Originally Posted by
MechEngr
True, but the magnetic heading can effectively change overnight. It's a weird side effect of the magnetic poles not staying put.
Edit: Came across the following website because a certain drone suddenly was unable to fly when the magnetic database changed on Jan1.
https://www.ngdc.noaa.gov/geomag/WMM/ I don't know the particular correlation to the avionics side of this but looking at the loops and whorls of the magnetic field leaves me dizzy. They track not only the current offsets but the rate at which the offsets change; apparently 9 dimensions all together.
I think the article related the bug to the true direction, not the magnetic direction of a runway.