I've used expensive ($1-2 thousand USD) compasses that work really quite well, particularly if you don't suddenly spin around. So the tech exists, at least if y

I've used expensive ($1-2 thousand USD) compasses that work really quite well, particularly if you don't suddenly spin around. So the tech exists, at least if you ignore power and size restrictions and up the price range a little. I imagine cellphones don't bother to get a good compass. External metals are always a problem, though. You can incorporate accelerometer data which kinda helps to protect against quick transients.