Data
WMM
The World Magnetic Model captures the deviation of the magnetic field lines on earth from the true geographic north pole.
Usually the magnetic field lines point to the magnetic pole, which is located somewhere in north Canada. So for a place in Europe this means a deviation of about 3 degrees from true north.
But there are areas on earth where the field lines have a large variation due to local abnormalies of the molten iron core of the earth, which generates the magnetic field.
More information on the WMM at NOAA and on the WMM project page.