A year ago Twitter was just a microblogging platform. Sure, it was a popular one, but it wasn't until the announcement of its geotagging API that it took its first step toward being a location platform. Since then, it has expanded its offering to include the four geo APIs I've categorized below.
Even before Twitter made it possible to geotag tweets, users and developers could find tweets by location using Twitter search. To do so, add near:cityname or near:lat,lng to a query. From the API side, the prefered method is to include the geocode parameter to get similar results. But be careful--most of your results won't be geocoded. Instead, they'll be based off the location provided in the user's profile, as described in the docs.
You'll know it's a geocoded tweet if the <twitter:geo> Atom field (simple "geo" in JSON) is filled in.
When most people think of Twitter geo APIs, they probably think of tying a latitude/longitude pair to a tweet. Though we've questioned its adoption in the past, there's no doubt that the geotagging feature improves the Twitter service. By keeping the input simple it provides a platform upon which many location-aware applications will no doubt be built--from crisis response to parking finders.
At Chirp, Twitter announced it is adding context to geotagged tweets with Points of Interest. This will allow not just coordinates, but actual landmarks, to be connected to tweets. Though Twitter claims to not be aiming to create a check-in service, the functionality is similar to location-sharing applications like FourSquare.
A latitude/longitude pair is much more useful than nothing, but knowing a bit more about a location helps make more sense of a tweet's context, especially to users. Perhaps that is why Twitter acquired the makers of GeoAPI (our GeoAPI profile) so early in the product's life.
Given a point, the API returns the boundary information: neighborhood, city, etc. Twitter recently integrated the service with its front-end to let users geotag an area as opposed to a specific point.
Another element of GeoAPI is the ability for developers to add layers of data and query it. For example, you could create a location finder for a chain of stores using GeoAPI as your back-end. The service remains, but it's hard not to be wary of its future. It's unclear whether Twitter wants to be a platform for private, persistent location data. With SimpleGeo's public release, we expect GeoAPI to be less relevant as a place to store your own data.
Nevertheless, with its many geo APIs, it's clear we need to keep an eye on Twitter as location sharing takes a larger role in the lives of users. For a look at location straight from Twitter, see Raffi Krikorian's Adding the "Where" to the "When" presentation, embedded below.