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Richard Grinell, Coventry, England
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THE SEARCH COST OF GEOGRAPHIC PROFILING

6/15/2019

 
PictureKim Rossmo
With the search for the infamous Zodiac Killer through DNA looking ever more threadbare, one very much overlooked tool in locating our offender is the 'science' of geographic profiling pioneered by Canadian criminologist Kim Rossmo, who utilizes the software geo-profiling tool Rigel to analyze the likely home residence of the Zodiac Killer. Rigel uses an algorithm called criminal geographic targeting (CGT) to identify hot and cold zones on a map and give the areas that our killer may (or may not) likely reside. In the case of the Zodiac Killer, it would take into account such things as the murder sites, phone calls, commuter routes, and geographic profiling models such as buffer zone, comfort zone, least effort principle, mental map, target backcloth and distance decay function. As Kim Rossmo points out, this is an accompaniment to law enforcement to narrow the pool of suspects, but takes a back seat to powerful forensic evidence. However, with limited forensic evidence available from the Zodiac crimes, this appears our best bet to narrow down the home location of our killer before venturing outwards. Searching for suspects the length and breadth of California is a thankless task, with many researchers identifying a potential suspect and ultimately layering weak circumstantial evidence upon weak circumstantial evidence, until they become convinced the totality of the evidence should now be reclassified as powerful and irrefutable proof of their suspect's guilt. There is a tendency to fit the suspect around the evidence, rather than let the evidence be the driver. 

Kim Rossmo stated: ​​"The San Francisco murder differs significantly from the Zodiac's other crimes. Up to this point he was hunting in locations that had a good probability of containing his desired victims. Target selection was a function of area, not of an individual. However, it is unlikely the Zodiac was successful in all his searches; serial killers typically engage in extensive hunting activities, and for every attack there are many unsuccessful search attempts. In San Francisco, however, the Zodiac controlled the situation through his selection of victim type. The need for such control could be indicative of the distance the Zodiac had to travel to the crime site. Criminals who travel longer distances to offend are less likely to use uncertain target selection techniques. In respect to the killer's home residence, he continued "There's a number of possibilities. But remember, Rancho Vallejo was possibly 26,000 people at that time, of which only 13,000 will be male, and only a certain proportion of those are of the correct age, and a certain portion of those are only going to be white. Then we start looking at the neighborhoods involved, we have a pretty small subset. Then we start combining that with some of the personal and behavioral descriptors, and then we work in the vehicle information. If this case was active today, it would not be too difficult to find this person." 

There is an excellent interactive online tool that has generated geo-profiling heat maps for killers such as Jack the Ripper, Dennis Rader (BTK), Atlanta Child Murders, Golden State Killer, Night Stalker, Yorkshire Ripper, Long Island Serial Killer and more. It can be found here at geographicprofiler.com. The Kim Rossmo heat map for the Zodiac Killer is located here. 
You can toggle the geoprofile using the sidebar to remove the heat map and identify the locations underneath the color. The red color is the most likely residence of the Zodiac Killer, with a graded scale to the least likely areas of purple and beyond. This is classified as a percentage search cost (link) - Suspects are ranked based on their search cost—the percentage of the map searched before reaching the suspect’s anchor point. The search is conducted from the highest point of probability to the lowest; therefore, the lower the search cost, the more viable the suspect. If the geographic profile is accurate, the offender will have a low search cost. In the case of search cost, “map” refers to only the portion of the profile containing the crime scenes. As a result, suspects can have a search cost greater than 100%, which means a space larger than the area containing the crime scenes would have to be searched before reaching their anchor point.

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Zodiac Killer (hot zones). Click to view the complete geo-profile on geographicprofiler.com
One can see that the search cost is lowest for the area just east of the Napa Junction, the area of Ridgeview Park, Mare Island Naval Shipyard and across the Carquinez Strait Bridge to the area of Crockett Hills (just east of Rodeo). The area I favor, around the Springs and Tuolumne payphone, is afforded a 4% search cost. Despite the majority of the Zodiac mailings originating from San Francisco, this area features low on the geo-profiling heat map, and backed up by the statements of Kim Rossmo above. He states that "geographic profiling can't solve a crime; there are only three ways to do that - eyewitness, confession or physical evidence - the role of geographical profiling is to allow a detective to get to that stage, where he or she can apply one of those techniques sooner, faster, more effectively and more efficiently". 

