Turn a corner, and the markers reflect the new scene. Click a marker for a restaurant, and you can see customer reviews and price information. Decide you'd rather be sightseeing? The indicators are easily changed to give information about the buildings you're passing.
This computer-enhanced view of the world is not just available to cyborgs in science-fiction movies. Increasingly it can be found on cell phones, for free or on the cheap, through programs that provide "augmented reality."
These applications take advantage of the phones' GPS and compass features and access to high-speed wireless networks to mash up super-local Web content with the world that surrounds you.
That means you can see available apartments on the block you're moseying down. You can view photos other people have taken at the park you're passing, or find the nearest bus stop or hotel room _ all by just holding your phone up and peering at its screen.
The possibilities for melding the virtual and actual worlds have just started to become apparent. The first phones with Google's Android operating system, which enables augmented reality, have come out in the past year. The iPhone became augmented-reality-friendly with the compass that debuted in June on the iPhone 3GS. Apple also recently joined Google in making it possible for software developers to overlay images on the phone's camera view.
As cell phones get even smarter and GPS and wireless networks improve, we may soon be spending more time in a virtually enhanced world, using information gathered from the Internet to inform everything from eating to playing video games.
One company working to make this happen is Amsterdam-based Layar, which recently released an augmented reality browser by the same name for Android phones. Layar lets you search for things on Google, but delivers the results based on your location, which it determines from the GPS readout. So you can search for, say, a bike shop or a pet store close to where you happen to be.
If you don't feel like actively searching, you can sign up to have certain kinds of information automatically appear on your phone screen. For instance, Layar lets other companies build on its system to overlay information about such places as skateboarding spots and local landmarks. A startup called Brightkite uses Layar to let people post virtual tags, with their locations and activities, that other people can see if they use the same app.
Layar's goal is to create a "serendipitous experience" that lets you can discover new things about your surroundings, says co-creator Maarten Lens-Fitzgerald.
The company is working on a 3-D function, too, that it hopes to release in November. That will allow virtual objects to be placed "on" actual locations. A guy might be able to put a virtual heart in front of his girlfriend's house for Valentine's Day _ and she would see it if she used the Layar app on her phone.
For a year, Yelp, a Web site with business reviews written by customers, had an iPhone app that used the device's GPS and wireless Internet connectivity to deliver local search results. But when the iPhone got a compass, bloggers wondered whether Yelp would go further and make its app overlay information onto a real-time view of the world. After noticing the speculation, Yelp quietly created such an app this summer, spokesman Vince Sollitto said.
The augmented-reality program, known as Monocle, was built for Yelp by an industrious intern and originally hidden in Yelp's app. (It was activated if you shook the iPhone three times.) Monocle is now a formal feature that combines the iPhone's camra view with tiny tags indicating the names, distances and user ratings of proximate bars, restaurants and more. Poke a floating tag on the screen with your finger and up pops detailed information about the business.
Among the other augmented reality programs that recently have hit Apple's App Store is Robotvision, a 99-cent program built by Portland, Ore.-based developer Tim Sears.
If you hold your phone parallel to the ground, Robotvision displays a map of your surroundings. Hold the phone up, however, and it goes into augmented-reality mode, highlighting places like coffee shops and bars. Robotvision also can search for other kinds of businesses with Microsoft's Bing search engine. You can view pictures that people took nearby and posted to Flickr with a "geotag" of the shot's physical location. Or you can see Twitter postings composed in the area.
Next Sears plans to update the application with local content from Wikipedia.
"Looking at the world around you is something everyone can get. That, to me, is what makes it so fascinating," he said.
Consumers may feel that way initially, too. But Blair MacIntyre, an associate professor who runs the Augmented Environments Lab at Georgia Tech, worries that the technological limitations these applications currently face will keep them from living up to what people imagine they can do. Similar disappointments followed early hype for virtual reality, a cousin of augmented reality in which the landscape is entirely computer generated.
Indeed, there are issues hindering augmented reality applications. Cell phones need to be more powerful, with improved cameras and graphics capabilities and more accurate GPS. The technology can generally pinpoint location to within 30 feet if a user is outdoors.
The limitations mean businesses you see on the screen are often not actually in front of you, though they are nearby. And often tags sometimes just kind of dart around on the screen, seemingly untethered to a physical place. Another problem: Using GPS for extended periods quickly sucks up the battery life on most phones.
Developers and industry watchers are optimistic, though, that in the next few years we might see everything from augmented reality video games to museum guide services that recognize paintings and can pull up videos showing the artist at work.
"Things are pretty cool right now," Sears says, "but they're definitely going to get better."