David Rumsey:
All the maps contain rich information about the past and represent a sampling of time periods (1680 to 1930), scales, and cartographic oart, resulting in visual history stories that only old maps can tell. Each map has been georeferenced, thus creating unique digital map images that allow the old maps to appear in their correct places on the modern globe.
Some of the maps fit perfectly in their modern spaces, while others (generally earlier period maps) reveal interesting geographical misconceptions of their time and therefore have to be more distorted to fit properly in Google Maps and Earth. Cultural features on the maps can be compared to the modern satellite views using the slider bars to adjust transparency.
Very cool.