Ask HN: Can you prevent me from hacking Facebook to check in from Siberia?

I'm sitting in Florida. Can you stop me from faking a Facebook check-in by altering the coordinates provided by JavaScript location services? I'm curious because I'm working on a Web app (geopackages.com) that relies heavily on JS loc services, and I hope to prevent tampering. No luck on Stack Overflow, so I'm trying here because this community always has creative ideas concerning security. Plus, I think it's an increasingly relevant issue, as more apps are using client-side loc services to offer special promotions, etc.