Which vintage of county boundaries is COUNTYNHG variable for?


I’m producing some county-level choropleths for each year from the US full-count censuses 1850-1940. I have read that I can use the IPUMS USA variable COUNTYNHG to combine census-level tabulations (eg. employment share by industry) with the NHGIS county boundary files. However, the county boundaries of course change over time, and I have not been able to find for which year (i.e. vintage) of county boundaries the COUNTYNHG variable is for–assuming it is for a single year, and not also changing each year (I was hoping to keep the county boundaries consistent across years). Would this be possible to clarify?

Thank you in advance for the help, and apologies if I missed something in the documentation.


The comparability tab for COUNTYNHG specifies that these codes are year-specific and the meaning of the code will vary based on census year. You can get the year-specific NHGIS boundary files via the NHGIS website (using the data finder, select the years of interest in the filters area and click on the “GIS Files” tab of the results). IPUMS does not offer harmonized county boundaries, you might be interested in my colleague Jonathan Schroeder’s approach for county crosswalks or these crosswalks from Ferrara et. al via Open ICPSR.

Thank you very much for the prompt and helpful response, and my apologies for missing that information earlier in the comparability tab for COUNTNHG.