I just found an interesting article that has me very intrigued. Time to order a compass.
http://www.extremetech.com/computing/132484-indoor-navigation-on-your-smartphone-using-the-earths-magnetic-field-just-like-a-homing-pigeon
I agree the concept looks interesting.
Along with the downsides they mention int the article, I would suggest that the maximum error, 200cm, might prove less than useful.