Question about CBSA GIS Files

I was comparing the 2018 cbsa shapefile I had downloaded from NHGIS to the US Census Bureau CBSA reference map and noticed a number of anomalies, primarily that the NHGIS file includes counties in CBSAs that the Census Bureau does not.

Here is the Census Bureau’s CBSA reference map.

The map you shared is for the March 2020 CBSAs.

NHGIS derives its shapefiles from the Census Bureau’s TIGER/Line files from the same year, so the NHGIS 2018 CBSA shapefile corresponds to the 2018 TIGER/Line files. According to the 2018 TIGER/Line documentation, “All legal boundaries and names are as of January 1, 2018.”

The OMB updates the official CBSA delineations at irregular intervals across time. The delineations that were in use for the 2018 TIGER/Line files (as of January 1, 2018) are the August 2017 delineations.

Thank you for your reply. I went through some of the reference maps and bulletins and now see that there was a pretty significant change to the CBSA definitions in 2018 which explains the differences.