This means we target the red hot zones first, before extending outwards to orange, yellow, green, blue etc.The hot zones will inevitably sometimes fall over an unpopulated area, so the areas around them become the focus of our search. The technique of geographic profiling is a supportive tool to narrow the suspect pool and search parameters, especially in a case over 50 years old with limited physical evidence - narrowing the search field in order to prioritize our efforts. The possible candidates for the Zodiac Killer can be massively whittled down, as shown in the case of Rancho Vallejo, highlighted by Kim Rossmo above. After all, he did state "If this case was active today, it would not be too difficult to find this person."
drew
6/16/2019 10:41:09 am

Very nice article, Richard. I wonder what the geo-profiling map would look like if two incidents on or around Telegraph Avenue were included. One involved two young women and a man who offered to give them a ride -- you also did an article on this event, Richard. The other involved a woman who said a clean-cut man driving a convertible attempted to lure her into his vehicle. However, he sped off when more people began appearing at the bus stop.

Richard
6/16/2019 11:21:18 am

That is a good question Drew, but I guess the bottom part of the red and orange zones would shift southwards a little. What would happen if we added Modesto and Lake Tahoe? It depends on the priority afforded the first two crimes, that may have been a working day for Zodiac. The later hour crimes of 11:10 pm and midnight may suggest his ability to reach home rather quickly, as opposed to 6:30 pm and 9:58 pm, affording him the travel time back home.

I checked out a couple of the other killers, including BTK, and his residence was just north of the hot zone. I love this aspect of profiling, rather than trying to analyze Zodiac's mind in particular. Most of us have comfort zones and use the least effort principle, so this form of profiling examines people as a whole. If Zodiac is ever identified however, and he lives nowhere near the payphone or even in Vallejo, I guess I'll have to hide away for a few years.

Richard
6/16/2019 11:29:22 am

Are you still in the same place Drew on Zodiac's residence or have you changed over the years?

drew
6/16/2019 03:23:35 pm

I have always felt that Zodiac actually did live in San Francisco, given that it would be easier and less of an effort to mail all those letters if he did live in the city. Given his interests in the arts, film and popular culture, San Francisco had a lot more to offer than places like Vallejo and thus seems like a likelier place of residence.

art
6/17/2019 08:15:58 am

Fascinating approach. Would be interested to learn the accuracy of this approach, i.e. as applied retrospectively to dozens of solved serial cases...has this been published perhaps? So what is the feasibility of extending this analysis to actual names of residents in the Vallejo red zone from that time period, late 60's? Are records available? Also, just out of curiosity, does the ALA residence fall into the red zone?

Richard
6/17/2019 09:06:34 am

Check some of the solved cases here art http://geographicprofiler.com/crimes
BTK is a good one to start with.lived at 6254 Independence Street in Park City.

The idea is to search the directories in 69-70 and see if any interesting names pop out in the hottest zones first. I have looked through streets north and south of the payphone, because that is where I personally believe Zodiac lived. But without knowing what these people looked like, their age, criminal record, vehicle etc, searching for random names is a thankless task. I wanted to cross check the names of people in a 0.5 mile radius with the residents living near the Riverside library, but unfortunately I don't have the directory for the south side of Terracina Drive.

Arthur Leigh Allen at 32 Freano St lies in the light green zone. The hot red zones have a search cost of 0%, whereas the light green zone is 0.5 %. In other words right in the frame. The spectrum runs from 0% to 90%, so less than 1% is fairly hot. As Kim Rossmo stated, if Arthur Leigh Allen is ruled out, we should be looking for somebody similar within these target areas.

Tom1
6/18/2019 06:00:22 am

Hundreds if not thousands of outdoorsmen, hunters and fisherman in those days would have been familiar with Lake Berryessa, BRS and Lake Herman Road. Many of those lived is San Fran.


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