My goal: to calculate a car's orientation with respect to true north. The car will be traveling thousands of miles across both Australia and North America (without stopping), so simple adding the offset for where I typically am is not an option, as per various other forums suggestions. I was attempting to avoid using the GPS sensor to calculate orientation using direction, because the accuracy of this data is rather important
What I've found: Using a Magnetosensor seems to be a very good way to find magnetic north, and there are ways of finding the offset needed to point to true north (judging by the various magnetic calculators found online such as NOAA). https://www.ngdc.noaa.gov/geomag-web/#declination
Complications that have arisen: I stumbled across various world maps of magnetic declination, and the magnetic field doesn't seem to follow any pattern or continuity (dang physics, why u no easy), meaning that a mathematic approach to this is (apparently) impossible.
Where I went from there: My next intention was to combine data taken from a magneto sensor and from a GPS module to lookup, in a table, the relative offset needed to counteract the magnetic declination of the car at its current latitude and longitude. But finding a table of various different lat,long coordinates and their declination proved unsuccessful.
Ideas? Opinions? Help? Where should I go from here. Is it even worth compensating for Magnetic North? Or should I simply leave all data as-is and disregard the drift that may arise. Is there a mathematic approach I can take to this, or should I consider dedicating onboard space to housing the offset table? Or is there another approach entirely I did not consider.