English: This is a visualization of US expansion in North America from 1700 to 1900, seen through changes in the spatial distribution of post offices.

Data from the USPS Postmaster Finder, with lat/long coordinates extracted from placenames through correlation with the USGS Geographic Names Information System.

Of course, there are a few caveats: the USPS admits that their post office data is a constant work in progress, so there are likely many offices that aren’t shown on this map. About 10% of the placenames in the USPS data failed to find any coordinate matches in the GNIS data, so those don’t show up either. Finally, the USPS data includes closing dates for many offices, but these aren’t represented on the map, which might give a false sense of density in some areas.

http://derekwatkins.wordpress.com/2011/08/06/posted/