Magnetic compass checks and the deviation card

A magnetic compass earns trust because it keeps indicating a heading without depending on the boat’s main electrical network. That independence does not make its reading automatically correct: the Earth supplies one correction, the vessel another, and careless checking can mix the two. A useful deviation card is therefore the result of controlled observations, not a comparison made once beside the berth.

Start by separating variation from deviation

Magnetic variation, also called declination, is the angle between true north and magnetic north. NOAA explains that it changes with both position and time, so a value taken from an old note or a distant cruising ground is not a sound basis for today’s conversion. Use the value and epoch shown on suitable current navigation information, or a recognised geomagnetic model for the place of the test.

Deviation is local to the vessel. Bowditch attributes it to magnetic influences around the installed compass, including magnetised material, metal structures and electromagnetic fields from direct-current circuits. Because those influences meet the Earth’s field differently as the boat turns, deviation changes with heading; it cannot be replaced by the variation printed on a chart.

Keep the conversion chain explicit: true direction, variation, magnetic direction, deviation, compass direction. Mark East or West beside every figure and use one sign convention throughout the worksheet. The familiar mnemonics used in navigation classes are only safeguards against arithmetic mistakes; they do not validate the source value or the observation.

Freeze the boat’s configuration before testing

Inspect the instrument before collecting numbers. The card should move freely and settle, the lubber line should be aligned as installed, and the bowl should not show a physical condition that makes the reading doubtful. A persistent bubble, sluggish card, damaged dome or loose mounting calls for inspection, not a larger correction pencilled onto the old card.

Record the electrical and physical setup. Batoo’s working sheet would note engine state and rpm, charging sources, battery switches, autopilot, radar, wipers, blowers, audio equipment, tablets, handheld radios and temporary metal gear near the helm. MCA guidance specifically warns that portable electrical equipment and steel objects can affect a magnetic compass, which makes repeatability more important than an unrealistically “clean” test arrangement.

Choose an independent reference whose uncertainty you understand. A charted transit can provide a true bearing when the vessel is exactly on the line; applying the local variation produces the magnetic bearing against which the compass is compared. Professional adjusters may use other established techniques. A GNSS course over ground is not automatically a heading reference: NOAA demonstrates that a cross-current can separate the ground track from the course steered.

Select open water with room to manoeuvre, modest traffic and conditions that permit a steady heading. If waves or gusts swing the bow faster than the card settles, the spread of readings mainly measures a poor test. Separating the helmsman and observer roles also reduces the temptation to steer toward the number one expects to see.

Observe headings around the full circle

Deviation on one heading says little about the rest of the compass. The underlying method is to steady the vessel on headings distributed around 360 degrees, compare the compass indication with the independent magnetic direction, and record the signed difference. Bowditch describes the remaining error after adjustment as residual deviation and places those values in a table for different headings.

For an owner’s diagnostic check, Batoo recommends beginning with the four cardinal and four intercardinal headings, while making clear that this is not a professional adjustment standard. Hold each heading long enough for a stable reading and repeat doubtful observations. Time, position, sea state and equipment configuration belong beside the numbers, because otherwise a later difference cannot be interpreted.

Calculate the error using the convention adopted on board, then run the conversion backwards as a sign check. An East-versus-West mistake is particularly dangerous because applying the wrong sign adds an error instead of removing it. If crew members use different worksheets, put a worked example on the card using invented training figures rather than leaving the convention implicit.

Resist the urge to create a smooth curve from scattered observations. Show the headings actually tested and the readings obtained; interpolate cautiously only where the pattern is stable. A sharp discontinuity, new asymmetry or poor repeatability is diagnostic information that should trigger another observation or an investigation of the installation.

Make the card operational rather than decorative

Identify the individual compass on the card. NOAA’s nautical terminology notes that each compass needs a separate deviation table, so readings from a steering compass should not be assigned to a hand-bearing compass or a transmitting magnetic sensor. Include the test date, area, method, relevant operating configuration and the residual East or West value for every measured heading.

Place the card where the person steering or checking the passage can use it. Its layout should support conversion in both directions: planning a compass course from a true course and recovering a true direction from a compass observation. The magnetic compass can serve as an independent cross-check on electronic heading only when its own residual error and current configuration are known.

Treat the card as dated evidence, not a lifetime certificate. Within its regulatory context, MCA guidance calls for properly adjusted required compasses and an available residual-deviation table; it also identifies installation, unreliability, structural work, nearby electrical or magnetic changes, excessive recorded errors and physical defects as reasons for adjustment. These are useful technical triggers on a private pleasure boat too, but the article does not convert UK commercial-ship guidance into a universal legal rule.

Know where an owner check must stop

An owner can detect, document and monitor error. Adjusting corrector magnets, soft-iron spheres or other compensating elements is a different job: changing one component may improve a heading while worsening another. Do not move correctors to chase a single observation, and do not hide an unstable instrument behind a freshly formatted table.

Seek a qualified compass adjuster when a compass is newly installed, errors become large or irregular, observations will not repeat, physical defects appear, or structural and electrical work has changed the environment around the instrument. Professional help is also the fail-safe choice when no independent reference of suitable quality is available. The exact legal requirement for who may adjust a compass depends on flag, vessel and operating regime.

After adjustment, keep the signed new card and retain the former one with a note of the intervening work. Short checks repeated under comparable conditions can then expose a trend rather than merely produce isolated numbers. Confidence in a magnetic compass should come from a transparent method, an honest uncertainty record and a clear threshold for escalation—not from its reassuringly analogue appearance.