The city of Philadelphia is deploying high-tech sensors and a new statistical model to help it combat the blight and attendant health problems of vacant and abandoned properties. The city’s Department of Licenses and Inspections first calculates which properties might be vacant and too hazardous for inspection. It then employs aerial surveillance with laser imaging to determine if they are at risk of collapsing roofs. After this data and other indicators are collected, the department uses a scoring system to evaluate the safety of the properties. Presently, department officials believe that about 4,800 vacant homes are unsafe and 230 are imminently dangerous.