Magnetic deviation is the error induced in a compass by local magnetic fields, which must be allowed for, along with magnetic declination, if accurate bearings are to be calculated. Relevant local magnetic fields include:
- the metal parts of the compass or the ship or vehicle it is travelling in.
- variations in the earth's magnetic fields caused by variations in the earth's crust and mantle
- variations caused by mountains, iron ore deposits, etc.
In nautical terminology deviation is the error caused by the metal parts of the ship, an error dependent on the magnetic heading (the angle the ship makes to magnetic north) whereas declination, also called variation, is the difference between true north and direction of the earth's magnetic field at that point without the ship. Of course variation in this sense is due to the location of the magnetic north pole as well as local geological conditions. The former sources of magnetic deviation vary from compass to compass or vehicle to vehicle. However, they are independent of location, and thus the compass can be calibrated to accommodate them. The latter sources of magnetic deviation vary according to location, and also vary over time (conditions in the earth's crust/mantle change over time.) Generally, they are indicated on maps, so you must take into account the deviation for your current location. In the short term, these changes are predictable, and maps generally indicate the year in which the deviation was determined, and an amount to add for each subsequent year, for a period of several years. However, after that period, the predictions would become increasingly inaccurate. Non-magnetic methods of taking bearings, such as astronomical observations, satellites (as GPS) or radio navigation, are not subject to magnetic deviation. Thus, a comparison of bearings taken with such methods with the bearing given by a compass can be used to compute local magnetic deviation.
