The Brooklyn Food Coalition held its data entry party on the Monday after Martin Luther King, Jr. Day. That night was especially dry and cold; even indoors, everyone was bundled in sweatshirts and scarves. Above couches on which eight volunteers sat with laptops perched on their knees, there hung a framed quote by Indian author Arundhati Roy: "Not only is another world possible, she is on her way. On a quiet day, I can hear her breathing."

The volunteers were collating the results of a recent survey of the 200-some greengrocers and convenience stores in Crown Heights, a neighborhood in Brooklyn, N.Y. As they tapped away on their keyboards, colorful dots appeared on their online map, The map shows the locations of all the food stores in a handful of central Brooklyn neighborhoods. Clicking on a location—one of the dots—brings up a dialogue balloon that displays the store's name and address and whether it carries fresh fruit and vegetables, whole grain bread, low-fat dairy and other healthy options. The volunteers plan to eventually map the entire borough.

Northern and central Brooklyn have some of the unhealthiest food stores in New York City, according to a paper published by New York City District Public Health Office researchers in January. "We want to get to a more specific and detailed description of what that looks like," says Jeffrey Heehs, who leads the mapping project. He hopes the map will help residents find fresh food and help policymakers assess food availability.

The Brooklyn mapping effort represents the intersection of two growing trends: mapping fresh food markets in U.S. cities, and private citizens creating online maps of local features in their neighborhoods. According to Michael Goodchild, a geographer at the University of California at Santa Barbara, citizen cartographers may make maps just for fun; because there is no good government-made map for the area, as in Port-au-Prince, Haiti; or to record problems such as potholes or burned-out traffic lights.

"All this has come out in the past four years," says Michele Ver Ploeg, a USDA economist who prepared a report to Congress in 2009 about "food deserts," or urban areas where the stores sell mostly packaged snacks or fast food instead of fresh produce.

Low-income neighborhoods, where residents are at higher risk for obesity and chronic disease, are often located in food deserts, according to recent research. Researchers have several theories about how food deserts arose. Linda Alwitt and Thomas Donley, marketing researchers at DePaul University in Chicago, wrote in The Journal of Consumer Affairs in 2005 that supermarkets often can't afford large enough properties for their stores in cities. Alwitt and Donley also have posited that when more affluent families moved out of many inner cities in the U.S. during the 1970s and 1980s, supermarkets left as well. City planning researcher Cliff Guy and his colleagues at the University of Leeds in the U.K. wrote in the International Journal of Retail & Distribution Management in 2004 that smaller urban groceries tend to close due to competition from suburban supermarkets.

As fresh food stores leave a neighborhood, residents find it harder to eat well and stay healthy. Food deserts are linked with lower local health outcomes, and they may be a driving force in the health disparities among lower-income and affluent people in the U.S. The issue attracted national attention in 2008, when that year's Farm Bill included, for the first time, funding to study and improve food deserts.

Now more U.S. cities are taking stock of their food landscapes. Last year, the USDA launched a map of food store densities in all the U.S. counties. Mari Gallagher, who runs a private consulting firm, says her researchers have mapped food stores and health statistics for the cities of Detroit, Chicago, Cincinnati and Washington, D.C. These maps help cities identify where food deserts are and, occasionally, have documented that people living in food deserts have higher rates of cancer and diet-related diseases.

The Brooklyn effort differs in that it's run by local core of five volunteers, not professional researchers, who have worked on the project for the past year. To gather data, they simply go to individual stores with pre-printed surveys in hand and check off boxes for the products for sale and whether the store accepts food stamp benefits. The team usually works with other local service groups or high school students looking to fulfill community service hours.

As citizen cartographers, they're a part of the second trend, one that geographers say is becoming increasingly popular. The movement is fueled by new technologies such as mapping apps and GPS-enabled smart phones, other handheld GPS devices, Google maps and OpenStreetMap, an open-source online map with a history of involvement in social justice. After Haiti's devastating January 2010 earthquake, for example, OpenStreetMap contributors built the first digital street map of Port-au-Prince. Brooklyn-based uses OpenStreetMap as its base. Such tools are "very, very simple to use and essentially available to everybody," says Goodchild.

Like the Brooklyn Food Coalition volunteers, many citizen cartographers use maps to bring local problems to official attention, Goodchild says. Heehs, the mapping project leader, says that after his group gathers more data, it will compare neighborhoods, draft policies to address local needs and then lobby New York City officials. hasn't caught much local or official attention yet, however. The Web site, which launched last month, doesn't yet track its hits so its creators don't know who's looking at it.

Experts who visited the Brooklyn group's site were optimistic but cautious. "This kind of detailed information could be very useful," says Ver Ploeg. To make the map more helpful to both residents and policymakers, she would like to see price data for healthy products, too. Karen Ansel, a registered dietician and a spokesperson for the American Dietetic Association, found the site confusing to navigate. "That said, with this information in place, Foodcensus has the tools to build a more user-friendly site that could be...very helpful to consumers," she says. The group also should ensure their map is available to those who don't have internet access at home, she adds. In fact, 27 percent of Brooklyn residents don't have internet at home and 8 percent rely on dial-up service, instead of high-speed, internet access, according to Gretchen Maneval, director of Brooklyn College's Center for the Study of Brooklyn. The Center's numbers come from data received from the New York City Mayor's Office.

"It's still very much a work in progress," Heehs says of the coalition's online map. They'll start advertising it online and by e-mail to other community groups, such as urban food garden associations, next month. He also hopes warmer days in the spring will draw out more volunteers to finish surveying—they have about two-thirds of Brooklyn left to cover—and to spread awareness.