By insideSail
Automated technical check · version 1 ·
True, magnetic and compass north
Three references to distinguish before comparing numbers.
In this guide
Original schematic illustration. Read the explanation and limitations in the lesson.
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When two instruments show different values, the first question may be: “Are they using the same reference?” True north relates to the geographical reference. Magnetic north relates to the Earth’s magnetic field locally. Compass north is the reference indicated by the installed compass.
Variation and deviation are different
Magnetic declination, also called variation in many English documents, relates true and magnetic north. It depends on place and date. Compass deviation relates a compass indication to the magnetic reference and can be associated with installation and local influences. The word “error” without context does not identify which difference is involved.
- Geography: which north is the reference?
- Magnetism: which place and date does the value concern?
- Instrument: which indication and documentation are being compared?
The diagram visually separates the concepts. The spacing between arrows is illustrative and does not represent variation in Portugal, the United Kingdom or elsewhere. It is not a deviation card, compass-adjustment procedure or steering instruction.
Concepts in this lesson
Compass north
The north reference indicated by an installed compass, which may differ from magnetic north.
Magnetic north
The north reference associated with the local direction of the Earth’s magnetic field.
True north
The geographical north reference, distinct from magnetic and compass references.
Sources and references
- Magnetic declination ↗
NOAA NCEI · Difference between magnetic and true north; place and date
Version: Institutional page. Jurisdiction: International.
Checked on - How does a compass work? ↗
U.S. Geological Survey · Historical explanatory audio; stable physical terminology · Magnetic field and local interference
Version: Historical explanatory audio; stable physical terminology. Jurisdiction: International.
Checked on
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