The "fine detail" magnetic field changes throughout the day (like the tide) and the underlying pattern itself shifts such that a new "map" is issued every five years which is actually a spherical harmonic equation with a depth of 24 (IIRC) terms at least.
(Addendum: degree 12, my bad .. (or degree 790 if you're aiming a missile)
https://en.wikipedia.org/wiki/World_Magnetic_Model or
https://en.wikipedia.org/wiki/International_Geomagnetic_Refe...
choose your model adventure, these are 'coarse' global models with five year epochs, see also:
The Enhanced Magnetic Model (EMM) is a sister product of the NGDC featuring a much higher amount of data to degree and order 790, giving a wavelength of 51 km as opposed to the 3000 km of WMM.Ideally you also want nine sensors, one for each XYZ axis at three spread out vehicle locations: typically tail + each wing tip on an aircraft to get that mag flux differential across the craft.
It also helps to have ground base station that can record the diurnal flux (daily changes as the earth turns) from a fixed position to subtract from the nine readings on a moving vehicle.
Oh, yeah, heading also plays a role, so you need a Kalman filter formed from a butterfly wing motion pattern in order to subtract the field induced on a the craft by motion that varies with heading.
It's fun stuff.