By KATE MURPHY

When Adam Savage, host of the American popular science television program “MythBusters,” posted a picture on Twitter of his automobile parked in front of his house, he let his fans know much more than that he drove a Toyota Land Cruiser. Embedded in the image was a geotag, a bit of data providing the longitude and latitude of where the photo was taken.

Hence, he revealed exactly where he lived. And since the accompanying text was “Now it’s off to work,” potential thieves knew he would not be at home. Privacy advocates have recently begun warning about the dangers of geotags, which are embedded in photos and videos taken with GPS-equipped smartphones and digital cameras.

Because the location data is not visible to the casual viewer, the concern is that many people may not realize it is there, and they could be compromising their privacy, if not their safety . “I’d say very few people know about geotag capabilities,” said Peter Eckersley, a staff technologist with the Electronic Frontier Foundation in San Francisco, “and consent is sort of a slippery slope when the only way you can turn off the function on your smartphone is through an invisible menu that no one really knows about.”

The Web site ICanStalkU.com provides detailed instructions for disabling the photo geotagging function on iPhone, BlackBerry, Android and Palm devices. Because of the way photographs are formatted by some sites like Facebook and Match.com, geotag information is not always retained when an image is uploaded, which provides some protection .

But experts say the problem goes far beyond social networking and photo sharing Web sites . “There are so many places where people upload photos, like personal blogs and bulletin boards,” said Johannes B. Ullrich, chief technology officer of the SANS Technology Institute, a Maryland company that provides network security training .

A handful of academic researchers and independent Web security analysts, who call themselves “white hat hackers,” have been trying to raise awareness about geotags. By downloading free browser plugins like the Exif Viewer for Firefox (addons.mozilla.org/en-US/firefox/ addon/3905/) or Opanda IExif for Internet Explorer (opanda.com/en/ iexif/), anyone can pinpoint the location where the photo was taken and create a Google map. Moreover, since multimedia sites like Twitter and YouTube have userfriendly application programming interfaces, or A.P.I.’s, someone with a little knowledge about writing computer code can create a program to search for geotagged photos in a systematic way. For example, they can search for those accompanied with text like “on vacation” or those taken in a specified neighborhood.

“Any 16-year-old with basic programming skills can do this,” said Gerald Friedland, a researcher at the International Computer Science Institute at the University of California, Berkeley. He and a colleague, Robin Sommer, wrote a paper, “Cybercasing the Joint: On the Privacy Implications of Geotagging.”

ICanStalkU.com warns about the dangers of geotags by displaying a stream of geotagged photos posted on Twitter; the person who posted the photo also gets a notification via Twitter. “The reaction from people is either anger, like ‘I’m going to punch you out,’ or ‘No duh, like I didn’t already know that’ or ‘Oh my God, I had no idea,’ ” said Larry Pesce, one of the site’s developers.