators, speakers, phones, toolboxes, and even a steel coffee mug near the binnacle can change the reading.

Critical properties

1. Deviation depends on the vessel — each vessel has its own residual magnetism and equipment layout.
2. Deviation depends on heading — as the ship turns, the relationship between the Earth’s field and the ship’s field changes, so the error changes.
3. You obtain deviation from a deviation table (or deviation card / curve) prepared by swinging the ship and recording compass vs magnetic (or true, then converting) on many headings.

How to read a deviation table (exam style)

Tables list ship’s head (usually by compass or magnetic—read the table header) and deviation E or W.

Example extract:

If you are steering compass course 180°, deviation is 3°W. If you need a value between tabulated headings, interpolate linearly unless the problem supplies a curve.

Never invent deviation from the chart. Charts give variation only. Using the rose number as deviation is a classic Master-exam trap.

Compass Error: Combining Variation and Deviation

Compass error (CE) is the total difference between true and compass:

Compass error = variation + deviation (with consistent east/west sign handling).

Many mariners assign East = positive (+) and West = negative (−) when combining:

• Var 10°E, Dev 3°W → CE = +10 + (−3) = +7° → 7°E
• Var 14°W, Dev 2°W → CE = −14 + (−2) = −16° → 16°W
• Var 8°E, Dev 4°E → CE = +8 + 4 = +12° → 12°E

Compass error answers the operational question: “How far is my compass card from the chart’s true meridians?” Variation alone answers “Earth.” Deviation alone answers “this hull on this heading.”

Worked combination examples

Example 1. Chart variation 15°W. Steering compass heading 090°; deviation table shows 2°E on that head.

CE = 15°W + 2°E → net 13°W (west is larger; subtract the east from the west, keep west label).

Example 2. Variation 6°E. Heading 270°C; deviation 4°W.

CE = 6°E + 4°W → net 2°E.

Example 3. Variation 0° (near the agonic line). Deviation 3°W.

CE = 3°W — all of the error is ship-made.

Magnetic North vs True North in Practice

Near-coastal Master work is almost always done on true charts (Mercator charts with true meridians). GPS tracks and most modern plotters default to true. The magnetic compass still speaks compass, and older coastal practice and many exam items still live in magnetic intermediate steps.

Practical implications for a Master LT 100:

• Plot courses in true on the chart, then convert to compass course to steer (CTS) using TVMDC.
• Take visual bearings carefully: a pelorus or bearing compass may be relative or magnetic; convert to true before plotting LOPs (Section 4.7).
• After any major steel work, new electronics installation, or cargo change that moves ferrous material, recheck deviation — your table can go stale.
• Keep magnetic material away from the compass; a deviation card is worthless if the last mate left a toolbox under the binnacle.

Exam Traps Specific to Q162

Quick self-check definitions

• True north: geographic North Pole direction along the chart meridian.
• Magnetic north: direction of the Earth’s magnetic field horizontal component at the vessel’s position.
• Compass north: direction the vessel’s compass card indicates as north after ship magnetism acts.
• Variation: true vs magnetic (chart).
• Deviation: magnetic vs compass (vessel + heading).
• Compass error: true vs compass (variation combined with deviation).

Master these six terms cold before you touch TVMDC memory rhymes. The rhyme only works if you know which number is variation and which is deviation.