[Qgis-user] US county shared boundaries

[Brent Blumenstein]

Thanks - I will explore immediately. Seems like I chose the wrong file
based on ignorance, which is not an uncommon event.

On Wed, Mar 11, 2026 at 10:21 AM Keith Jenkins via QGIS-User <
qgis-user at lists.osgeo.org> wrote:

> 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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-- 
Brent A. Blumenstein, PhD
Trial Architecture Consulting
Port Saint Lucie Florida USA
www.TriArcConsulting.com
bab at TriArcConsulting.com
mobile: +1 (206) 390-0081
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