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Compass deviation refers to the error in the readings of a magnetic compass caused by local magnetic fields that may be present in the vicinity of the compass itself. This local magnetism can be influenced by various factors, such as the materials and electrical equipment nearby, which can distort the magnetic field that the compass relies on for accurate direction finding.

The correct understanding of compass deviation is important for navigators and pilots, as it highlights the inaccuracies that may arise in navigation due to environmental influences rather than the inherent properties of the compass. By recognizing and compensating for these deviations, users can achieve more accurate navigation and avoid potential dangers associated with inaccuracy during travel or flight.

In contrast, the other choices relate to different concepts in navigation. The difference between true north and magnetic north is referred to as magnetic declination. The angle of elevation of the compass is not a defined term in relation to compass reading. The rotation of the compass in flight may refer to the compass card's movement but does not define deviation. Thus, the selection regarding local magnetism captures the essence of what compass deviation is.