Food deserts—areas where residents have limited options for purchasing fresh foods—are not easy to quantify. Access to food depends on a number of factors, from geography to transportation to the choices of individual grocers. One simple way to sketch out food desert boundaries is to chart those regions where supermarkets are scarce. The map below does just that for South Dakota, marking out areas that are more than 10 miles, and in many cases 20 miles, from a supermarket. The map comes from a 2009 USDA report to Congress, "Access to Affordable and Nutritious Food: Measuring and Understanding Food Deserts and Their Consequences." A somewhat different look at the nation's food deserts is available through the USDA's Food Desert Locator Web site.

The USDA researchers used a different metric for urban areas, where residents may be more reliant on walking and public transit. The map below of the Washington, D.C., area shows where residents are more than one mile from a supermarket.

Credit: Economic Research Service/USDA