Police investigations "à la carte"
So why use the raster method, if it is heavier than the vector method? Simply because this approach offers alternative but complementary possibilities in terms of data processing vis-à-vis the vector approach. ‘We privileged it for two reasons,’ explains Marie Trotta, co-author of the publication. ‘First of all because it allows the application of propagation algorithms. Propagation means that passing from one pixel to another will increase the value of the itinerary by a process of addition. As if each pixel had a value determined beforehand. To pass from one pixel to another, you have to ‘pay’.’ For example, in the context of this study, the resolution of each pixel corresponds to 20 metres by 20. On moving from one pixel to another next to it, one crosses a cost surface of 20 metres. The cumulated value of the second pixel is thus 40 metres. Normally, a propagation can be carried out in any direction starting from the original pixel. Yet the criminals had a car. To calculate their trajectory the researchers had to use the road network. In adding this network restriction to the logic of propagation it thus became possible to calculate the shortest cumulated distance between each pixel of the network and the different sites the crimes took place. Developing different scenarios...Once the maps had been digitalised the criminals’ journey needed to be imagined. All in all four scenarios were suggested. The first was a simple adding up of the distances covered between the criminal events. A loop between the thefts and the car being abandoned. This loop only measured 86 kilometres. This scenario did not suit, thus, as the expected 100km had not been covered. The second scenario integrated a withdrawal zone, with a round trip between this landmark and each of the crime scenes. |
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© 2007 ULi�ge
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