University of Liège geomaticians have applied a numerical crime mapping methodology to the simulation of a police investigation. A simulation based on real events, in other words a series of crimes orchestrated by a single gang using a single car. On the basis of information divulged by the Federal Police Force, they had to imagine the most plausible scenario concerning the gang’s itinerary and thus designate on a map the zones which could potentially accommodate their withdrawal site. And the results are astonishing.
Directed by Professor Jean-Paul Donnay, the ULg’s Geomatics Unit from time to time works with the police. One of its most recent research studies sprang from the dissertation by a student, Kenneth Broxham, and was subsequently picked up by two doctoral students. At the outset it was the police who commissioned this study to in order to test the department’s analytical capacities. And the study ended up going beyond our borders to appear in an American reference work (1). ‘The results of the dissertation were presented during a conference on crime mapping, at Washington D.C.,’ explains Jean-Paul Kasprzyk, the publication’s first author. ‘At the end of this presentation, the American Michael Leitner (NB: an internationally recognised researcher in the discipline) suggested we write an article on it.’
This working together is not surprising. If the USA, together with England and Canada, is the main cradle of crime mapping, the present publication offers an innovative methodology transposable to other enquiries, and more widely to all the disciplines requiring the study of cartographic data.
A case of operational crime mapping
The case in question is thus the simulation of an operational investigation. The police divulged to the University department information about a series of acts committed by a band of criminals.
The information provided was the following: the same group of criminals committed 5 crimes near to Charleroi initiated by the theft of a car, and which took place between May 30 and June 3, 2005. The chronology, the nature and the locations of these acts were known. The time when the car was rediscovered was also established. A final piece of information, and not the most insignificant, the distance travelled by the car between the theft and its being abandoned. ‘It’s a piece of data which isn’t often known,’ explains Jean-Paul Kasprzyk. ‘In the present case, the vehicle’s owner remembered the mileage at the time the car was stolen. We could thus deduct that the criminal gang had travelled approximately 100 kilometres in four days.’
On the basis of this data the researchers registered the cartography of the region in a numerical model, established the most plausible scenarios of the trajectories travelled, and put forward the hypothesis of the existence of a withdrawal zone. The final stage was to point out different locations where there was a chance this zone could be located.
Vector method and raster method
There exist two ways of storing and utilising geographic data numerically. The vector method and, which was the method used in this study, the raster method, which consists of cutting up an image into several information units, cells, or pixels.
Raster method has a slight disadvantage, being heavier than the vector method. ‘In a vector approach, we only define the objects which are of interest to us,’ says Jean-Paul Kasprzyk. ‘In an image all the territory is represented, and each pixel must contain information, a value. In the case of roads, for example, a simple binary value is enough to mark the layout of the roads in the image’ (see illustration below).
(1) J.P. Kasprzyk, M. Trotta,
K. Broxham, J.P. Donnay, Reconstitution
of the journeys to crime and location of their origin in the context of a crime
series. A raster solution for a real case study, in Michael Leitner (Ed.), Crime Modeling and Mapping Using Geospatial
Technologies, Springer, 2012