US cities are battling the problem of vandalism head on with a hi-tech system that analyses and tracks graffiti and its perpetrators.
In the US, cleaning up graffiti is estimated to cost about $10bn (£5bn) per year.
Rather than simply obliterate the graffiti, the system keeps a permanent record of it which allows police to compile a database of similar daubings.
This surveillance of graffiti has angered some privacy advocates.
Gang graffiti
Graffiti Tracker, the brainchild of graduate student and crime analyst Timothy Kephart, uses global positioning systems (GPS), digital photography and computer databases to track and catch graffiti artists.
The system - dubbed Graffiti Analysis/Intelligence Tracking System (GAITS) - takes pictures of graffiti, using GPS cameras that record the date, time and exact location.
It then extracts information from the photographs and provides reports of each incident of graffiti which can be matched against other graffiti stored on a computer database in an effort to track down the perpetrator.
Keeping a database of known graffiti means offenders can be charged with multiple counts of vandalism.
The ability to locate where graffiti occurs means work can also be tracked. In one case, the system showed that graffiti was located in close proximity to a suspect's house, the park he used and the school he attended, providing compelling evidence for the police.
Mr Kephart devised the system after he analysed more than 450 gang graffiti photographs for his master's thesis.
He identified a series of trends, particularly relating to gangs which US police authorities believe are responsible for much of the graffiti in cities such as Los Angeles.
It is estimated that there are 720 street gangs in Los Angeles with some 40,000 members.
Urban regeneration
Much of graffiti contained coded messages to rival gangs as well as codes to identify the particular gang responsible for it, Kephart discovered.
So far, 20 cities in California, Nevada and Nebraska have bought the system and Mr Kephart expects sales to double in the next 12 months.
While the authorities regard graffiti as vandalism, others are not so sure the high-tech system is the best way to tackle the problem.
"I have never heard anything so hilarious," said Simon Davies, head of Privacy International.
"The money they are spending on this should be reinvested in urban regeneration, using graffiti as an art form.
"Surveillance tracking destroys self-esteem and there are more effective solutions to urban dysfunction," he said.
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