I produced this animation for an independent study in Geographic Information Systems when I was a grad student at Southern Illinois University. I used available census data sets for 1900-1990, and hand-entered all county population data from a reference for 1790-1890 (about 3000 counties times 10 entries per county) (dataset available). The data table was then linked (ARC/Info GIS software) to a standard coverage county boundaries to produce a series of images, which I then tied together using a GIF animation tool. Geographers may notice that while the population data changes over time, so too should the county boundaries! But correcting each map in the time series for historic county boundaries was beyond the scope of this project. Back in 1997, this was pretty impressive--but you can get things like this online pretty easily now from NASA or the USGS.