Defeating VPN location-spoofing by mapping network delays (thestack.com)

You should have linked the source instead of a very short summary. There are plenty of interesting details in the PDF document: https://sce.carleton.ca/~abdou/CPV_TDSC.pdf

At first I thought this technique sounded like it would work very well for determining if, for example, a European user was using an American IP, but not work well for bordering countries, simply because I assumed the error margin would be fairly large. However, according to the source paper, the error margin is only a few kilometres. Impressive.