I'm using twitter4j to collect tweets from Twitter based on location, specifically using the geoCode() method which uses lat/lon measurements to localize the returned tweets. The geoCode() method works fine: If i search "weather" limited to San Diego's lat/lon, the tweets are about it being sunny, if i search somewhere else, its rainy, etc. However, if I then do a getGeoLocation() for each individual tweet, it returns null almost every time. SO, that means, whatever info the geoCode() is using must come from somewhere else.

According to the documentation @  
http://twitter4j.org/en/javadoc/index.html  ,

the geoCode() method:
"returns tweets by users located within a given radius of the given latitude/longitude, where the user's location is taken from their Twitter profile"

But I don't understand this. Users type in their location themselves, so how could twitter 4j take that data and connect it to the lat/lon measurements it requires?

What I really want to know is where does the geoCode() method get the data it needs from a  tweet to determine if it is near the specified location? And is it possible to access that info within any tweet, regardless of if a geoCode was set in the first place?