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County GIS identifier, United States


NHGISJOIN is a 7-digit numeric variable.

A small number of cases in NAPP cannot be matched to the NHGIS. These are coded as 9999999 in NHGISJOIN. These were non-county areas such as Indian reservations and military installations. There were 6 such counties in 1900 and 1 in 1910.

A small number of counties in NHGIS cannot be matched to NAPP data for the United States. In all cases, these were very small counties that were not captured by the IPUMS sampling procedures. All of these counties contained fewer than 1,000 people; many were administrative areas with no population at all. There were 9 such counties in 1850, 56 in 1860, 32 in 1870, 33 in 1880, 2 in 1900 and 1910, and none in 1920 and 1930.


NHGISJOIN is a county identifier that can be used to match NAPP data with the county-level boundary files and aggregate data from the National Historical Geographic Information System (NHGIS). The corresponding variable in NHGIS is called "GISJOIN2."

Researchers can use this variable to match NAPP data with NHGIS boundary files (to make maps) or to match NHGIS county-level data with NAPP microdata for the United States (to facilitate multi-level analysis). For user convenience, copies of the NHGIS boundary files are available on the IPUMS County Codes page. NHGIS aggregate data are available via the "Data Finder" link on the front page of the NHGIS homepage.


NHGISJOIN is year-specific. Codes can have different meaning depending on the census year. Researchers should be careful to match NAPP and NHGIS data only from the same year.

NHGISJOIN is only available from 1850-1910.


  • All households and group quarters.


  • United States: 1850a, 1850b, 1860, 1870, 1880a, 1880b, 1900, 1910