Using a compass indoors is a challenge because all the metal in the buildings makes finding "north" almost impossible. But, what if you could take advantage of just those magnetic disturbances, specifically, map them, and then use a compass ot navigate by them? That's the vision of IndoorAtlas,
which was spun out of the University of Oulu in Finland by computer scientist Janne Haverinen and colleagues. In 2009 Haverinen told New Scientist how he built a robot that could navigate by magnetic fields after reading about a lobster with a similar ability. Now he's taking advantage of the digital compasses found in modern smartphones to do the same for people.
To make the map, the mapper aligns the floorplan with real world coordinates (and north) then walks the indoor areas to literally map the disturbances. Users can then download the map and use their smartphone's compasses for navigation. The big plus for this sort of indoor navigation: there's no additional infrastructure required. Most other solutions require some sort of beacon to send out signals (like little GPS satellites, really).