[Qgis-user] US county shared boundaries

[Brent Blumenstein]

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On Wed, Mar 11, 2026 at 11:45 AM Brent Blumenstein <bab at triarcconsulting.com>
wrote:

> 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
>


-- 
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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