A smartphone app using magnetic fluctuations to map indoor locations can guide users where global positioning systems can't penetrate, Finnish researchers say.

Developed by a company called IndoorAtlas, which was spun out of the University of Oulu in Finland, the app takes advantage of the digital compasses found in modern smartphones, NewScientist.com reported Monday.

Such compasses don't normally function inside buildings as metallic structural elements disturb the Earth's magnetic field, making it impossible to reliably find north.

The IndoorAtlas turns that problem to its advantage, developer Janne Haverinen said, by using these disturbances to create a unique map within each building.

Mapmakers can align a building's blueprint with a traditional map, then walk along designated paths while a smartphone charts the magnetic variations.

"In principle, a non-uniform ambient magnetic field produces different magnetic observations, depending on the path taken through it," Haverinen said.

"In IndoorAtlas' location technology, anomalies [fluctuations] of ambient magnetic fields are utilized in indoor positioning."

Once a map is created, visitors to the building can download it to their own phone and navigate through the indoor environment.

Unlike other proposed indoor navigation systems based on WiFi and Bluetooth, IndoorAtlas doesn't require any additional infrastructure, the developers said.