I can also do this, though I'm better outdoors than in. I can pretty much always get north to within 45 degrees. I can also pretty much look at a map once, memorize the key features, and then mentally navigate on that map, mentally keeping the map orientated to north even if I'm travelling in some other direction. Few people I've talked to seem to do this.
I'm not convinced any of this is magnetic though. I've travelled a fair bit, and I've noticed several failure modes in my navigation ability:
- In countries near the equator, I get north and south switched round fairly often.
- In the southern hemisphere, until I get used to it, I consistently swap north and south.
- In cities where the grid is at 45 degrees to north, I sometimes get north out by 90 degrees.
- I'm not as accurate at night and indoors, though I'm still pretty good.
Because my navigation is normally so good, when I do get it wrong, I really believe my error for quite a while, which is not so great.
The north/south swap in the southern hemisphere leads me to believe that the dominant factor is to do with the position of the sun in the sky. I don't do it consciously though, and I live in London, and it still works on frequent cloudy days, so however it works, it's subtle.
The 45 degree grid shift one is strange and very disconcerting. I think what happens is my accuracy is only to the nearest 45 degrees, and I mentally orientate the grid to north. The mismatch between mental model and reality combined with my limited accuracy can cause the whole mental model to jump 90 degrees. Too much thought - should just trust the instinct.
The fact that it all still works at night and indoors could just be that I'm pretty good at dead reckoning. But maybe there is a magnetic aspect - if so it's not the dominant factor.