Pancyprian Compass & Instruments cheat sheet

A marine sextant measures the angle between two objects (e.g., horizon and celestial body) using a movable index mirror and a fixed horizon mirror; the arc reading is the observed altitude.

Sextant index error is the error on the arc or off the arc; on the arc it is subtracted from readings, off the arc it is added; index correction = -(IE) when on the arc.

The vernier scale on a sextant allows reading to 0.1' or 0.2'; the main arc is graduated in whole degrees and minutes.

An aneroid barometer uses a sealed flexible capsule that expands/contracts with atmospheric pressure; a barograph continuously records pressure on a rotating drum.

A hygrometer measures relative humidity; a psychrometer uses wet-bulb and dry-bulb thermometers and the depression to find dew point and humidity.

A chronometer is a precision timepiece with a temperature-compensated balance; its rate is the daily gain or loss, applied to obtain GMT/UT.

A log measures speed and distance through the water; a Doppler log measures speed over ground by frequency shift of acoustic pulses reflected from the seabed.

An echo sounder measures depth by timing the round trip of an acoustic pulse; depth = (velocity of sound in water × time)/2, with velocity about 1500 m/s.

A rate-of-turn indicator (gyro or magnetic) shows angular velocity of the vessel; a rudder angle indicator shows rudder position relative to the fore-and-aft line.

A magnetic compass aligns with the horizontal component of Earth's magnetic field; the directive force is proportional to H·cos(dip) and decreases near the magnetic poles.

A freely suspended magnet settles along the magnetic meridian; compass north is the direction indicated by the compass needle, affected by deviation and variation.

Deviation is the angle between compass north and magnetic north, caused by the vessel's own magnetic field; it changes with heading and is recorded on a deviation card.

Variation (magnetic declination) is the angle between magnetic north and true north at a place; it changes slowly with time and position and is shown on charts.

A gyrocompass uses a rapidly spinning rotor with two degrees of freedom; Earth's rotation and gravity cause the spin axis to align with true north (the meridian).

Gyrocompass north-seeking depends on latitude; it is unreliable near the poles where the horizontal component of Earth's rotation is small.

A gyrocompass has a settling time after starting (typically 1–4 hours) and must be checked against a known true bearing or the sun's azimuth.

A magnetic compass is affected by the ship's induced and permanent magnetism; a gyrocompass is unaffected by the ship's magnetism but has latitude and speed errors.

Conversion rule: True = Compass + Deviation + Variation (east positive, west negative); rearrange to find compass from true: Compass = True − Deviation − Variation.

Compass error = deviation + variation; when both are east, the compass error is east; when mixed, subtract the smaller from the larger and take the sign of the larger.

Deviation is found from a deviation card or by comparing the compass bearing of a distant object with its true bearing; deviation = compass bearing − magnetic bearing.

Variation is taken from the chart's compass rose, adjusted for annual change to the year of use; east variation means magnetic north is east of true north.

A magnetic compass is subject to deviation that varies with heading; a deviation card lists residual deviation on each cardinal and intercardinal heading.

Gyrocompass error includes latitude error (tangent latitude effect), speed error, and ballistic deflection; speed error is proportional to speed × cos(latitude) and is east or west depending on course.

A gyrocompass speed error: for a vessel on a northerly course in the northern hemisphere, the gyro axis tilts and the compass reads low (west error); the error is maximum on north/south courses.

Magnetic compass deviation can be caused by permanent magnets, induced magnetism in soft iron, and vertical soft iron; adjustment reduces but does not eliminate deviation.

Magnetic variation is also called magnetic declination; the line of zero variation is the agonic line, and the line of equal variation is an isogonic line.

Compass adjustment (swinging the compass) uses corrector magnets: fore-and-aft and athwartship magnets correct semicircular deviation; vertical magnet corrects heeling error.

Quadrantal deviation is corrected by soft iron spheres (Kelvin's spheres) or soft iron correctors placed athwartships and fore-and-aft.

The Flinders bar corrects quadrantal deviation caused by vertical soft iron (e.g., the ship's funnel and masts) when the ship changes latitude or heel.

Semicircular deviation is caused by permanent magnetism and horizontal soft iron; it is corrected by permanent magnets in the binnacle.

The deviation card is produced after adjustment by swinging the ship through 360° and comparing compass bearings of a distant object or celestial body with true bearings.

Residual deviation after adjustment is recorded on the deviation card; it should be applied to all compass readings, not ignored.

Heeling error is caused by vertical permanent magnetism and is corrected by a vertical magnet in the binnacle; it changes with latitude and should be checked after large latitude changes.

Adjustment should be carried out with the ship upright and, ideally, on a steady heading; correctors are placed in the binnacle and secured.

A compass adjuster records the residual deviation on a card; the card should be dated and signed and kept near the compass.

A magnetic compass consists of a bowl filled with liquid (alcohol-water mixture), a float, a graduated card, and magnets; the liquid dampens oscillation and reduces friction.

The compass card is graduated in degrees clockwise from north; the lubber line marks the fore-and-aft line of the vessel and is the reading line.

A binnacle houses the compass and contains corrector magnets (fore-and-aft, athwartships, vertical) and soft iron spheres; it should be sited on the centreline, away from magnetic material.

The compass bowl is mounted in gimbals to keep the card horizontal as the ship rolls and pitches; the gimbal axis should be fore-and-aft and athwartships.

Compass liquid should be free of bubbles; a bubble indicates a leak or low liquid and should be rectified by a compass adjuster.

A magnetic compass should be kept away from electrical equipment, loudspeakers, radios, and steel structures; a minimum distance from magnetic influences is required.

A gyrocompass is installed near the centre of roll and pitch to minimise acceleration errors; the master compass sends signals to repeaters at the steering position and on the bridge wings.

Gyrocompass repeaters should be checked against the master compass and against a known true bearing; the gyro should be run continuously to maintain accuracy.

A magnetic compass is checked for deviation at regular intervals and after any structural change, cargo that is magnetic, or a lightning strike; the deviation card is updated.

A sextant is used for celestial observations; the observed altitude is corrected for index error, dip, refraction, semi-diameter, and parallax to obtain true altitude.

A chronometer provides accurate time for celestial navigation; its rate is applied to obtain UT, and it should be checked against radio time signals.

A log and echo sounder provide speed and depth; the log distance is used in dead reckoning, and the echo sounder is used for pilotage and safety.

A gyrocompass repeater and a magnetic compass are compared to check for deviation and gyro error; the difference between true and compass bearings is the compass error.

An automatic identification system (AIS) and radar are electronic aids; they do not replace compass checks but provide position and collision avoidance information.

A course recorder and a rudder angle indicator assist in monitoring the vessel's track and steering; they are not compasses but are related bridge instruments.

A barometer and barograph are used to predict weather changes; a fall in pressure of 1 hPa/h or more indicates a developing depression.

A hygrometer and thermometer are used to determine dew point and fog risk; the difference between air and sea temperature can indicate advection fog.

A magnetic compass and a gyrocompass are both required on SOLAS vessels; the gyrocompass is the primary heading reference, and the magnetic compass is the backup.