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Sydney’s rail system uses high-tech sensors to detect tagging in progress.

The age-old cat-and-mouse game between police and graffiti artists has gotten a lot more interesting lately, with the advent of new technologies. Earlier this month, CityLab reported on the use of drones in public art and vandalism—most notably, in tagging a prominent Calvin Klein billboard in New York. But artists aren’t the only ones with tech on their side: now Sydney’s rail authority is deploying a next-generation anti-graffiti system to catch vandals in the act.

The tool, aptly named Mousetrap, uses electronic chemical sensors to detect vapors of permanent markers and spray paint in real time. As soon as Mousetrap sniffs out a tagging in progress, it alerts transit staff monitoring CCTV feeds of Sydney rail stations. Then transit police can arrest the vandals.

"Those who [create] graffiti across the network can now be sprung immediately,” Sydney transport minister Andrew Constance told the press. “With can in hand, marker in hand, doing the damage, you will get caught."

Mousetrap isn’t a perfect solution. According to the Sydney Morning Herald: “The sensors do register other smells, and it can be difficult to differentiate between the vapour of graffiti or, say, someone transporting house paint.” But since the current trial began in mid-2014, the system has already nabbed more than 30 offenders. And with a price tag of about $500,000—compared to the $34 million Sydney spent cleaning up graffiti last year—Mousetrap seems like a bargain.

One thing’s for sure: would-be taggers will have a hard time evading the long nose of the law.

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