Pancyprian Compass & Instruments common mistakes
Pancyprian Compass & Instruments · Understand the traps and how to avoid them.
When computing deviation from a deviation table or card, the candidate applies the tabulated deviation for the ship's heading directly to the magnetic course without interpolating for the actual heading, or applies it with the wrong sign (adds instead of subtracts, or vice versa).
Deviation tables are usually tabulated at 30° or 45° intervals, and candidates often grab the nearest value without interpolation, especially when the actual heading falls between two tabulated headings. Sign errors occur because the candidate forgets whether deviation is east or west and how it combines with variation, or because they memorise 'deviation east, compass least' without understanding the relationship. Under exam time pressure, they skip drawing a simple diagram to confirm the sign.
What to do instead
Always draw the compass error diagram: true north, magnetic north, compass north, with deviation and variation arrows. For interpolation, use linear interpolation between the two nearest tabulated headings, and state the sign explicitly (east deviation means compass north is east of magnetic north, so compass heading is less than magnetic heading). Practise converting among true, magnetic, and compass headings using the mnemonic 'True Virgins Make Dull Company' (True + Variation = Magnetic; Magnetic + Deviation = Compass) and its reverse, and verify each answer by working backwards.
The candidate confuses variation and deviation, attributing variation to the ship's magnetism or deviation to the Earth's field, and then applies the wrong correction to the wrong heading.
Both are angular differences between norths, and the symbols and formulas look similar. Candidates often memorise definitions without anchoring them to physical causes: variation is caused by the Earth's magnetic field and depends on geographic position and time; deviation is caused by the ship's own magnetic field and depends on the ship's heading. In exam questions that describe a change of position or a change of heading, the candidate applies the wrong correction because they did not link the cause to the effect.
What to do instead
Build a clear mental model: variation changes when you move on the chart or when the chart's annual change is applied; deviation changes when the ship's heading changes or when magnetic material near the compass changes. For every problem, first ask 'what changed?' If position changed, update variation from the chart's compass rose (including annual change). If heading changed, look up deviation from the ship's deviation card. Write the cause next to each correction in your working.
When asked to explain the principle of a magnetic compass, the candidate describes only the 'compass needle points north' idea and omits the role of the directive force, the horizontal component of the Earth's field, the damping fluid, and the pivot arrangement, or confuses magnetic dip with deviation.
Candidates often study the compass as a black box and memorise operational procedures without understanding the underlying physics. The exam expects an explanation of how the compass card aligns with the horizontal component of the Earth's magnetic field, why the card is immersed in fluid (to damp oscillations and reduce pivot wear), and why the compass is mounted in gimbals (to keep the card horizontal). Magnetic dip (inclination) is often confused with deviation because both involve the vertical component of magnetism, but dip is a property of the Earth's field at a location, not a ship-induced error.
What to do instead
Write a one-page summary of the magnetic compass principle: the compass card's magnets align with the horizontal component (H) of the Earth's magnetic field; the vertical component (Z) causes dip, which is why the card is balanced with a dip corrector or why the compass is designed for a specific latitude zone; fluid provides damping; gimbals keep the card horizontal. Distinguish dip from deviation by location: dip varies with latitude and is corrected by the compass manufacturer's balancing, while deviation is ship-specific and corrected by adjustment. Test yourself by explaining why a compass at the magnetic equator has no dip and why a compass near the magnetic pole is unreliable.
The candidate treats the gyrocompass as if it were a magnetic compass and fails to explain why it seeks true north, or describes the gyrocompass as 'pointing to the North Star' or 'using the Earth's rotation' without explaining the mechanism of precession and the effect of latitude and speed.
Gyrocompass questions often ask for the principle of operation, and candidates who have only memorised 'it uses a gyroscope' cannot explain the interplay of the gyroscope's inertia, the Earth's rotation, gravity, and the damping mechanism. They also confuse the gyrocompass with a gyroscope that simply maintains a fixed direction in space, missing the north-seeking property. Questions about latitude error, speed error, and settling time are then answered incorrectly because the underlying principle is not understood.
