Census Geography Division

Description:

Most of the boundary files on PolicyMap come directly from the Geography Division of the U.S. Census. Block Group, Tract, County, State, County Subdivision, Place, Core-based Statistical Area (CBSA), Metropolitan Division, Congressional District, and Zip Code Tabulation Area (ZCTA) boundary files are all publicly available TIGER files. See: https://www.census.gov/geographies/mapping-files/time-series/geo/tiger-line-file.html.

Every ten years, following the Decennial Census, there are major updates to many of the Census boundaries based on a new population count. Additionally, there are minor updates annually reflecting changes to local geographies (for example, in 2014, the county-equivalent Bedford City, Virginia, was combined with its adjacent county). These minor updates also reflect more accurate surveying techniques by the Census.

Because of the significant changes that were made to underlying geography files in 2010, PolicyMap displays different data at Census 2000, 2010, and 2020 boundaries. In some cases, a single dataset will display some years of data at the different vintages. For example, Census 2000 data is shown at the 2000 Census boundaries while the most recent American Community Survey (ACS) data is displayed at the 2010 boundaries.

In cases where a single dataset relies on multiple boundary files, percent changes from 2000 to 2010 are calculated using relationship files and other materials provided by the Census. For more, see: https://www.census.gov/geographies/reference-files.html. In most cases the materials provided by the Census were adequate to make the calculations. However, the process for calculating Block Group percent changes was more complex because the Census does not provide Block Group relationship files. PolicyMap relied on the Census Block relationship files for these calculations using a family, household, or population multiplier to crosswalk the 2000 data to 2010 boundaries to make the calculation. Percent changes that involve both 2000 and 2010 vintages are all displayed using 2010 Census boundary files. For additional information on percent change calculations please see data directory entries for individual datasets.

In cases where a single dataset relies on multiple boundary files, percent changes from 2010 to 2020 and 2000 to 2020 are calculated using relationship files created by PolicyMap from Census TIGER boundaries. These PolicyMap relationship files match census boundaries across different vintages (such as 2000 to 2020) as equivalent if both boundaries contain 98% or more of the area for an individual boundary that shares the same unique identification code (often called a FIPS code). Change over time was also calculated across boundaries if each boundary vintage was comprised of more than 50% of both vintages.

PolicyMap has the following geographies with distinct boundaries for the 2000, 2010, and 2020 vintages: block group, tract, county, county subdivision, and place.

The state boundaries do not have substantial changes from 2000 to 2010 or from 2010 to 2020 (changes are simply related to improved surveying), so only the latest 2020 state file is loaded in PolicyMap.

Core-based Statistical Area (CBSAs, or “Metro Areas” on PolicyMap) and Metropolitan Division boundary files have been released on a different schedule than the other boundaries, as they are delineated by the Office of Management and Budget (OMB). Significant changes were made in 2003, 2013, and 2019. PolicyMap uses 2010 CBSAs (which are based on the 2003 OMB delineation), 2013 CBSAs, and 2019 CBSAs. CBSA encompasses both metropolitan and micropolitan areas. For additional information on metro areas and their historical and current delineations please see: https://www.census.gov/programs-surveys/metro-micro.html.

Places (identified as “Cities” on PolicyMap) are defined by the Census as either incorporated places or Census Designated Places (CDPs). Incorporated places are legally established to provide governmental functions for a concentration of people. An incorporated place is usually a city, town, village, or borough, but can take other forms. CDPs are statistical entities created by the Census “to provide data for settled concentrations of population that are identifiable by name but are not legally incorporated under the laws of the state in which they are located”. For more on the difference between incorporated areas and CDPs, see: https://www.census.gov/geo/reference/gtc/gtc_place.html The vintages of Place boundaries used in PolicyMap are 2009, 2010, and 2020. The 2009 cities are labeled on PolicyMap as 2000 for the sake of consistency and ease of use, because they are used for Census 2000 data.

Zip Code Tabulation Areas (ZCTAs) are based on US Postal Service ZIP Codes, but they are not identical, and in many cases, may be significantly different. Though ZIP Codes change continuously, ZCTAs are set in 2010 and 2020, and remain unchanged throughout the decade.

The congressional districts from 2004 to 2020 in the boundary menu reflect the congressional boundaries changing to redistricting in the 108th, 112th, 113th, 115th, 116th, and 117th sessions. On PolicyMap, election data displays the 110th Congress boundaries for 108th, 109th, 110th, 111th, and 112th Congresses, which are years 2004 to 2012. Congressional sessions 113th is for 2014, 2016 maps to 115th, and 2018 maps to 116th Congress. The boundary menu maps the 117th congressional boundaries with updated districts in North Carolina for the 2020 election cycle. The North Carolina boundaries are from the North Carolina General Assembly, which became law in 2019.

The vintage – or year – of the boundary used can be found in several places throughout the site. On the Maps page, the boundary type and boundary year is shown in the legend once data is loaded on the map. There is also an option to overlay additional 2000, 2010, and 2020 boundaries by clicking on the “Boundaries” menu.

When you search for a geography using the Set Location Bar, a dropdown menu will appear allowing you to select the vintage of the boundary you would like to see. If the boundary for the area of your search has changed, you can select either the 2000, 2010, or 2020 boundary definition. If the boundary has not changed there will only be one item in the dropdown labeled “2000 and 2010 and 2020 boundary”.

PolicyMap ZIP codes are licensed from Precisely. See: Precisely ZIP Code Boundaries.