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 < [email protected]> 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 > <[email protected]> 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. > _______________________________________________ > QGIS-User mailing list > [email protected] > List info: https://lists.osgeo.org/mailman/listinfo/qgis-user > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user > -- Brent A. Blumenstein, PhD Trial Architecture Consulting Port Saint Lucie Florida USA www.TriArcConsulting.com [email protected] mobile: +1 (206) 390-0081
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