U.S. tech giants Google and Apple are in the race to come up with aerial maps so detailed and precise that they can practically find you in your backyard.

Google says it has already sent planes over cities while Apple has acquired a firm using spy-in-the-sky technology that has been already tested on at least 20 locations, including London.

Apple's cameras are understood to be so powerful they could potentially see into homes through skylights and windows. The technology is similar to that used by intelligence agencies in identifying terrorist targets in Afghanistan, Daily Mail reports.

Google will use its spy planes to help create 3D maps with much more detail than its satellite-derived Google Earth images. Nick Pickles, director of Big Brother Watch, warned that privacy risked being sacrificed in a commercial ‘race to the bottom.'

“The next generation of maps is taking us over the garden fence,” he warned. “You won't be able to sunbathe in your garden without worrying about an Apple or Google plane buzzing overhead, taking pictures.”

He said householders should be asked for their consent before images of their homes go online. Apple is expected to unveil its new mapping applications for its iPhone and other devices — along with privacy safeguards. Its 3D maps will reportedly show for the first time the sides of tall buildings, such as the Big Ben clock tower.

Google expects by the end of the year to have 3D coverage of towns and cities with a combined population of 300 million. It has not revealed any locations so far.

Current 3D mapping technology relies on aerial images taken at a much lower resolution than the technology Apple is thought to be using. This means that when users ‘zoom in', details tend to be lost because of the poor image quality.

Google ran into trouble when it emerged that its Street View cars, which gathered ground-level panoramic photographs for Google Maps, had also harvested personal data from household Wi-Fi networks.

Apple's spy planes are believed to be equipped with technology developed by the military to guide missile strikes. Each plane is equipped with multiple cameras taking high-resolution photographs of buildings and landmarks from every possible angle, which are then compiled to make three-dimensional images.

The military-grade images are taken at a height of around 1,600 ft — meaning people below are very unlikely to realise they are being photographed. The cameras can be installed on planes, helicopters or even unmanned drones, although there are safety restrictions on the use of the latter in Britain.

More In: Technology | Sci-Tech