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Census Bureau webinar outlines 'small geographies' and shows how to map them on data.census.gov
Summary
U.S. Census Bureau trainers explained what constitutes a 'small geography' (under 65,000 people), differences between Decennial and ACS data, and demonstrated filters, map layers and table downloads on data.census.gov, including how to access block-level occupancy data from the 2020 Decennial Census.
Krystal Jimerson, a training specialist at the U.S. Census Bureau, opened a webinar that walked users through the agency's definitions of small geographies and a live demonstration of the data.census.gov mapping interface. Jimerson said the session was being recorded and that materials and Q&A would be posted to the Census Academy page within 30 business days.
David Kraiker, of the Census Bureau's Data Dissemination and Training Branch, described the practical threshold the agency uses for reporting: "anything under 65,000 in population... that's a small geography," meaning such areas generally do not receive 1-year American Community Survey (ACS) estimates and instead rely on ACS five-year releases or Decennial counts. He contrasted the Decennial Census as "100% data" collected at a point in time with the ACS, a sample survey that reports characteristics over a period and is subject to margins of error.
Kraiker defined and differentiated the common units users encounter: census blocks (the smallest unit, derived from visible features and only available from the Decennial Census), block groups (collections of blocks used for many ACS products but often accompanied by large margins of error in low-population areas), census tracts (relatively stable statistical areas, averaging about 4,000 people, that can be subdivided when population growth occurs), counties and county equivalents (parishes/municipios/boroughs), county subdivisions and Census County Divisions (CCDs), incorporated places and Census Designated Places (CDPs), and Zip Code Tabulation Areas (ZCTAs), which approximate postal ZIP codes but are constructed from blocks.
On the portal side, Kraiker recommended selecting the year first (he used 2024 ACS data during the demo), then specifying geography. He demonstrated the mapping tools (layers, boundaries, basemaps), how to select geographies interactively (for example, all tracts that touch a highway), and how to open profile tables or create choropleth maps. He warned that some summary-level codes and geo IDs are important for GIS users and advised checking margins of error for small geographies and small ZCTAs.
During Q&A, Kraiker clarified a technical rule about tract boundaries: "the census tracts have to nest within a county. They cannot expand across county lines." He also recommended the Missouri Census Data Center's geocorr crosswalk tool for users who need to map ZIP codes to tracts, counties or other geographies. When asked about using P.L. 94-171 (Decennial redistricting) block data for allocating people to housing units, Kraiker demonstrated how to select the 2020 Decennial year, pick block geography and open occupancy/vacancy tables to inspect block-level counts and download the data.
The webinar closed with a reminder that the presentation slides and a transcript of the Q&A will be posted on the Census Academy page, and that Kraiker will present a follow-up session on large geographies on April 23.
