Oh lord - do you really want to be that bored to oblivion? Ok - no problemo... it'll be a bit, but I'll have a not-so-brief brush-over of what geomagnetic field observations are, how they are observed, and some resources for you to look at. I'll make sure to look over my resources and ensure at least one of 'em is a non-government individual. A lot of them are either university guys or industrial gone private.
For now, to be brief and have something to ponder think of it this way... you have molten rock with some ferrous materials in it. It's just a floatin there waiting to cool. Since there is nothing else to do, all the particles in the molten material that are magnetic start to line up with the ambient magnetic field. The substance finally cools and sets into rock, ore, pyroclastic leftovers of whatever kind... and you end up with an absolute indicator of the N/S alignment of the magnetic field of the earth when that sample was created.
So, take that a step further - you go to several sites (heck, three will do) along a general N/S track on any given land mass. You look at the orientation of the deposits at each site. You draw your line 'down' the map. Now everyone tends to agree that we at least know the general shape of most continents... and if not, then pick an island.
Here is where the problem comes in... when you draw your line, the North and South orientation would end up going from North to an arbitrary spot at an angle to the rim of the Flat Earth, rather than 'straight down' to the familiar 'Polar' South depending on where you are on the Flat Earth and how far South you are as it will be more and more exaggerated as the geomagnetic data would show increasing rotation discrepancies around the circumference of the plane the closer you get to the rim - or the farther you get from the center.
Hopefully, that made a bit more sense with a bit more detail. I'll work on the data stoofs tomorrow. Just realized I gotta hit it. Gotta go get tomatoed by my class tomorrow and Thursday.
Take care,
- Hap