What is the angular difference between true north and either magnetic or grid north called?

The angular difference between true north and either magnetic or grid north is referred to as declination. This concept is important in navigation and map reading because it accounts for the discrepancies that arise from the Earth's magnetic field and the definition of true north.

True north is the geographic north pole, the fixed point in the Northern Hemisphere around which the Earth rotates, while magnetic north refers to the direction a magnetic compass points, which is influenced by the Earth’s magnetic field. Grid north, on the other hand, pertains to the orientation of maps based on a grid system.

Understanding declination is crucial for accurate navigation, as failure to consider this angle can lead to significant errors in determining direction. By knowing the degree of declination for your area, navigators can adjust their bearings to ensure they are moving towards true north rather than being misled by magnetic north or grid north.

Inclination, deviation, and orientation refer to different concepts. Inclination pertains to the angle at which magnetic field lines enter the Earth’s surface. Deviation is about compass error due to local factors like nearby metal objects. Orientation generally refers to the process of determining one's position or direction but does not specifically define the angular difference between true north and other north references.