Our guide focuses on three geographies that will be most useful for your work and are the smallest geographies available through the Census Bureau data portal: counties, census tracts, and census block groups. Before we delve into our step-by-step data case studies and tutorials, let’s review these areas and describe how they are related to each other. To learn more about the geographies that the ACS covers, see the complete listing of Census Bureau geographic hierarchies on the agency’s Geographic Program website. As you know, counties are legal units that make up a state. In some states, the equivalent units are called parishes, boroughs, or municipalities, and some states have independent cities in addition to counties. The ACS refers to this level of geography as “county or county - equivalent.” The Census Bureau data portal provides access to information on two geographic areas that are smaller than a county. Census tracts are statistical subdivisions that are smaller than counties but larger than census block groups. Their populations range in size from 1,200 to 8,000, with an average of about 4,000 residents. Breaking census tract areas down further, census block groups are statistical subdivisions of tracts that describe areas of 600 to 3,000 individuals. They are smaller than tracts but larger than individual census blocks. We will be using county, census tract, and block group data in our case studies, as block data is unavailable through the Census Bureau portal.
Figure 1: County, census tract, and census block group maps of Patrick County, Virginia.
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