Hi, Brent. In your original message, you mentioned the cb_2020_us_all_500k.zip boundaries, which are clipped to the shoreline.
You might also be interested in looking at the Tiger/Line shapefiles that are available here: https://www.census.gov/cgi-bin/geo/shapefiles/index.php These boundaries are not clipped to the shoreline, so the polygons for counties like San Juan and Whatcom extend out into the water and actually share a border, and thus are truly adjacent polygons. I'm attaching a map image to illustrate -- the thick borders are the Tiger/Line shapefiles. The US Census Bureau also publishes a county adjacency file which I think is based on the Tiger/Line polygons. https://www.census.gov/geographies/reference-files/time-series/geo/county-adjacency.html Keith On Wed, Mar 11, 2026 at 3:18 AM Brent Blumenstein via QGIS-User <[email protected]> wrote: > > Thank you for the response. Yes I agree that some "handwork" could be used > here but I think I should pursue the automated solution until I cannot > implement it. I have found that the using the census data does indeed find > counties where the coordinates indicate isolation, so my plan to focus on > isolated counties is what I am pursuing at the moment. > > However, I found that the census data does not provide a consistent > identification of isolated counties across states. There is a huge difference > between Masachuchetts and Washington states. The census data does identify > Dukes County as isolated but does not identify San Juan County as isolated. > In fact, San Juan county is connected to Whatcom and Skagit Counties but does > not identify San Juan County as connected to Island, Clallam and Jefferson > Counties, which are clearly neighbors even though a little further away. > Dukes County and San Juan Counties have roughly the same distance to the > nearest county, so there must be some distance parameter at work when the > dataset was created, but this theoretical parameter seems to implemented > differently in these two states. > > This finding complicates the algorithm I dream about refining. > > BTW: The coordinate files from xxx.yyy say that San Juan County has five > neighbors {Whatcom, Skagit, Island, Clallam and Jefferson Counties}. This > result seems correct to me geven the goals of the project. Similarly, xxx.yyy > identifies Dukes County as having mainland counties as neighbors. > > I have attached maps for reference.
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