The planet on which we live is as a big magnet: with the two poles north and south different and opposite, they are not the same as the geographic poles and their positions changes from time to time. This makes the north measured with the compass is different from the geographic one called magnetic declination. Lines that connect points with the same magnetic declination, are called isomagnetics and they are representative of the not regular distribution of the magnetic field.
The true cause of the terrestrial magnetism is unknown yet: ancient theories explained the inside of the earth as done of magnetic material. Now, this sounds impossible, because of the high temperature in the fluid center of the earth. The magnetic field is originated by electric currents produced from the flux of the fluid center of the earth.
The plane show the geomagnetich chart of the Italy.