Magnetic deviation
![Slots for small magnets in a binnacle. The magnets are positioned to reduce the effect of the ship's permanent magnetization on the compass.](/Images/godic/202501/25/CompassAdjusting0134912.JPG")
![Diagram of a 19th-century binnacle housing a compass. It has two soft iron spheres (Q) to correct for induced magnetization.](/Images/godic/202501/25/PSM_V34_D323_The_compensating_binnacle4912.jpg")
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. (More loosely, "magnetic deviation" is used by some to mean the same as "magnetic declination". This article is about the former meaning.)