I have census demographic data at the census block group level. The key in the data that I'm using looks like this: 010010205.001

Which translates as:

01 - State ---> Alabama
001 - County ---> Autauga County
020501 - Tract ---> 020501
1 - Block Group ---> 1

I also have a shapefile that corresponds to this census block group and is matching on the key (GEOID). This is all good - basically, demographic data at the census block group level and a corresponding shapefile at the same census block group level. The software I've written has been using this "corresponding data" for a while now and it works just fine.

Now, I'm trying to update the demographic data and the shapefiles to current data, but I'm getting shapefiles for census block groups that I don't have census demographic data for and vice versa.

For instance, I don't have a new shapefile that corresponds to 010010205.001, but I do have seven (7) shapefiles with these keys:

010010205.011 ---> Tract = 020501; Block Group = 1
010010205.012 ---> Tract = 020501; Block Group = 2
010010205.021 ---> Tract = 020502; Block Group = 1
010010205.022 ---> Tract = 020502; Block Group = 2
010010205.031 ---> Tract = 020503; Block Group = 1
010010205.032 ---> Tract = 020503; Block Group = 2
010010205.033 ---> Tract = 020503; Block Group = 3

If I'm reading this correctly, the original 010010205.001 no longer exists in the shapefile data and has instead been split into three (3) separate tracts, each with two block groups. Is that a correct reading/assumption?

I may just have a fundamental misunderstanding of how this data is organized and changes over time.

What am I missing here? Based on what I'm seeing in the shapefile data (from the Census Bureau), there should be 3,924 census block groups for Alabama (01), but my demographic data only has 3,438 census block groups.

1 Answer 1


The U.S. Census Bureau updates their enumeration geographies (blocks, block groups, tracts) for each decennial census. If you are using 2020 census data or 2020 American Community Survey (ACS) data, you need to also update your block group polygons to the 2020 version.

  • Thank you for your insight! I was indeed using the 2021 shapefiles. I went back and downloaded the 2020 shapefiles for Alabama and reprocessed them. There's still a mismatch between the shapefiles and the demographic data. Turns out the source demographic data is still using the census block groups from 2010 (but the actual data is from 2020). Sigh. Back to the provider to fix their stuff. Thanks again. Apr 28, 2022 at 19:30

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