New answers tagged county
You can find a script inspired by MvG's answer and a few examples here : https://github.com/laem/regions-topojson Sadly, not all geonames administrative relations are tagged with iso-3166-2...
Just use a Spatial Join. You can do this with QGIS (free to download; you may already be using it). Here is a tutorial that does exactly what you are trying to do using nursing homes as points (you'd use your GPS locations instead) and boroughs as polygons (you'd use Counties). Tutorial
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