Magnetic Compass

General Information

• Note that there are two types of bearings, relative and absolute.  A relative bearing refers to the angle of an object relative to the ship’s head with zero degrees representing dead ahead.  An absolute bearing refers to the angle of the ship’s head relative to either magnetic north or true north with both magnetic or true north representing zero degrees.
• The magnetic compass is usually only used as a backup for the gyro compass because the magnetic compass doesn’t require power
• The magnetic compass points to magnetic north.  The north magnetic pole is a feature of the earth that moves over time and doesn’t represent a fixed geographical location
• Error between magnetic north and true (geopositional) north increases the further north you go
• The magnetic compass error is a combination of two influences:
  • The error produced by magnetic north and true north being in different locations
  • The error produced by the magnetism of the ship itself
• The magnetic error is frequently checked during navigation by the OOW.  As part of this, the helmsman may be requested to provide magnetic headings
• An expression called “boxing the compass” refers to the act of reciting all 32 divisions of the compass (see Point System)