Adjustment: The process of eliminating compass error sometimes called Compensation.

Athwartship:  Across the ship from side to side or, in other words perpendicular to the centerline of the vessel.

Card: The compass part imprinted with the heading graduations, commonly referred to as the dial.

Built-in correctors (BIC): An assembly of two brass or aluminum rods with magnets that can be rotated to create a magnetic field, equal in strength but opposite in polarity to the boat’s disturbing magnetic field, thereby eliminating compass deviation.

Cardinal points: N 000°, S 180°, E 090° and W 270°

Course over ground: The track over ground of the vessel’s course vector combined with leeway and drift vectors.

Dead Reckoning: Determining the position of a vessel by adding to last fix the vessel’s surface course and speed for a given time.

Deviation: The angle between the compass heading and the corresponding magnetic direction.  The differences caused by magnetic influences onboard the boat, and the change in the boat’s heading.

Intercardinal points: NE 045°, SE 135°, SW 225° and NW 315°

Lubber Line or Lubber’s Line: The vertical reference line where the user reads the heading.

Magnetic Dip: The angle between the horizontal and the lines of force of the earth’s magnetic field at any point.

Pelorus:  A device with a compass-like card and rotatable pointing and sighting vanes used to take bearings.

Real Time Heading Data: continuously delivering accurate heading data.

Swinging Ship: The process of placing a vessel on various headings and comparing magnetic compass readings with the corresponding magnetic directions to determine deviation.

Transient Deviation: Changes in deviation resulting from variance in the magnetic potential of a disturbing field source or its change in position in relation to the magnetic compass.

Variation: The horizontal angle between magnetic north and true north at a particular location.