Essentially ... "Modern buildings with reinforced concrete and steel structures produce have unique, spatially-varying ambient magnetic fields that can be used

Essentially ... "Modern buildings with reinforced concrete and steel structures produce have unique, spatially-varying ambient magnetic fields that can be used for positioning, in much the same way (albeit on a much smaller spatial scale) as animals use the Earth's magnetic field," the paper's authors say. "In principle, a non-uniform ambient magnetic field produces different magnetic observations, depending on the path taken through it."