What to do instead
Study the gyrocompass as a system: a spinning gyroscope with three degrees of freedom, constrained by gravity (via a pendulous mass or mercury ballistic) so that the Earth's rotation causes precession toward true north; damping brings it to rest at the meridian. Learn the errors: latitude error (the gyro settles slightly east or west of true north depending on latitude), speed error (caused by the ship's own velocity adding to the Earth's rotation), and ballistic deflection during course or speed changes. Practise explaining why a gyrocompass does not work well at high latitudes or during rapid acceleration, and how the speed error correction is applied (usually by a manual setting or automatic input from the log).
When describing compass adjustment procedures, the candidate lists the correctors (B, C, D, E spheres, Flinders bar, heeling magnet) but mixes up which corrector addresses which deviation component, or states that adjustment eliminates deviation completely rather than reducing it to a small residual value.
The names and functions of correctors are technical and easily confused: fore-and-aft B correctors, athwartship C correctors, quadrantal D and E correctors, the Flinders bar for vertical induced magnetism, and the heeling magnet for permanent vertical magnetism. Candidates often memorise the list without understanding the magnetic theory (hard iron vs soft iron, permanent vs induced). They also overstate the outcome, believing adjustment removes all deviation, when in fact it only reduces it to a few degrees, and a deviation card is still required.
What to do instead
Create a table mapping each corrector to the type of magnetism it compensates: B and C correctors for permanent (hard iron) horizontal magnetism; D and E correctors for induced (soft iron) horizontal magnetism; Flinders bar for vertical induced magnetism from the Earth's field; heeling magnet for permanent vertical magnetism. Learn the adjustment sequence (usually with the ship on specific headings) and the fact that residual deviation is recorded on a deviation card. Practise explaining why adjustment cannot eliminate deviation completely (because the ship's magnetic field changes with heading, cargo, and time) and why a deviation card is still needed.
The candidate fails to account for the annual change of variation when extracting variation from a chart's compass rose, or applies the annual change in the wrong direction (adds when it should subtract).
Chart compass roses state variation with an annual change (e.g., 'Variation 4°15' W (2015), annual change 8' E'). Candidates often use the stated variation as if it were current, ignoring the year of the chart and the current year, or they apply the annual change without checking whether it is east or west and whether it increases or decreases the variation. The arithmetic is simple but the sign convention is easy to get wrong under exam pressure.
What to do instead
Always note the chart's edition year and the current year (given in the question). Calculate the number of years elapsed, multiply by the annual change, and apply it in the correct direction: an east annual change reduces west variation and increases east variation; a west annual change does the opposite. Write the calculation explicitly: 'Variation 2015: 4°15' W; annual change 8' E; years elapsed 10; total change 80' E = 1°20' E; variation 2025 = 4°15' W - 1°20' = 2°55' W.' Double-check by asking whether the variation should be larger or smaller.
When describing the installation and care of a magnetic compass, the candidate places the compass without considering the magnetic environment, or omits key checks such as the effect of nearby electrical equipment, the importance of the binnacle being non-magnetic, and the need for regular swing and adjustment.
Candidates often treat installation as a purely physical task (mount it on the centreline, away from windows) and forget the magnetic requirements: the compass must be located where the magnetic field is as undisturbed as possible, away from speakers, motors, radios, and ferrous structures; the binnacle must be non-magnetic (brass, aluminium, or fibre-glass); and the compass must be checked and swung after installation or after any structural change. Exam questions about 'why is the compass mounted here?' or 'what care is required?' are answered with generic statements rather than specific magnetic reasoning.
