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Smart, but I don't know how granular you could get unless you had known servers at every possible datacenter.

For me it roughly the right continent where I am (if I am on VPN, otherwise it also include greenland and arctic regions).

But with servers in more locations, and possibly accounting for network topology (could be mined with traceroute?), accuracy could go quite up.

Just use a CDN with decent edge coverage. I'm sure those results can be improved drastically.

You did notice that the backend code is Go code snippet that needs to run at the end points?

So? I'm sure some CDN out there allow you to run your own code, hence it could be more granular (as OP was interested in).