Vehicles that move slowly down the street, pausing regularly to take photographs with remote-controlled cameras, tend to make the police a bit nervous. But one trailer loaded with imaging equipment that made its way through the streets of central Philadelphia wasn't spying - although at first, Secret Service agents had their doubts.

Both the vehicle and a plane that flew over the same area were taking authorized pictures of each building and its surroundings, at the behest of the downtown improvement district. Now the terabytes of imaging data are being used to build a three-dimensional model of central Philadelphia, down to the last cornice, mailbox and shrub.

The city model can then be integrated with other information, like listings of shops and rental space, so that one day people who'd rather be in Philadelphia will be able to be there virtually, from their computers. Apartment seekers, for example, will be able to click their way through the neighborhood, taking a virtual walk and checking out the view from the windows of apartments that strike their fancy.