What to do instead
Study the installation requirements as a checklist: location on the fore-and-aft centreline, as high as practicable, away from magnetic interference (electrical cables, loudspeakers, radar, ferrous objects), with a clear view of the horizon; binnacle constructed of non-magnetic material; provision for correctors; access for adjustment. For care, include: regular checks of the deviation card, swinging after dry-docking or structural alterations, checking the fluid level and bubble, avoiding nearby magnetic objects, and keeping the compass cover clean. Practise answering 'why' for each requirement, not just 'what'.
The candidate confuses the functions and reading methods of related navigational instruments, such as the fluxgate compass, the gyro repeater, the magnetic compass, and the GPS compass, or incorrectly describes how a repeater is synchronised and what errors it may have.
Modern bridges carry several heading sensors, and candidates often learn only the main compass. They may not know that a fluxgate compass measures the Earth's magnetic field electronically and feeds repeaters, that a gyro repeater is a slave to the master gyrocompass and can have its own alignment error, or that a GPS compass derives heading from multiple antennas and is not affected by magnetism but can be affected by multipath. Questions that ask 'which instrument would you use?' or 'what is the source of error?' are answered by guessing rather than by matching the instrument's principle to the situation.
What to do instead
Build a comparison table for heading instruments: magnetic compass (direct magnetic, affected by deviation/variation), gyrocompass (north-seeking by Earth's rotation, affected by latitude and speed error), fluxgate compass (electronic magnetic sensor, feeds repeaters, still subject to deviation), gyro repeater (slave, alignment error possible), GPS compass (satellite-derived, no magnetic error, possible multipath). For each, note the principle, the typical error sources, and the checks (e.g., comparing repeaters with the master, checking the azimuth ring alignment). Practise scenario questions by identifying the instrument and its error source before answering.
The candidate calculates compass error using the formula 'Compass Error = Deviation + Variation' but forgets that the signs must be treated algebraically (east positive, west negative), or uses the wrong formula for converting from compass to true versus true to compass.
The relationship between true, magnetic, and compass headings is simple but sign-sensitive. Candidates often memorise 'add east, subtract west' without understanding which direction the conversion is going, or they mix up the order of operations. When the problem involves both variation and deviation, they may add them with the wrong signs, especially when one is east and the other west. The result is a heading that is off by twice the error, which is a common distractor in multiple-choice questions.
What to do instead
Use a consistent sign convention: treat east as positive (+) and west as negative (-). Then True = Magnetic + Variation (with sign); Magnetic = Compass + Deviation (with sign); so True = Compass + Deviation + Variation. For the reverse, subtract. Always draw the north diagram to confirm the direction of each correction. Work through several examples with mixed east/west errors and check by converting back. Memorise the mnemonic 'True Virgins Make Dull Company' and its reverse 'Can Dead Men Vote Twice' (Compass + Deviation = Magnetic; Magnetic + Variation = True) to keep the order straight.
When answering questions about compass swinging and the production of a deviation card, the candidate describes the procedure as a single step or omits the need to swing on multiple headings, to use a known reference (such as a transit or the gyrocompass), and to record residual deviation after adjustment.
Candidates often know that a compass is 'swung' but not the details: the ship is turned through a series of headings (usually eight or twelve), the compass error is measured against a known true bearing (from a transit, celestial observation, or a gyrocompass with known error), correctors are adjusted to minimise deviation, and the residual deviation is recorded on a card. They may also forget that the swing should be repeated after major structural changes or cargo changes, and that the card must be dated and signed. Exam questions that ask 'what is the purpose of swinging?' or 'what must be done after adjustment?' are answered vaguely.
What to do instead
Learn the swing procedure as a sequence: prepare the ship (secure loose magnetic objects, note cargo), choose a calm day with good visibility, use a known reference (transit bearings, gyrocompass, or celestial azimuth), turn through the cardinal and intercardinal headings (eight headings) or more, at each heading compare the compass bearing with the true bearing to find the error, adjust correctors to reduce deviation, then record the residual deviation on a deviation card. Emphasise that adjustment reduces but does not eliminate deviation, and that the card must be updated after any change that affects the magnetic field. Practise describing the procedure in your own words with the reasons for each step.