For all of these reasons, Jefferson sent out the Lewis and Clark expedition to discover everything about the West, taking a journey that at that time seemed like a very long distance. (Ambrose) The Lewis and Clark expedition lasted for two years, from 1804 to 1806, and while some of the preparations had been made for such a journey and therefore it was not the first journey made across the continent, it was the first American expedition across land that would travel to the Pacific coast and return to the East coast
Finally, they would need to be able to accurately map out the terrain of the land. (Rick et al