Well...for hundreds of years, magnetic fields of the Earth were the primary means of determing your bearing using an ordinary compass. Navagation isn't affected by magnetic fields so much as it is facilitated by magnetic fields provided the fields are due to the Earth's magnetic 'dipole' or 'bar-magnet' field. These fields can, however, be affected by solar storms which cause' magnetic storms' that can change the local direction to the magnetic pole by many degrees for periods of several hours or even days.