This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. At any point, the Earth's magnetic field is characterized by a direction and intensity which can be measured. From these these elements, all other parameters of the magnetic field can be calculated. To measure the Earth's magnetism in any place, we must measure the direction and intensity of the field. D and I are measured in units of degrees.
Magnetic declination is the angle between magnetic north and true north.
D is considered positive when the angle measured is east of true north and negative when west. Magnetic inclination is the angle between the horizontal plane and the total field vector. Yes, the magnetic field is different in different places. It is so irregular that it must be measured in many places to get a satisfactory picture of its distribution. This is done at the approximately 200 operating magnetic observatories world-wide and at several more temporary sites.