Old Maps Online
oldmapsonline.org
A search engine for historical maps aggregated from major collections worldwide, allowing geographic and time-period filtering across hundreds of thousands of maps.
Why it is useful
Solves the problem of historical maps being scattered across dozens of institutional collections with incompatible search interfaces. A single query returns maps of the same location across multiple centuries from multiple archives, making it invaluable for tracking how cities, coastlines, and borders changed over time. The zoom quality on most maps is excellent.