The compass heading of a vessel differs from the true heading due to what factor?

The compass heading of a vessel differs from the true heading primarily due to compass error, which includes factors that affect how a magnetic compass functions. Compass error itself is the difference between the true heading (based on geographic north) and the heading indicated by the magnetic compass.

Within compass error, two specific components contribute: variation and deviation. Variation refers to the difference between magnetic north and true north, which can change based on geographic location. Deviation is the error introduced by local magnetic influences, such as electronic devices, steel constructions, and other magnetic fields onboard the vessel, which can alter the compass reading.

Thus, when navigating, understanding that the compass heading can be affected by these various factors is crucial. However, when specifically discussing why the compass heading differs from true heading, compass error encompasses both deviation and variation, but the term itself indicates the overall discrepancy in navigation referenced against true heading.