Navigation and Seamanship

2.1.6. Compass error

The sum of variation and deviation is called as compass error. The navigator must understand thoroughly how to apply variation, deviation, and compass error, as he is frequently required to use them in converting one kind of direction to another.

Direction can be expressed three ways;

True direction, when referred to the true (geographic) meridian as the reference of measurement.

Magnetic direction, when referred to the axis of the magnetic meridian as the reference of measurement.

Compass direction, when referred to the axis of the compass card as the reference of measurement.

Any given direction may be expressed in all three of these ways, if it is understood that:

True differs from magnetic by variation.

Magnetic differs from compass by deviation.

Compass differs from true by compass error.