hi! I was wondering why there is such a stark difference in Gisjoins across years, especially in the decentennial censuses. Is there a way to merge the gisjoins from the 2010 census with, say, the 2013 ACS file? For example, when just doing a 1:1 merge from 2010 to 2020, only about 60% of the Gisjoins go through, with the rest being unmatched (about an even split between years). Thanks!
GISJOINs differ across time because the areas identified in Census and ACS summary data change. The definitions and boundaries of census tracts, blocks, block groups, ZIP Code Tabulation Areas, urban areas, and some other areas are updated with every decennial census. The definitions of many other areas (counties, places, county subdivisions, metropolitan statistical areas, etc.) sometimes also change between censuses. Changes in county definitions also affect the GISJOIN identifiers for any units that nest within counties, like census tracts, block groups, and county subdivisions, which all use county codes to distinguish areas in different counties.
To support users who’d like to analyze data across time for static geographic areas, NHGIS provides geographic crosswalks and geographically standardized time series tables. These don’t cover all periods, levels, and subjects in U.S. census data, but they can help with the 2010-to-2020 time frame you mentioned in your message.
Thank you! Do you know if the GISJOIN of the 2006-2010 ACS would match the 2010 decentennial census? and same for 2016-2020? or are they entirely separate?
You can generally determine this type of information from the Census Bureau’s summary of ACS Geography Boundaries by Year. Most of the 2010 ACS release’s geographies correspond to 2010 Census geographies, and most of the 2020 ACS release’s geographies correspond to 2020 Census geographies, but not, e.g., for urban areas and ZIP Code Tabulation Areas.
For census tracts and block groups, the 2010-2019 ACS releases generally use 2010 Census definitions, and the 2020-forward ACS releases generally use 2020 Census definitions, but there are some exceptions.