Vertical magnetic line (say, magnetic axis) is quite unstable. It keeps on shifting according to changes in electric currents produced in the core of the Earth; with the result that magnetic North Pole also keeps on shifting all the time.
The geographic North Pole has remained stationery for centuries but magnetic North Pole has played a ‘catch me if you can’ game over an extensive area of the world. Around the year 1000 AD magnetic North pole was situated in western Russia. Thereafter, it gradually shifted eastward reaching the Pacific Ocean in about 1300 AD and continued westward arriving in Canada around 1900 AD. Today magnetic North Pole is situated north of Canada at 82.7ºN latitude and 114.4ºW longitude — i.e., southward and far away from the geographic or true North Pole. Amazing conclusion is that if compass is placed on geographic North Pole, the needle of compass will point towards south, not north. If the same compass is placed on magnetic North Pole, it obvious that its needle will not point towards any particular direction. Magnetic North Pole keeps on shifting from its location slowly but steadily so who knows where it will be after some decades!
It needs to be noted that the needle of compass in ships and planes always points towards magnetic North Pole whereas aerial flight or voyage of ship has to be charted according to geographic directions. Therefore, navigators calculate angle between two Poles and adjust direction accordingly.