Originally posted by Darryl Kenyon
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Sure. That would look like this:
The Rigel routines include a "buffer-zone" in the calculations, meaning the offender is thought to be "close" to offense locations but not "too close". It also bases distances on "Manhattan distances", so you total the horizontal and vertical distances between two points (as the cab drives) rather than the "Euclidian distance", which is the distance in a straight line (as the crow flies). This is why the Rigel outputs tend to have sharp corners and such, as the underlying geometry is based upon "squares" rather than "circles".
Other routines, like those in "Dragnet" (by Canter), do not include a buffer zone (hence the highlighted areas right around offenses; this doesn't always happen, but does sometimes, as in this example), and tend to be based upon Euclidian distances. For example, if I do the above using Dragnet routines, we would get this, which produces a more rounded looking output due to the underlying geometry is based upon circles:
And my own routines ("Dr. Watson"), also employ Euclidian distances, but the analysis is based upon a number of different underlying patterns, which then get combined. One pattern is a general "distance decay from the offenses", but uses a different calculation from Dragnet, and because it involves other spatial aspects, it doesn't tend to produce "spikes" at individual crime locations .
All three sets of routines work equally well, meaning if I give them all a bunch of different cases, on the whole they produce similar results over the entire set. Obviously, for any individual case, one routine may work better than another, but that is neither here nor there when evaluating the generality of the routines.
And while at first glance the areas look different, in the end, they are all highlighting very similar regions really. At some point I want to try and take all three routines and combine their output to see if that in anyway improves performance. It's like getting three opinions, and looking for where those opinions agree. Whether or not that improves things is an empirical question though, and at the moment I've not started on that.
- Jeff
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