In 2010, the USDA released this Food Access Atlas, which aims to map food deserts across the US. Its methodology isn't perfect-- the highlighted areas above are census tracts where a significant portion of residents live more than one mile away from a grocery store (in urban areas) or more than 20 miles away from a grocery store (in rural areas). In my opinion, a few more variables (such as availability of fresh food, vehicular and public transportation access and routes, price of food, poverty, etc.) need to be considered before declaring an area a food desert. Still, this map is a fascinating starting point to consider the geography of hunger in one's own city.