Deviation refers to the response of the compass to local
magnetic fields caused by the presence of iron and electric currents.
These vary with Latitude and also with the components
used in the aircraft construction and the electrical equipment on board.
Because the Earth's magnetic field's inclination and
intensity vary at different latitudes, compasses are often balanced during
manufacture.
Compasses are then 'swung' or adjusted to minimise the
effect of deviation and these are noted at each cardinal heading with any
corrections being written onto a 'compass card' which is displayed in the
aircraft, usually near the magnetic compass.