Deviation in navigation refers to the error in reading a bearing from the compass caused by the magnetic influence of some nearby object, such as a metal post or an engine. In the case of boats, deviation can also be due to the materials from which the boat is constructed, and electric currents close to the compass may create a magnetic effect. Objects on a boat, such as radios, mobile phones, and engines, create their own magnetic fields, which can interfere with the compass and cause deviation. Deviation is unique to each vessel and changes depending on the boat and its heading. To minimize deviation, the compass should be kept away from all magnetic influences, and careful mounting of the compass can help reduce deviation. A compass corrector can adjust the steering compass to minimize errors and then produce a deviation card showing the remaining deviation. When converting from true to compass, the degrees of deviation should be added if they are west and subtracted if they are east, and the opposite should be done when converting from compass to true.