Reference

Compass correction

Convert between true, magnetic and compass bearings using variation and deviation. The essential CADET rule explained.

This is the core of Day Skipper navigation. Once you understand the three-step relationship — True → Magnetic → Compass — compass work becomes straightforward.

Related reference

COLREGS — rules of the road Navigation lights Passage planning checklist

The three types of bearing

True (T)
T
Relative to
geographic North Pole
↔
± Variation (V)
from chart
Magnetic (M)
M
Relative to
magnetic north
↔
± Deviation (D)
from compass card
Compass (C)
C
What your
compass shows
BearingWhat it referencesWhere you get it
True (T)Geographic North PoleMeasured from a chart using parallel rules and compass rose
Magnetic (M)Magnetic NorthApply variation to true bearing
Compass (C)What your compass readsRead directly from your compass

Variation

Variation is the angle between True North and Magnetic North at a given location. It is printed on charts and updated annually.

Example: variation 2°W, true bearing 085°T → magnetic = 085 + 2 = 087°M

Deviation

Deviation is the error in your onboard compass caused by local magnetic influences — engine, electronics, steel fittings. It varies with the vessel’s heading.

Example: deviation 3°E, magnetic bearing 087°M → compass = 087 − 3 = 084°C

The CADET rule

Memory aid for converting Compass → True (adding easterly errors):

C A D E T Compass Add Deviation East True CADET: going from Compass to True, Add East errors. Reversed: True to Compass, Subtract East.

True Virgins Make Dull Company (True → Variation → Magnetic → Deviation → Compass)

True to Compass (T → C): subtract East, add West Compass to True (C → T): add East, subtract West Apply variation first (T ↔ M), then deviation (M ↔ C).

Calculator

Enter any one bearing plus variation and deviation to find the other two.

Worked example

Situation: You plot a course of 090°T. Variation 3°W, deviation 2°E. What do you steer?

Step 1 — True to Magnetic: 090°T, variation 3°W → ADD west variation 090 + 3 = 093°M Step 2 — Magnetic to Compass: 093°M, deviation 2°E → SUBTRACT east deviation 093 − 2 = 091°C Steer 091° on your compass.

Quick reference — what to add or subtract

Direction of conversionEast errorWest error
True → MagneticSubtractAdd
Magnetic → CompassSubtractAdd
Compass → MagneticAddSubtract
Magnetic → TrueAddSubtract

Memory: going towards Compass (T→C), subtract East. Going towards True (C→T), add East.