<Li> D = deviation caused by magnetic material (mostly iron and steel) on the vessel; </Li> <Li> CC = compass course . </Li> <P> Magnetic deviation is the angle from a given magnetic bearing to the related bearing mark of the compass . Deviation is positive if a compass bearing mark (e.g., compass north) is right of the related magnetic bearing (e.g., magnetic north) and vice versa . For example, if the boat is aligned to magnetic north and the compass' north mark points 3 ° more east, deviation is + 3 ° . Deviation varies for every compass in the same location and depends on such factors as the magnetic field of the vessel, wristwatches, etc . The value also varies depending on the orientation of the boat . Magnets and / or iron masses can correct for deviation, so that a particular compass accurately displays magnetic bearings . More commonly, however, a correction card lists errors for the compass, which can then be compensated for arithmetically . Deviation must be added to compass bearing to obtain magnetic bearing . </P> <P> Magnetic declination has a very important influence on air navigation, since the most simple aircraft navigation instruments are designed to determine headings by locating magnetic north through the use of a compass or similar magnetic device . </P>

At a specific location the value of magnetic variation