The dataset represents the 2020 Decennial Census Urban Areas (UA) within the County of Sonoma, California.

Urban areas represent densely developed territory, and encompass residential, commercial, and other nonresidential urban land uses. Each urban area must encompass at least 2,000 housing units or at least 5,000 people. Notes: (1) This is a change from the previous minimum of 2,500 people which had been in place since the 1910 Census. (2) The Census Bureau will no longer identify an individual urban area as either an "urbanized area" or an "urban cluster", but we will refer to all qualified areas as “urban areas” regardless of population size.

The US Census designates certain blocks as urban based on certain criteria. This layer includes polygons that represent all of the census blocks that were designated as urban as of 2020.

Urban areas are defined primarily based on housing unit density measured at the census block level. Three housing unit densities are applied during the delineation process:

1. Initial urban core: at least 425 housing units per square mile. Based on the national average of 2.6 people per occupied housing unit, this density threshold is similar to the 1,000 people per square mile used in 2000 and 2010 when delineating initial urban cores.

2. Remainder of urban area: at least 200 housing units per square mile. This is similar to the 500 people per square mile density used for the 2000 and 2010 Censuses, based on the national average of 2.6 people per occupied housing unit.

3. At least one high-density nucleus of at least 1,275 housing units per square mile required for qualification. This ensures that each urban area contains a high-density nucleus typical of what one would expect to find within an urban area. In addition to the change in minimum thresholds for qualification and the change to use of housing unit density, the Census Bureau also will no longer distinguish between urbanized areas of 50,000 or more people and urban clusters of less than 50,000 people.

Significant Dates:

The proposed 2020 Census Urban Area criteria were published in the Federal Register on February 19, 2021.

The criteria for defining 2020 Census Urban Areas were published in the Federal Register on March 24, 2022.

Final criteria clarifications were published in the 2020 Census Qualifying Urban Areas and Final Criteria Clarifications Federal Register Notice on December 29, 2022.

Resources:

Difference between Final 2020 Census Urban Area Criteria and the 2010 Census Urban Area Criteria: https://www2.census.gov/geo/pdfs/reference/ua/Census_UA_CritDiff_2010_2020.pdf

Federal Register Notice for the 2020 Census urban area final criteria: https://www.govinfo.gov/content/pkg/FR-2022-03-24/pdf/2022-06180.pdf

Federal Register Notice containing 2020 Census Urban Area final criteria clarifications: https://www.census.gov/programs-surveys/geography/guidance/geo-areas/urban-rural.html

Proposed 2020 Census Urban Area criteria Federal Register Notice: https://www.federalregister.gov/documents/2021/02/19/2021-03412/urban-areas-for-the-2020-census-proposed-criteria

Census Glossary: https://www.census.gov/glossary/?term=Urbanized+Area+%28UA%29

American National Standards Institute (ANSI) and Federal Information Processing Series (FIPS) Codes: https://www.census.gov/library/reference/code-lists/ansi.html