[Qgis-user] US county shared boundaries

[chris hermansen]

David, Brent and list,

On Tue, Mar 10, 2026 at 12:43 PM David Strip via QGIS-User <
qgis-user at lists.osgeo.org> 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.
>

Disclaimer - not from the USA so my opinions here may not count.

Having said that, I'm not comfortable with the exhaustiveness of David's
proposal below.


>
> 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.
>
>
I'm assuming that there are more than one island in the USA that contains
more than one county; certainly Long Island incorporates four counties:
Nassau, Suffolk, Queens and Kings.  As well, Queens and Kings are also
boroughs of New York City.

Given what I understand of Brent's interest, it seems to me that there is a
kind of "adjacency" relationship between Queens, Kings (Brooklyn) and
Manhattan, Bronx and Richmond counties, that is, the boroughs of New York
City.  I don't know if Brent would consider Nassau and Suffolk counties to
be "adjacent" to Manhattan...

Also there is the question of adjacency across state lines, I suppose...
For example, Long Island looks pretty adjacent to Connecticut to me.  Does
that count?

-- 
Chris Hermansen · clhermansen "at" gmail "dot" com

C'est ma façon de parler.
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