On 30 October of this year, security guards outside the Grosvenor House Hotel in London saw a man taking photos of CCTV cameras on the hotel’s outer walls. They performed a citizen’s arrest, then called the police.

Once the police arrived, they asked for ID and conducted a search, before telling the offending artist that carrying a camera in central London was automatic grounds for suspicion. Under the 1968 Theft Act, they said, carrying a camera could be seen as “going equipped”: casing a joint for burglary. By email, the man in question, James Bridle, said they also “repeatedly suggested that any suspicion was reasonable based on the ongoing threat of terrorism”.

Under “Search Grounds”, the receipt reads: Male concerned has been seen by security to take photographs of security cameras at Grosvenor House Hotel, Park Lane.

They probably didn’t realise it at the time, but the police actually had good reason to worry about James Bridle. That’s not because he was planning a robbery of the Grosvenor House hotel (he isn’t). Rather, he’s trying to raise awareness of London’s CCTV cameras, the data they collect and the way that data is used as part of an online residency at the Southbank Centre’s Mirrorcity art exhibition.

That day, he was attempting to take a photo of every CCTV camera he could see around the periphery of London’s congestion zone. Though the run-in with police put an end to the walk (for that day at least – Bridle hopes to finish the trip at some point over the next month), Bridle compiled the photos into a Flickr photo album and animated heatmap.

During the 8km section of walk he completed (the coloured area on the left of the map), Bridle found and photographed no less than 427 cameras. If that rate were to continue for the entire 18m length of the route, he would have photographed around 1,000 in total. Bridle says he only photographed those cameras he could see:  he believes the map above is a “gross underestimation of the total”.

Through his residency as a whole, Bridle hopes to explore the role of the state in London – “the Other, largely invisible, side of the city”. In blogpost on the project, titled All Cameras are Police Cameras”,  he puts forward the theory that these cameras represent a “third city wall” for London: the first was the Roman Wall erected in the 2nd century; the second the roadblocks and sentry boxes built after bombings by the Provisional IRA in the nineties.

These “walls” are fairly conceptual, but these cameras and the data they provide are a little more concrete. At first, access to data from the ring of license-plate recognition cameras in the congestion zone was tightly restricted, and historical data regularly deleted. But since their installation it has been quietly reclassified as “crime” data. As Bridle puts it in his blog:

As this data is not considered to be “personal data” within the definition of the law, the police are under no obligation to destroy it, and may retain their ongoing record of all vehicle movements within the city for as long as they desire.

As a result, vehicle tracking data collected from the CCTV cameras is freely available to police and security services for as long as they like. Carrying a camera in central London, though – now that’s suspicious.