They are New York City's nearly 600 secret oases: plazas, courtyards, rooftop gardens and patches of greenery carved into private buildings. Privately owned public spaces, often referred to as POPs, are free and open to the public — at least on paper. But controversially, they’ve often existed in obscurity.

As with many conundrums, there’s now an app for that. NYC Public Space is a years-in-the-making civic experiment developed by Chris Whong, a mapping-software developer who used to work in the city's planning department and has spent time thinking about what constitutes a public space. Whong and Matt Wing, a political strategist, talk to WNYC about how to find these places and reclaim public space in private hands.