Refer to figure.
The Earth has a magnetic field that has the same properties as any other simple magnet you may have seen. Because of this, the Earth’s magnetic field consists of magnetic field lines flowing from one pole to another. These lines have some properties that are very important to understand as they greatly affect our navigation on Earth and thus, our navigation instruments such as the magnetic compass. In fact, these magnetic lines are not straight lines spread equally around the Earth. They are straight and horizontal close to the equator, but the higher the latitude, the more these lines tend to converge towards the Earth’s poles. Therefore, they will become more and more inclined until being fully vertical at the pole. As a consequence, the pendulous magnet will also increasingly tilt towards its vertical and produce greater instrument errors (turning, acceleration…).

The magnetic lines converge at the magnetic poles, which is different from the geographic pole (the axis around which the Earth is spinning). Another thing to consider is that the magnetic poles continuously change location. For instance, right now the magnetic North pole is situated approximately in Northern Canada, but it is moving at a rate of about 50km/year towards Siberia. On the other hand, the South magnetic pole is moving at around 15 km/year, the same rate at which the North magnetic pole used to move as well as a few years ago.