The Hound of the Data Points

The lion migrates to a new area. It hunts down the length of a river course; its cover is the vegetation belt. The killer drives to downtown Chicago. He plies the "trapline" of the entertainment strip; his cover is the crowd.



Rossmo was learning that predators -- of whatever stripe -- have "mental maps," an "awareness space" within which is a "focus of activity." Looters, inner-city gang members and even shoppers are all hunters of a sort, and research has proven their predation patterns to be highly formatted. On the face of it, a psychotic serial killer would seem to be another matter. Someone like Richard Trenton Chase, the "Vampire Killer" who slayed six in Sacramento County in the late 1970s and drank their blood (believing his own blood supply was being siphoned off by aliens) would appear to pose an impossible challenge to a geographic profiler. And yet when Rossmo applied Rigel retroactively to Chase's case, while testing the algorithm for his doctoral thesis, it pegged Chase's home within 1.7 percent of the total hunting area.



A retroactive analysis Rossmo did of data points associated with the 1977-78 Los Angeles "Hillside Strangler" murders revealed similar correlations between the crime-related spots and the daily habits of the two cousins -- Angelo Buono and Kenneth Bianchi -- who were ultimately convicted of the crimes.
"You can see the influence of their non-criminal activities," Rossmo says. "The encounter sites are closer to their homes than the body-dump sites. And we have a cluster down where Angelo Buono used to go to make out when he was a teenager. I don't think that we've ever had a case where someone wasn't rational geographically."

A serial killer is scissored by competing forces whenever he leaves his home to hunt: the desire to remain within a kind of comfort zone and the desire not to be caught. The first force pulls him back home, and the second pushes him away. That relationship, expressed mathematically, is the very heart of Rigel.



What Rossmo hoped to do with his algorithm was to add rigor to the traditionally somewhat "soft" science of profiling, to create something that, once the crime sites were established, leaned more on deduction than induction. (Here's the difference: When Sherlock Holmes notices that the tips of your fingers are yellow and concludes you are a smoker, he's being inductive; when he concludes that if you are a smoker you cannot be the killer, because the killer is known to be fatally allergic to cigarette smoke, he is being deductive.)



"Induction is what most science is: You record observations and make generalizations about them," Rossmo says. "The only true deductive system is mathematics." You might think of Rossmo out walking his faithful hound, Rigel. Rossmo himself is "soft science" -- a sleuth out gathering data from crime sites -- while Rigel represents "hard science." The dog is off like a shot, programmatically, when the evidence is placed under his nose.

On the surface, the Beltway sniper case seemed a perfect candidate for geographic profiling, if only by default. Here was a serial killer against whom the arsenal of high-tech forensic tools -- the mass spectrometers and gas chromatographs and scanning electron microscopes that can practically pull a DNA sample from an errant thought -- appeared useless; whoever it was seemed to glide across the landscape without leaving a trace. What the sniper was leaving, in every pool of blood in every suburban gas station or parking lot, were data points. And Rossmo knew what to do with those.