"It just got way, way easier to search and browse the US Geological Survey’s collection of historical topographic maps, thanks to a new online map viewer. These maps—more than 178,000 of them—date back to 1880, and they cover the entire country. Best of all, they’re free to download..."</p>
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http://ngmdb.usgs.gov/maps/Topoview/
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http://ngmdb.usgs.gov/maps/Topoview/
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