According to government agencies assigned to look at food availability as part of Michelle Obama's Let's Move! campaign, a "food desert" is defined as a low-income census tract with limited-access to healthy food. In urban areas, that means it's more than 1 mile away from a large grocery store or supermarket or 10 miles in the case of rural ones. The map of these census tracts show how common these food deserts are across the United States.

The graphic is an overview from the recently launched Food Desert Locator, which provides detailed information about food access. According to the USDA's Economic Research Service, 13.5 million people fit the criteria of living in one of these areas. But don't let the area on the map fool you: the vast majority of these people live in urban areas--82 percent according to the ERS.

Here's a fuller look at the map: