INDEPTH: SNIPER ATTACKS
Geographic Profiling
John Bowman, CBC News Online | Updated October 10, 2003
It's a stereotypical scene in television crime dramas: the map of the city with coloured pushpins stuck in it, one for each scene in a baffling series of crimes.
It may help the cops (and the audience) to keep track of a killer's moves, but does it help solve the crime?
Geographic profiling is a lot more sophisticated than a corkboard map. The technique combines geography, psychology and mathematics to help investigators narrow down the search of a suspect's home base. The technique can also help identify where a suspect is likely to strike next.
Kim Rossmo |
From theories developed by Kim Rossmo, a former Vancouver police officer, in his PhD criminology thesis, geographic profiling has evolved into a computer program called Rigel.
The program takes the locations of a series of crimes and creates a three-dimensional "jeopardy surface." The surface's peaks indicate a greater probability of a location being the suspect's home base. Superimposing the surface on a city street map gives investigators a good idea of where a suspect is working from.
"It doesn't give you an 'X' that marks the spot but it does allow you to focus investigative efforts," said Rossmo in a 1998 presentation.
With five or six data points, Rossmo says his technique can reduce the search area for a suspect's house by 90 per cent. That means a suspect list of 1,000 people can be reduced to 50.
The computer program and equations are based on the principle of least effort, a psychological theory that crops up in such disparate fields as linguistics, city planning and graphic design.
In general, the principle means people won't put any more effort into an activity – pronouncing a series of words, finding a restaurant or reading a sign – than they feel they need to.
Rossmo says geographers call this the nearness principle, and use it to determine such things as the optimal location for a new hospital or fire hall.
In geographic profiling, the principle suggests that a serial criminal – such as a killer, rapist or arsonist – will commit his crimes in familiar areas not far from his home base. But the criminal will not commit his crimes too close to his base, either, creating a buffer zone surrounding it.
Rossmo says the inspiration for the theory came from his years of walking beats in Vancouver as well as from books on psychology and the hunting behaviour of African lions.
But Rossmo's technique doesn't begin and end with the computer program. Using traditional investigation techniques such as surveying crime scenes and interviewing witnesses, Rossmo can place more weight on certain points on the map, or discard others as red herrings. The weighting of the different points can shift the jeopardy surface and point in a new direction.
The technique has been used by police agencies around the world, including the RCMP and Scotland Yard. Rossmo is now with the Police Foundation in Washington, D.C., investigating the D.C.-area sniper killings.
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