THE city of New York owns thousands of slivers of unused land, and about a year ago, a group of Brooklyn gardeners had an idea: identify all the vacant lots in the borough, then help neighborhood residents take them over. They built an online map, then a mobile app, with information about the plots, including the names and phone numbers of the agencies that owned them. They called themselves 596 Acres, after the total area of unused public land in Brooklyn, according to city data.

On a recent Saturday, Paula Z. Segal, 34, a founder of the group, loaded up a bicycle trailer with handwritten wooden signs and set off for points on her interactive map, starting with 406 Nostrand Avenue in Bedford-Stuyvesant, a narrow ribbon that belongs to the Department of Housing Preservation and Development. Passersby stopped to watch Ms. Segal and a fellow volunteer, Eric Brelsford, hang signs on the chain-link fence.

“This lot is public land,” read one of the signs. “It’s very likely that they would let you and your neighbors do something nice here — maybe a farm or an outdoor movie theater.” When a woman pushing a shopping cart said she might call the agency’s phone number, Ms. Segal gently steered her in another direction.

“Calling the number is an O.K. place to start,” she said. “It’s better to talk to your neighbors and see what they want to do here.”