[Qgis-user] US county shared boundaries

[Keith Jenkins]

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
<qgis-user at lists.osgeo.org> 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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