[Qgis-user] US county shared boundaries

[Brent Blumenstein]

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.



On Tue, Mar 10, 2026 at 3:43 PM David Strip <qgis-user at stripfamily.net>
wrote:

> If you are determined to have an automated method, you can find neighbors
> using this approach. <https://gis.stackexchange.com/a/418824/4449>
> Then look for counties with no neighbors. For these counties look for the
> closest counties in the same state and add these to the list.
>
> But that seems like a lot of work for a small problem. A cursory search
> suggests the number of island counties is quite small and you can patch up
> the data by hand:
>
> 1. Hawaii (All 5 Counties)
>
> As the only state located entirely on an archipelago, all its counties are
> island-based.
>
>    -
>
>    *Hawaii County:* The "Big Island."
>    -
>
>    *Honolulu County:* Oahu and the Northwestern Hawaiian Islands.
>    -
>
>    *Kalawao County:* A small peninsula on Molokai (the smallest county in
>    the US).
>    -
>
>    *Kauai County:* Kauai, Niihau, and others.
>    -
>
>    *Maui County:* Maui, Lanai, Molokai, and Kahoolawe.
>
> 2. Washington (2 Counties)
>
> While most of Washington is on the mainland, its northwestern corner is
> home to:
>
>    -
>
>    *Island County:* Comprised primarily of Whidbey and Camano Islands.
>    -
>
>    *San Juan County:* Comprised of the 170+ islands that make up the San
>    Juan archipelago.
>
> 3. Massachusetts (2 Counties)
>
>    -
>
>    *Nantucket County:* Coextensive with Nantucket Island and its tiny
>    neighbors (Tuckernuck and Muskeget).
>    -
>
>    *Dukes County:* Comprised of Martha's Vineyard and the Elizabeth
>    Islands.
>
> 4. New York (2 Counties)
>
> Technically, two of New York City's boroughs are their own island counties:
>
>    -
>
>    *Richmond County (Staten Island):* Comprised of Staten Island and
>    surrounding smaller islets.
>    -
>
>    *New York County (Manhattan):* Comprised of Manhattan Island,
>    Roosevelt Island, and others. (Note: A tiny 0.1 sq mile neighborhood called *Marble
>    Hill* is physically on the mainland due to a canal being dug in 1895,
>    but it remains legally part of the island county).
>
> 5. Rhode Island (1 County)
>
>    -
>
>    *Newport County:* While some of its towns are on the mainland, the
>    county itself is largely defined by *Aquidneck Island*, *Conanicut
>    Island*, and *Prudence Island*. However, it is often excluded from
>    "entirely island" lists because a portion of the county (the town of Little
>    Compton and Tiverton) is physically attached to the Massachusetts mainland.
>
> 6. Florida (1 County*)
>
>    -
>
>    *Monroe County:* This is a "hybrid" case. While the populated *Florida
>    Keys* are entirely islands, the county boundaries actually extend onto
>    the mainland to include a large, uninhabited portion of the Everglades.
>
>
>

-- 
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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