People like to know what goes on in their neighborhoods. Most of us want to know if a new family is moving in down the block, if a store is closing or a new business opening, and, perhaps more than these, we want to know if any crimes have been committed nearby. And when it comes to something as important as crime, we want that information from a credible source. While police departments across the country record this information, it is generally only used internally by police personnel. In recent years a relatively small number of city police departments have started making the data available to the public.
Philadelphia’s Police Department (PPD) is now one of these select police departments. In response to widespread public concerns about crime in the city, Mayor Nutter and Police Commissioner Ramsey charged the Police Department with creating a public website where city residents can map the incidence of major crimes in Philadelphia. Based on our previous work with crime analysis applications (such as Crime Spike Detector and PhiCAMS), the PPD selected Azavea to develop the system. Working closely with the Police Department and the Mayor’s Office of Information Services (MOIS), we were able to get the application up and running in just six weeks.
The emphasis of the site is on simple, accurate display of crime occurrence across the city in a “pin map” style. All crime data is fed nightly to the site directly from Philadelphia Police Department’s databases. Up to 30 days worth of crime can be viewed simultaneously and a data download feature enables anyone to extract and download the data for more rigorous analysis.
One of the greatest challenges in creating the site was the need to display even relatively high volume of crimes at every scale. For example, theft is the most numerous of the so-called “Part 1″ crimes (the more serious crimes). Viewing thefts city-wide, for a typical thirty-day period, may result in 3,000 or more data points. The map depicting this situation would simply be a mass of undifferentiated points, which is not useful to anyone.
To address this concern, the site uses a common cartographic technique of “aggregation” – taking many points concentrated in the same geography and lumping them together into a single larger point, sized proportionately to the number of points it represents. This is analogous to the size of points used to represent the population of cities in many atlases. The website computes these new aggregations “on-the-fly” depending on how close or far one has zoomed into the map. There are several techniques for accomplishing this type of task. We used the ‘K-means clustering‘ approach which is a method for finding the centers of natural clusters.
We are excited by this new initiative and hope the public will find it useful. Visit http://citymaps.phila.gov/crimemap to check out the application or your neighborhood.
