Twitter's plans to let its users attach geographic coordinates to their messages have external developers very interested at the possibilities this new geolocation functionality will open up for their applications.

By the company's own admission, geotagging in the popular microblogging and social-networking service has been a rudimentary and inexact affair, dependent on a text field that users can leave blank or fill in with anything they like.

However, Twitter announced last week that it will soon let users stamp their postings with precise location data, and give external developers access to that data via a new geolocation API (application programming interface.)

The news has thrown many developers into an intense brainstorming mode, thinking up ways to enhance existing applications and build new ones that use this location data.

Michael O'Connor is extremely excited about the potential of the Twitter geolocation API to enhance his application Myallo HotList, which is awaiting approval for the iPhone application store.

Myallo HotList shows the locations of users' friends and favorite places on a radar-style map, whose blips change color -- from bright red to light blue, with shades of orange, yellow and gray in between -- to reflect their "hotness" depending on how close or far they are. The location data comes from the one stated on Twitter profiles.

"That's why Twitter's improvements in geolocation mean so much to me. The more people who report their locations on Twitter, and the more timely those locations, the better and more accurate Myallo HotList will get," said O'Connor, president and developer at Leptonic Systems, in an e-mail interview.

O'Connor expects that the geolocation API will make it easier for his application to determine whether a location is exact, vague, or just a funny phrase. It will also be useful to know the date and time of a geotagged Twitter message.

"With each tweet, a location, date and time could be available, and you could map the route of a person as they travel," O'Connor said, adding that he has no current plans to add this to his application.

One feature he does expect will be enhanced is Myallo HotList's ability to "discover" nearby entities by calling up data from the Yelp customer reviews site and displaying them as hotter or colder on the map based on distance and reviews.

"It appears I can do a very similar thing with Twitter. I could have it scan for nearby tweets and put them on the radar. And the contents of the tweet and/or profile could help rate the hotness of the person," said O'Connor. For example, if a user has told the app that he is interested in tweets related to iPhone, and a nearby user sends a Twitter message such as 'I like my new iPhone,' it might highlight that person with this similar interest in the neighborhood.

"Finally, I think this will encourage many more people to include location information. The more the merrier!" he said.

Twitter's geolocation functionality could have a significant impact on developers and users, because it has the potential to bring together the online and the real worlds, a big step forward in social networking, said IDC analyst Al Hilwa.

"Combined with mobile devices, it allows people to bring their online persona with them into the pub, the public square or even to the next-door neighbor. It really could spark a whole host of new applications that are scenario-specific," Hilwa said via e-mail.

Joel Strellner, founder and CEO of Twitturly, sees a big opportunity to improve the company's inView application, which is now in private beta.

InView lets businesses monitor their brand mentions, as well as competitors' brand mentions, on Twitter, by tracking keywords, domain names and URLs, and sending notifications in real time via instant message, e-mail and other methods.

"In our case, we'll be able to expand inView so that businesses using it can know where the users are in the world that are tweeting about their brand," Strellner said in an e-mail interview.

Another interested developer is Dennis Crowley, cofounder of FourSquare, a location-based social-networking application that integrates closely with Twitter and Facebook.

"The Twitter geolocation API will offer a whole new data set to play with," he said.

Crowley looks forward to the geolocation API helping to automatically filter out a lot of the Twitter "noise" in the massive Twitter data stream that has no relevance for FourSquare, which can be used from PCs or mobile devices.

The developers interviewed give the thumbs up to Twitter's decision to make the geolocation functionality an opt-in feature for end-users, so that people feel in control of whether and when to share their location.

They also hope Twitter will give users granular controls for sharing location, so that they can make the information available to certain contacts and not others, if they so wish.

"I haven't looked closely at the proposed API yet, but I assume it will at first be like it is currently: Twitter locations are totally public. Anyone can see the location field in your profile," O'Connor said.

It would be much better if users get the option to make their location available only to those they "follow" or who follow them on Twitter, or to hand-picked contacts, he said.

It will also be important for Twitter to closely monitor how developers use the location data their applications collect. "The area that I am most concerned about at this point is how the data could be used," Strellner said.

Applications that request geolocation data should get an approval from Twitter before getting access to it, he said. "This would allow Twitter to block applications using this data in a way that could be malicious."

Crowley, founder of the Dodgeball mobile social network that Google acquired in 2005, acknowledges that location-based services are tricky to design so that users feel comfortable with them. However, he feels confident Twitter will get it right.

"Twitter will be really careful with this. They know what they're doing. They're making all the right moves," Crowley said.