Historical Context of Lewis and Clark's Cartographic Work
-by Al Bredenberg

Mapping a New Territory

When Captains Meriwether Lewis and William Clark started up the Missouri River on May 14, 1804, they carried with them, among other maps, one showing what was then known about the American Northwest.

And yet Nicholas King's map, created especially for the Lewis and Clark expedition, was compiled from the best geographic sources of the time. (One of King's primary sources was the Aaron Arrowsmith map of 1802, based partly on information from an Indian named Ak ko mo ki.)

At the time of the expedition, the United States of America had just acquired from France a vast area of the North American interior. Called the Louisiana Purchase, this area included much of what are now the 15 states of Montana, Wyoming, North Dakota, South Dakota, Colorado, Nebraska, Kansas, Minnesota, Iowa, Missouri, New Mexico, Texas, Oklahoma, Arkansas, and Louisiana.

U.S. President Thomas Jefferson and his supporters wanted to learn more about this new area of the country, its waterways, its inhabitants, its natural resources, and its possibilities for settlement by Americans. One of the chief purposes of the Corps of Discovery expedition was to map the new Northwest territory, to fill in the empty spaces on the maps of the time.

Jefferson was an avid geographer from his early years. He took steps to accumulate the best geographic lore available and to pass that along to Lewis, along with all existing maps that might help the expedition. He even sent Lewis to Philadelphia and Lancaster County, Pennsylvania, to study surveying and astronomical observation and to get key geographic and cartographic information from scholars there.

Jefferson's Instructions to Lewis

In one of the first instructions set out in the letter, Jefferson directed Lewis "to explore the Missouri river, and such principal streams of it, as, by its course and communication with the waters of the Pacific ocean, whether the Columbia, Oregan, Colorado, or any other river, may offer the most direct and practicable water-communication across the continent, for the purposes of commerce."

Jefferson hoped that Lewis and Clark would find a water route across North America. Up to that point, trade with India and the Orient was only possible by sailing south around Africa or South America, a long and arduous journey. For hundreds of years, explorers had searched for a way to cross the continent by a water route, sometimes know as the Northwest Passage or the Passage to India.

Jefferson and most other geographers of the time were not aware of the size and extent of the Rocky Mountains. They believed that the source of the Missouri River lay in a low range of mountains or hills or perhaps a plateau (often referred to as a "height-of-land") somewhere in the northwest region of the newly acquired Louisiana territory. They believed also that just to the west, not far from the source of the Missouri, was a great "River of the West" (perhaps the Columbia) that emptied into the Pacific Ocean. The distance between the two rivers was supposed to be relatively short, perhaps only a one-day portage (overland trek carrying boats, supplies, and goods).

(The 1802 Soulard map, used by Lewis and Clark, shows the Rocky Mountains as a narrow band, as well as the fabled River of the West emptying into the Pacific Ocean.)

If Lewis and Clark were able to discover such a route, this would provide tremendous economic benefits for the new nation and would solve an ancient geographic mystery.

As it turned out, Lewis and Clark discovered that the Rocky Mountains were much more extensive and rough than expected (over 300 miles across in some places) and that the upper reaches of the Missouri River were not navigable. The supposed one-day portage was over 100 miles. It took the party 11 days on foot and horseback to cross the Bitterroot Mountains on the Lolo Trail, in an ordeal that almost cost them their lives. The hoped-for water route across the continent simply did not exist.

Importance of Mapping

Notice from Jefferson's instructions how he emphasized the importance of careful mapping during the upcoming journey: "Beginning at the mouth of the Missouri, you will take observations of latitude and longitude, at all remarkable points on the river . Your observations are to be taken with great pains and accuracy; to be entered distinctly and intelligibly for others as well as yourself ."

Although it was Lewis who received Jefferson's instructions, it was ultimately William Clark who did most of the actual surveying and mapmaking during and after the expedition.

Jefferson wrote his set of instructions before the U.S. had actually taken possession of Louisiana. However, it's evident from what he wrote that Jefferson was thinking ahead. He told Lewis to collect information that would add to the nation's strategic and military knowledge and that would help assess the economic value of the new territory.

Besides mapping the land, Lewis was to acquaint himself in detail with the native inhabitants of the territory, as well as the soil and terrain, animal and plant life, mineral resources, and climate. Jefferson anticipated that, as time went on, the nation would need more space for expansion, especially for agriculture. He wanted a better understanding of the potential of Louisiana for meeting that need.

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