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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