Conquer Compass Error:
Gyro vs. Magnetic, Deviation Demystified
From cadet to captain, mastering compass error is fundamental to safe navigation and exam success.
Every navigator faces it: the subtle but critical difference between what the compass says and where you're actually going. Compass Error is the umbrella term for all these discrepancies.
This guide will cover the causes of compass error (Variation and Deviation) and show you how to calculate corrections for both gyro and magnetic compasses.
1. Variation: The Earth's Magnetic Field
Definition
Variation: The angular difference between True North (geographic pole) and Magnetic North (Earth's magnetic pole) at your location.
The Earth's magnetic field is not uniform. The magnetic pole is located far from the geographic pole, resulting in a difference that varies by location and changes slowly over time.
- Where to Find It: Variation is printed on the compass rose of nautical charts. It also included in the ECDIS database.
- Annual Change: Because the magnetic field shifts, the variation has an annual change value (e.g., 6°E (2025) decreasing at 0.1° annually).
2. Deviation: The Ship's Magnetic Field
Unlike Variation (which is external), Deviation is caused by the vessel itself. The ship's metal hull, electrical wiring, and onboard equipment all create magnetic fields that interfere with the compass.
Permanent Magnetism
Caused by the hull's alignment with the Earth's magnetic field during construction.
Induced Magnetism
Temporary magnetism caused by the Earth's magnetic field acting on the ship's soft iron.
3. TVMDC: Converting Headings
Memorize this mnemonic: True Virgins Make Dull Company
This helps you convert from one heading to another.
- True (Heading)
- Variation (Apply East is Least, West is Best)
- Magnetic (Heading)
- Deviation (Apply East is Least, West is Best)
- Compass (Heading)
| Heading Type | Value | Calculation |
|---|---|---|
| True Heading (T) | 090°T | Starting Point |
| Variation (V) | 5°W | Apply "West is Best": 090° + 5° |
| Magnetic Heading (M) | 095°M | Result of Variation |
| Deviation (D) | 2°E | Apply "East is Least": 095° - 2° |
| Compass Heading (C) | 093°C | Final Compass Reading |
Stop the Manual Math
Let our Compass Error Calculator instantly convert between True, Magnetic, and Compass headings. Perfect for quick checks on the bridge.
Open Compass Error Calculator4. Calculating Gyro Error (Advanced)
Gyro compasses are generally more accurate than magnetic compasses, but they are still subject to error. Gyro error is the difference between True Heading and Gyro Heading.
The most accurate method for determining Gyro Error involves celestial observation (Azimuth or Amplitude). This requires:
- Taking a bearing to a celestial body (Sun, Moon, star, or planet).
- Calculating the body’s True Bearing using the Nautical Almanac and sight reduction tables.
- Comparing the True Bearing to the bearing observed on the gyro compass.
The formula: Gyro Error = True Bearing - Gyro Bearing.
5. The ABC Method
In celestial navigation, ABC tables are used to simplify the calculation of Azimuth. This calculation allows for gyro error to be found.
The formula to find Azimuth using ABC tables is
`tan Z = B / (A + cos Lat)`