Answer:
Many European explorers, including Columbus, sought a water route from Europe to Asia that was more economical than sailing around Africa. As the New World was colonized, Americans and Europeans still used the traditional African route or sailed around the tip of South America. Both of these routes were slow, dangerous, and expensive, and the search for a water route to Europe and Asia became a search for a water route across North America, known as the Northwest Passage.
For hundreds of years, the fabled Northwest Passage inspired explorers as they tried to find a navigable route through North America. President Thomas Jefferson still envisioned such a water route when he instructed Meriwether Lewis, "The object of your mission is to explore the Missouri river, & such principal stream of it, as, by it's course & communication with the waters of the Pacific Ocean, may offer the most direct & practicable water communication across this continent, for the purposes of commerce" (DeVoto, 1953).
At the time of Lewis and Clark and the Corps of Discovery, maps were "rare, expensive, and available only to the leaders of society." The process required to make a map involved a great deal of time and skill, and most available maps actually were published in England or France (Ehrenberg, 2000). Jefferson made great efforts to ensure that Lewis and Clark had access to the most recent maps and exploration narratives of their time.