The magnetic compass, compass adjustment and the deviation card

What SOLAS and the standards require

Besides the properly adjusted standard compass (V/19.2.1.1), SOLAS requires a pelorus or compass bearing device (2.1.2), a means of correcting heading and bearings to true (2.1.3) and, from 150 GT, a spare compass (2.2.1). The IMO performance standard, resolution A.382(X) of 1977, asks for each compass to be properly compensated, with its residual deviations available near it, and for correcting devices in the binnacle. The detailed design and testing standard today is ISO 25862 (marine magnetic compasses, binnacles and azimuth reading devices), whose Annex G advises when to adjust.

When to adjust

UK guidance (MGN 610, Annex D) and the guidance in ISO 25862 Annex G, as quoted by AMSA, agree on the main triggers. A compass should be adjusted:

• when it is first installed;
• when it becomes unreliable;
• after structural repairs or alterations that could affect the ship’s permanent or induced magnetism;
• when electrical or magnetic equipment close to the compass is added, removed or altered;
• when the compass shows physical defects;
• when a record of deviations has not been maintained, or the recorded deviations are excessive;
• and, in the ISO guidance as quoted by AMSA, when the master considers it necessary for the safety of navigation, and no less often than every two years, at every dry-docking or after significant structural work.

MGN 610 adds checks after magnetic cargoes, electromagnetic lifting, a casualty involving severe contact or electrical charges, and lay-up.

Who may adjust

SOLAS leaves this to the flag. The UK expects a holder of the UK Certificate of Competency as Compass Adjuster (MGN 279 requires workshop experience, adjustment of at least 12 sea-going ships and examinations). If none is available and the master considers it necessary, a Master (Class 1) holder may adjust, with re-adjustment by a qualified adjuster at the next opportunity.

The deviation card, the compass book and the certificate

A good record set has three parts:

1. Deviation card or curve, posted near the compass, showing residual deviations on at least the cardinal and intercardinal headings, the date, the ship condition (for example loaded or ballast) and the corrector positions.
2. Compass deviation book, with each observation: date, time, ship’s head, observed and calculated bearing or heading, deviation. The STCW Code watchkeeping provisions require the standard compass error to be determined at least once a watch and, when possible, after any major alteration of course.
3. Adjustment certificate from the adjuster. Panama specifies its minimum content: ship name, IMO number, call sign, flag, place, adjuster’s name, stamp and signature, compass maker, sea and weather conditions, method used, and numerical and graphical results of deviation after correction.

If a transmitting magnetic compass (TMC) feeds heading to other equipment, MGN 610 advises separate deviation cards for the standard compass and the TMC repeater, because the correctors affect the two differently; the error can be as much as 2.5 degrees.

A note on coefficients (theory)

Adjusters split deviation into coefficients: A is constant on all headings; B and C vary once around the compass (semicircular deviation, corrected with magnets and the Flinders bar); D and E vary twice around (quadrantal deviation, corrected with the spheres). From the eight cardinal and intercardinal headings, approximately: A = mean of the eight; B = (E − W) ÷ 2; C = (N − S) ÷ 2; D = (NE + SW − SE − NW) ÷ 4; E = (N + S − E − W) ÷ 4. This analysis is in the UK compass adjuster syllabus; it helps you read an adjuster’s report, but the adjustment itself is for a qualified adjuster.