As geolocation technology improves, websites are increasingly blocking groups of visitors and carving the Web into smaller chunks — in some cases, down to a ZIP code or employer.

Type “dentist” into Google from New York, and you’ll get ads for dentists in the city. Try watching a Cubs baseball game from a computer in Chicago, and you’ll be stymied. Pre-existing local TV rights block the webcast.

The same technology is also being used by a British casino to keep out the Dutch and by online movie distributors to limit viewing to where it’s permitted by license, namely the United States.

To privacy advocates like Jason Catlett, that technology can detect users’ whereabouts isn’t the most disturbing aspect of this trend. Rather, it’s the fear that websites will try to mislead visitors.

A company, for instance, might show different prices when competitors visit; a political candidate might highlight crime-fighting in one area, jobs in another.

“The technical possibilities do allow a company to be two-faced or even 20-faced, based on who they think is visiting,” Catlett said.

Alan Davidson, associate director for the Washington-based Center for Democracy and Technology, worries that governments will try to employ the technology to enforce their laws within artificial borders they erect. Such concerns, not entirely new, have grown with the technology’s reliability, he said.

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