Magnetic Variation and Deviation
Magnetic variation and deviation are the two sources of error in a magnetic compass. Understanding them and applying corrections is a fundamental navigation skill for deck officers.
Answer in Brief
- Variation: Difference between True North and Magnetic North — a fixed geographic error at any location, caused by Earth’s magnetic field
- Deviation: Error caused by the ship’s own magnetic fields — differs with compass heading
- Compass Error = Variation + Deviation
- True = Compass ± Error (use the mnemonic CADET/TVMDC to apply corrections)
Magnetic Variation
The Earth’s magnetic pole is not at the geographic North Pole. The magnetic north pole is located in northern Canada and moves over time. The angle between True North and Magnetic North at any location is called variation (also called declination in terrestrial navigation).
Key facts about variation:
- Measured in degrees East or West of True North
- Varies by geographic location — shown on nautical charts as compass roses with the variation value and annual rate of change
- Changes slowly over time (annual rate of change printed on charts)
- Completely outside the navigator’s control — it’s a property of Earth
Applying variation:
- Variation East → Magnetic < True (compass reads less than true)
- Variation West → Magnetic > True (compass reads more than true)
Memory aid: “Error East, Compass Least; Error West, Compass Best”
Deviation
Deviation is the difference between Magnetic North and the direction the compass needle points on a particular ship heading. It is caused by the ship’s own magnetic field — from structural steel, electrical wiring, cargo, machinery.
Key facts about deviation:
- Changes as the ship’s heading changes (because the ship’s magnetic field changes orientation relative to the Earth’s field)
- Recorded on a deviation card — a table showing deviation for each compass heading
- Can be reduced (but not eliminated) by compass adjustment (swinging ship with a compass adjuster)
- After loading or significant changes in cargo/equipment, the deviation card may need updating
Deviation card: Displayed near the compass. Shows deviation East or West for each 15° or 30° of compass heading. The OOW must consult this to correct compass readings.
Total Compass Error
Compass Error = Variation + Deviation
If Variation = 5° West and Deviation = 3° East on the current heading: Compass Error = 5°W + 3°E = 2° West
TVMDC mnemonic (True-Variation-Magnetic-Deviation-Compass): To convert True to Compass: T → V → M → D → C (apply each correction) To convert Compass to True: C → D → M → V → T (apply each correction in reverse)
Rules:
- Name East means compass heading is less than magnetic/true
- Name West means compass heading is more than magnetic/true
Compass Bearing Correction — Practical Example
A lighthouse bears 045° by compass. Deviation on this heading: 2° East Variation: 3° West
Magnetic bearing = Compass + Deviation (East positive): 045° + 2° = 047°M True bearing = Magnetic + Variation (West negative): 047° – 3° = 044°T
The lighthouse bears 044° True.
Checking Compass Error at Sea
By celestial observation:
- Compare compass bearing of sun at rising/setting with tabulated amplitude
- Compare gyro and magnetic compass headings (gyro error from transit or celestial must be known)
By transit:
- When two charted objects come into line (transit), compare the charted transit bearing (True) with the compass bearing. The difference is the total compass error.
Frequently Asked Questions
Q: Why does deviation change with heading? The ship’s steel and magnets are fixed to the hull. As the ship turns, their orientation relative to Earth’s magnetic field changes, altering the local field that deflects the compass needle.
Q: Do modern ships still use magnetic compasses? Yes — SOLAS requires a magnetic compass as a backup to gyro compass. The magnetic compass requires no power and cannot be jammed or suffer software failure.
Q: What is compass swinging? A professional compass adjuster boards the ship, moors at a buoy or in open water, and takes the ship around all compass headings while adjusting small magnets and soft iron correctors to minimise deviation.
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