I was thinking how birds and animals navigate long-distance. It's a proven fact that cats can return home from hundreds of miles away, homing pigeons take flight straight towards their home from over a thousand miles away, migratory birds find their nests from up to ten thousand miles away each year. Satellite tracking show, that the birds are going more or less straight towards their destination. Now, birds can't ask directions, they never seen the road before (homing pigeons). Dead reckoning by memory is not viable. Natural compass alone won't help. Longitude/Latitude determination by star observation alone sounds too complicated for a bird to do, and do not account for bad weather. Smell seems to be unused by some species, therefore redundant. That leads towards some kind of natural GPS, and leads the question into more physical area - Earth magnetic and gravitational field. Basically, what i wanted to know, is there any position-dependent and continuous variations in the terrestrial magnetic or gravitational field large enough to be detected by modern or near-future hardware? Something you can measure without looking at the sky and satellites, and tell more or less where you are, with about a few dozen miles precision?