Apple has acquired a US start-up that uses nearby WiFi networks to discover the location of a smartphone indoors. 

The service is aimed at businesses that operate large public spaces, such as shopping malls or venues, and allows them to offer precise, location-based services. 

It uses WiFi networks because GPS tracking, which is used for conventional geo-location, relies on high-frequency radio broadcasts that do not pass through walls. 

WiFiSlam says its technology is purely software-based, the only hardware required being a company's existing WiFi access points. 

Reuters has reported that the deal is worth around $20 million, and that Apple declined to comment on its intentions for the company. 

Google already uses WiFi signals to allow users of its Maps application to pinpoint their precise location, if the data is available. 

It was while collection data for this feature, using its Street View camera cars, that Google also collected private WiFi data, leading to a number of regulatory actions around the world.