These websites contain numerous high-resolution maps:

• https://www.davidrumsey.com/
• http://www.oldmapsonline.org/
• British Library Online Maps
• University of Texas Libraries Map Collection
• http://www.bigmapblog.com/
• John R. Borchert map library at the University of Minnesota

If you are looking for satellite imagery, the Earth Observing System’s Land Viewer is excellent and user-friendly. It does, however, cap the number of satellite images you can view at ten per day, so plan accordingly. You could also use Planet Explorer, or Landsat Lens.

Sample Data Sets

• Abby Mullen, Quasi-War Encounters. See related map of this data. (Source credit: Lincoln Mullen)
• Civil War Washington has both GIS data and .csv data
• Railroads and the Making of Modern America
• The Trans-Atlantic Slave Database
• Historical GIS Forum has tons of data organized by historical topic
• Digital Atlas of Medieval and Roman Civilizations
• UN Data
• Teaching with Data
• The Correlates of War Project concerns militarized conflict around the world
• Search for data sets at https://data.world/ (free account required)
• Search for data sets at Google Tables