Peter wrote:
The fluxgate would have still produced “some” value so it must intentionally do this above a certain latitude.
It is probably intentional but sufficiently close to the magnetic poles the inclination is large enough that the horizontal component of the magnetic field is very weak.
In the US, airways, VORs are rarely ever updated since installation, many going back to the 1960’s for the magnetic variation for radials. Airports and IFR procedures are updated more frequently, but most still use the 1990’s for variation. Pilot don’t know much about this but some do wonder why the GPS has a different track than the VOR radial or the final approach course on an IAP. 6 degrees is not uncommon in the US for the radials when compared to a GPS track using current magnetic variation.
