Cover by Toby Segaran, Jeff Hammerbacher

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Cracking the Nut

Oakland CrimeWatch is an application that serves crime report information in on-demand images with relatively primitive cartography and cartoon-like icons. CrimeWatch is optimized for data display, and follows from a development approach that focuses on predicting user needs rather than making raw ingredients available. The user experience of the application is informed by "wizards," user interfaces where the user is presented with a sequence of dialog boxes that lead through a series of steps, performing tasks in a specific sequence. The steps required by CrimeWatch are:

  1. What: select the type or types of incidents.

  2. Where: search near an address, within an administrative boundary, or near a feature, such as a school or park.

  3. When: how far into the past to search.

CrimeWatch responds with a static image showing iconic representations of individual reports. These can be clicked for more information.

My interest in CrimeWatch was first piqued when I began to think about a way to reverse the server-side merging process, to start with a static image and extract crime report information with explicit location information attached: latitude and longitude values compatible with those used by other geographic software systems, commonly called geolocation. This kind of simple recognition problem is fairly well understood, and there are well-established techniques for visual feature extraction.

First, we need to get an image to work with. This is actually more complicated than it ...

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