How did Lewis & Clark know where to go?

On May 14, 1804 The Corps of Discovery led by Captains Merriweather Lewis & William Clark began their journey across western North America, into completely unexplored and unmapped territory. At least that’s what is commonly thought. But was the west really unexplored?

Social Media
------------------------
Map Shop - http://thegeographygeek.com/
Instagram - Instagram: thegeographygeek
Patreon - Patreon: geographygeek
Email Newsletter - http://eepurl.com/hSeKpj

Book Sources
--------
"Mapping the West with Lewis & Clark" by Ralph E. Ehrenberg & Herman J. Viola
"Undaunted Courage" by Stephen Ambrose

RareMap.com Sources
-------
1718 Guillaume De L'Isle - https://tinyurl.com/4u2zx6w9
1814 Lewis & Clark Map - https://tinyurl.com/3e9fjf2h
1804 Aaron Arrowsmith - https://tinyurl.com/3unk2tm6
Aaron Arrowmith Maps - https://tinyurl.com/2wum3sju