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Is someone listening in to the signal from your wireless computer network, photographing your house or putting a detailed map of your neighbourhood online for anyone to see?

In a new initiative, Microsoft has dispatched cars to trawl many city and suburban streets across the US to locate the signals sent out by millions of short-range home and office wireless (or WiFi) networks.

The unusual move, now being repeated in the UK and some other countries, is part of a plan to create a ground-based location system as an alternative to the GPS satellite system. This echoes an effort by A9, a search engine owned by Amazon.com, the online retailer, to use trucks with cameras mounted on the roof to photograph millions of store fronts in the US.

Microsoft says it has a database containing the whereabouts of “millions” of WiFi networks, while A9's website gives access to 26m pictures from 20 US cities.

Microsoft has also used low-flying aircraft to catch big urban centres on film, while the software company and Google, the search company, are racing to make widely available the most detailed satellite images of every corner of the earth's surface.

These and other initiatives are now being extended internationally, as the internet companies vie to attract users.

Lee Tien, a senior staff attorney at the Electronic Frontier Foundation, a US pressure group, said, taken together, some of the powerful new mapping and tracking tools on the internet could represent a threat to privacy. The internet companies say their satellite and aerial photos many of which are already available publicly, even if not over the internet do not provide a high enough level of detail to identify individuals or car licence plates.

Microsoft said it had collected only the unique identifier, known as a MAC address which each WiFi network broadcasts. This could not be traced to an address or an individual user.

Microsoft said that, by recording the position of every MAC address on a giant map, it had created a positioning system that would make it possible for anyone with a WiFi-enabled laptop computer to identify their location to within 30.5 metres.

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