Aviation Formulary V1.43
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Geodesic distance between two Latitude/Longitude points using Vincenty ellipsoid formula in JavaScript
Using filters with GPSBabel - OpenStreetMap Wiki
gpsbabel -i gpx -f in.gpx \ -x transform,wpt=trk,del \ -x radius,distance=1.1K,lat=40.01,lon=10.001,nosort,exclude \ -x transform,trk=wpt,del \ -o gpx -F out.gpx