Tapping Earth's Magnetic Field for Indoor Navigation

Interesting concept. Migratory animals follow lines that are always changing to some degree though, which is why they can get to a general area but not the same square meter as the year before by just magnetic fields. If someone turned on a window fan in a room, for example, would that change the companies magnetic map. What if the building had to switch to off grid mode or had a power outage? I wonder if the apps would still work as well in those conditions?