What is the most important item that determines how we measure deviation?

• A. Latitude
• B. Heading
• C. Speed
• D. Depth

Answer: (B)

Understanding compass deviation relies on having a true reference direction to compare against. Deviation is the error in the magnetic compass caused by the ship’s own magnetic field. To quantify that error, you compare the direction the vessel is actually pointing—the heading—with what the compass shows. The heading provides the true direction, so the difference between the heading and the compass reading is the deviation. Latitude, speed, and depth don’t set this angular difference: latitude relates to variation (a separate factor tied to location), while speed and depth affect other navigational aspects but not the compass error. For example, if the vessel’s heading is 090 degrees but the compass indicates 095 degrees, the deviation is 5 degrees. The heading is the essential reference for measuring deviation.