You mean, that the same text book that tells you the Earth is round tells you the shape of the magnetic flux lines. Wow, what a coincidence, it must be correct. 
You can prove the shape of magnetic fields to yourself. Get a bar magnet, place a piece of paper on top and sprinkle iron filings on the paper, the iron filings line up with the magnetic field lines.
Now go out and get a cheap compass, you will find that it mostly points north, but if you look closely you will see that the needle doesn't sit quite flat, ( unless you are on the equator ) that tilt is called magnetic inclination and it varies depending on where you are on earth. With the flat earth model the bar magnet field lines will intersect the earth surface at right angles. So on a flat earth a compass will point up and down, not north and south.
The traditional flat earth model ( see FAQ ) overcame this problem by proposing the earth's magentic field was like a speaker magnet with the south pole all the way around the disk, But since we know the South Magnetic pole is a single point ( and you can go there yourself to prove it ) the magnetic disk model can't be correct. So the flat earth model falls flat.
Flat earth does not reflect reality, whereas round earth does. Draw your own conclusions.
Or, you could actually think for yourself, for a change.
That's a curious response, considering I just showed you how to do exactly that. ...
What about, you go and buy a bar magnet and do the experiment for yourself, that way you can prove the magnetic field direction for yourself.
I'll say again that
every magnet is
not an axially magnetized bar magnet.
You're saying:
1. Bar magnets act a certain way.
2. Earth acts as a magnet.
Ergo, the Earth acts as a bar magnet.
This is wrong to assume.
Consider instead:
1. Bar magnets act a certain way.
2. Arc magnets can act in a number of different ways, one of them being consistent with how the magnetic field of a flat Earth acts.
3. The Earth acts as a magnet.
Ergo, it is possible that the Earth acts as a series of arc magnets, and possible that it does not act as a simple axially magnetized bar magnet.
The way I see it is this. The earth is a flat disc on an infinte plane that forms the base or floor of the universe. Above the earth the galexy and the univeres rotate on an axis formed between the north pole and the north star Polaras. Between the north pole and the north start a magnetic field is formed. The north star is magnetic. The north pole is actually the magnetic south pole(even the REer's believe this) and the north start is magnetic north. compasses on earth are pulled into this magentic field. The REer's "south pole" does not exist at all becaue there is no certain location on the ice ring that surround the earth for the south pole and if there were a magentic south pole in antarcitca in a flat earth, everything woulf be turned on its head.
I'm sure Occam would disagree.
It's much easier and simpler to agree that the flat Earth's magnetic field acts as a series of arc magnets.