Terrestrial Magnetism, Variation, Deviation and Compasses
Earth’s magnetic field
Terrestrial magnetism is modelled by a dipole whose magnetic poles are offset from the geographic poles. The field has horizontal (H) and vertical (Z) components; dip is the angle of the field to the horizontal, zero at the magnetic equator and 90° at the magnetic poles. A compass aligns with the horizontal component, so dip causes turning and acceleration errors. Isogonals join places of equal variation; the agonic line has zero variation. Variation is the angle between true north and magnetic north, east or west. It changes with position and slowly with time, so use the chart/date stated.
True, magnetic and compass
Apply variation and deviation in the correct order. From true to magnetic: magnetic heading = true heading − east variation, or + west variation (‘variation east, magnetic least’). From magnetic to compass: compass heading = magnetic heading − east deviation, or + west deviation. Deviation is compass-specific and depends on heading; it is found by compass swing and recorded on a deviation card.
Direct-reading compasses are simple magnetic compasses; indirect-reading systems use a remote flux valve or gyro-magnetic unit to supply a stable display (e.g. RMI/HSI). Both are subject to magnetic errors, but the gyro system reduces short-term oscillation.
Worked example
True heading 120°, variation 8°E, deviation 3°W. Magnetic heading = 120 − 8 = 112°. Compass heading = 112 + 3 = 115°. Check: from compass to true reverse the signs: 115 − 3 = 112; 112 + 8 = 120.
Traps
The most costly error is reversing variation/deviation. Remember ‘east is least’ for both conversions. Do not use a deviation value for a different heading. Chart variation may be several years old; apply annual change. In the Northern Hemisphere, acceleration error is often summarized as ‘accelerate north, decelerate south’; turning errors are largest passing north/south. Do not quote these as universal without hemisphere.