While there are many ways to define a food desert, this indicator considers a low-income census tract where a substantial number or share of residents has low access to a supermarket or large grocery store. To qualify as a food desert tract, at least 33 percent of the tract's population (or a minimum of 500 people) must have low access to a supermarket or large grocery store. A low income tract is defined as one with a poverty rate of at least 20 percent or a median family income lower than 80 percent of the metro area median family income.
Low access to a healthy food retail outlet is defined as being more than 1 mile from a supermarket or large grocery store in urban areas and more than 10 miles from a supermarket or large grocery store in rural areas.
Food Deserts in the greater Portland region, 2011
Source: US Department of Agriculture
US Department of Agriculture Economic Research Service. http://www.ers.usda.gov/data/fooddesert/index.htm
Data on population and income come from the 2000 Census of Population and Housing. The 1-km square grid data come from the Socioeconomic Data and Applications Center. Census population data, which are released at the block group level, are first allocated to blocks and then allocated aerially down to the grid level.
Information on supermarket and large grocery store locations comes from a directory of supermarkets and large grocery stores (food stores with at least $2 million in sales that contain all the major food departments found in a traditional supermarket). The directory was developed from a list of stores authorized to receive Supplemental Nutrition Assistance Program (SNAP) benefits, augmented by data from Trade Dimensions TDLinx (a Nielsen company), a proprietary source of individual supermarket store listings, both for the year 2006. Details on these data sources can be found in the 2009 ERS report, Access to Affordable and Nutritious Food: Measuring and Understanding Food Deserts and Their Consequences.
The distance to supermarkets and large grocery stores is measured by the distance between the geographic center of the 1-km square grid that contains estimates of the population (number of people and other subgroup characteristics) and the nearest supermarket or large grocery store. Once the distance to the nearest supermarket or large grocery store is calculated for each grid cell, the estimated number of people or housing units more than one mile from a supermarket or large grocery store in urban tracts (or 10 miles for rural census tracts) is aggregated to the census tract level. (A census tract is considered rural if the centroid of that tract is located in an area with a population of less than 2,500, and all other tracts are considered urban tracts.) If the aggregate number of people in the census tract with low access is at least 500 or the percentage of people in the census tract with low access is at least 33 percent, then the census tract is considered a food desert.
