As more mobile phones tap into the Internet, people increasingly turn to them for location-centric services like getting directions and finding nearby restaurants. While Global Positioning System (GPS) technology provides excellent accuracy, only a fraction of phones have this capability. What's more, GPS coverage is spotty in dense urban environments, and in-phone receivers can be slow and drain a phone's battery.

To sidestep this problem, last week Google added a new feature, called My Location, to its Web-based mapping service. My Location collects information from the nearest cell-phone tower to estimate a person's location within a distance of about 1,000 meters. This resolution is obviously not sufficient for driving directions, but it can be fine for searching for a restaurant or a store. "A common use of Google Maps is to search nearby," says Steve Lee, product manager for Google Maps, who likened the approach to searching for something within an urban zip code, but without knowing that code. "In a new city, you might not know the zip code, or even if you know it, it takes time to enter it and then to zoom in and pan around the map."

Many phones support software that is able to read the unique identification of a cell-phone tower and the coverage area that surrounds it is usually split into three regions. Lee explains that My Location uses such software to learn which tower is serving the phone--and which coverage area the cell phone is operating in. Google also uses data from cell phones in the area that do have GPS to help estimate the locations of the devices without it. In this way, Google adds geographic information to the cell-phone tower's identifiers that the company stores in a database.

Another approach is used by a startup called Plazes. This Swiss location-tracking service has, over the past few years, established a relatively small database of Wi-Fi hot spots around the world, manually geotagged by Plazes users. Now, in a relatively large city, it's possible to log on to Plazes using a Wi-Fi connection, and have the software guess where you are because previous users have logged the Wi-Fi hot spots' location, which can be an address or a business name.

In addition, researchers at Intel and the University of Washington developed research software that uses a combination of Wi-Fi and cell-phone tower radios to pinpoint a person's device. The now-complete project, called Place Lab, takes advantage of any radio a person is using, whether it's Wi-Fi on her handheld or laptop, or a cellular signal from her phone, to triangulate location.

Google expects that over time, My Location's accuracy will improve. As the database grows, says Lee, the service will become more accurate. It will never be as accurate as GPS, but he expects that it could eventually find a person within a couple hundred meters. And even at that level of accuracy, there's still a lot of searching that Google can do. "Search is really important," he says. "This product is searching based on a map, but there are other types of local searching and advertising and other products that can be made relevant" with the technology, Lee says.

A few years ago, the Federal Communication Commission required cell-phone companies to find a way to locate people making 911 calls so that rescue workers could find them. The approach that most cellular providers take is to use triangulation, which works if a person's phone is visible to two or more cell-phone towers. But while a cellular carrier can use information from any of its towers, Google and other companies can't. The software available to them on a cell phone only has access to the tower that the phone is using at any given time, not to any neighboring towers.