When Philadelphia released its crime data last month, this was part of the plan. Make an application programming interface (API) available so that the city's growing corps of civic hackers could make something better with it.

"It's the beginning," said Chief Data Officer Mark Headd, speaking at the police headquarters press conference. "It's the beginning of an outreach process to the technology community to get them to use this data to do interesting and valuable things with it."

A month later, GIS developer Dave Walk has stepped up to the plate.

Walk has created PHL Crime Mapper, a handy interface for seeing where serious crimes are being committed. You can draw your own geography using a computer. Or you can search nearby crimes using your smartphone's GPS.

Interestingly, Walk works for the city's Office of Information Technology, the department that put out the data. His site adds functionality not available via the city's map portal.

Walk says PHL Crime Mapper is "100 percent a pet project."

"I work on projects in my free time," Walk said. "I thought the data was interesting and I just wanted to build something."

It's an interesting demonstration of open data in action: government makes the stuff available and tech-savvy citizens make it more useful on their own dime.