I built a tool yesterday to observe geolocated tweets as they came in on a world map. Its kind of cool to zoom in on specific cities. After just 30 seconds I gathered quite a few in New York. Clicking on a marker reveals the profile pic and the tweet.
And then zooming out I could see clusters along cities in the Northeast.
After several minutes I zoomed out to see the world map, but at that point there were too many tweets and my browser crashed. This is where selective coarsening would be useful.
You can find the project on github.