Many places that have a tile ceiling would also have a tile floor. And while you see more of the ceiling, the landmarks on the floor would be a lot if you get up close to them. As you say, the accuracy of the system drops as you raise the roof (bad joke, but I amuse myself), but the ground clearence would usually be about the same, very low compared to the ceiling. I just think you can get a better, more reliable, more flexable system with the floor. I would love to see it work no matter which way use choose. Now what would really be "out there" would be to navigate on the earth's magnetic fields..... Birds do it, so how hard can it be....
