George Vancouver. “A Chart Shewing Part of the Coast of N.W. America.” In A Voyage of discovery to the North Pacific ocean, and Round the World. London, 1798. The Tracy W. McGregor Library of American History

George Vancouver accompanied Cook on his voyage around the world in 1772-1774 and served as a midshipman on Cook’s explorations along the West coast of North America. As commander of the ship Discovery, he embarked on a mission to survey the coast of the American Northwest and search for a water connection to the eastern part of the continent. He produced superb charts of the Northwest coast and wrote a lengthy account of his voyage, A Voyage of discovery to the North Pacific ocean, and Round the World (1798). The map shown here is from the atlas that accompanied Vancouver’s book.

Vancouver, like Cook before him, initially missed the Columbia River on his voyages along the coast but later learned of the great river from an American sea captain, Robert Gray. Vancouver’s flagship Discovery was unable to make it past the sandbar blocking the mouth of the Columbia. However, Lieutenant William Robert Broughton succeeded with his smaller ship, the Chatham. Broughton advanced nearly 100 miles to a site opposite present-day Portland, Oregon.

Vancouver’s and Broughton’s reports of a navigable Columbia River encouraged Jefferson and others who were planning Lewis and Clark’s transcontinental journey. Since Vancouver’s publication was too bulky to carry on the expedition, Meriwether Lewis traced Vancouver’s charts so that he could have them on the voyage. In addition, Nicholas King relied on Vancouver’s charts when he prepared his map for the expedition. Broughton’s accurate chart of the lower Columbia proved especially useful to Lewis and Clark as they approached the Pacific coast.

Alexander Mackenzie. “A Map of America between Latitudes 40 and 70 North, and Longitudes 45 and 180 West, Exhibiting Mackenzie’s Track.” In Voyages from Montreal, on the River St. Laurence, through the Continent of North America to the Frozen and Pacific Oceans; In the Years 1789 and 1793. London, 1801. The Tracy W. McGregor Library of American History

Scottish-born Alexander Mackenzie made voyages to the Arctic and Pacific oceans for the North West Company. In 1801 he returned to London to publish Voyages from Montreal, on the River St. Laurence, through the Continent of North America which chronicled his exploits in western Canada.

Mackenzie was the second explorer, after American Peter Pond, to reach Lake Athabasca and the Great Slave Lake in present-day northwestern Canada. He based his route on Pond’s prediction that a river led from the Great Slave Lake to the Pacific Ocean. In 1789, Mackenzie followed this river (which later bore his name) and reached the Arctic Ocean instead. Four years later he crossed over the Continental Divide and completed his journey by traveling over land until he reached the Pacific Ocean at present-day Bella Coola, British Columbia. Mackenzie thus became the first European to reach the Pacific coast north of Mexico by traveling from the east.

Jefferson read the detailed account of Mackenzie’s voyage in 1802. Mackenzie’s description of an easy crossing of the Continental Divide helped convince Jefferson of the feasibility of an American expedition across the continent. Moreover, Mackenzie’s urgent recommendations that the British government secure control of the Pacific Northwest probably hastened President Jefferson’s authorization of an expedition to the Northwest.

Andrew Ellicott. “Map of the Mississippi River.” In The Journal of Andrew Ellicott. Philadelphia, 1803. The Tracy W. McGregor Library of American History

Andrew Ellicott was the foremost surveyor of his day. He conducted numerous surveys to establish state and territorial boundaries, including the boundary between the United States and the Spanish possessions in Florida. He also surveyed the site for the nation’s capital at Washington, D.C.

The Journal of Andrew Ellicott (Philadelphia, 1803) includes a map of the mouth of the Mississippi River, two maps of the Mississippi below the mouth of the Ohio River, and two maps of the Ohio. Ellicott mapped the upper Mississippi River to the Great Lakes and located the position of the mouth of the Missouri River quite accurately. The Journal, which was published at the time of the Louisiana Purchase, is also noteworthy because Ellicott argues for the United States’s acquisition of Louisiana as a way to keep the western states in the Union.

When Secretary of the Treasury Albert Gallatin commissioned Nicholas King to produce a new map of North America for the Lewis and Clark Expedition, he instructed King to incorporate Ellicott’s work on the Mississippi River. Jefferson turned to Ellicott for advice when planning the expedition. Jefferson knew that no other American could match Ellicott’s experience in making astronomical and field observations under trying conditions. Ellicott supplied Jefferson with a list of equipment that should be taken on the expedition. He also instructed Meriwether Lewis in the use of the sextant and octant, regulated Lewis’s chronometer, and devised a new type of artificial horizon for making field observations on the expedition.

David Thompson. “A map showing the Great Bend of the Missouri.” 1798. David Thompson worked as a surveyor for the North West Company. In 1811, he became the first European to descend the length of the Columbia River. With Peter Fidler, he surveyed and mapped 16,000 miles of waterways in western Canada and the northwestern United States. Beginning in 1816, Thompson led a ten-year commission to survey the border between Canada and the United States. Although he was one of the outstanding geographers of his day, he received little recognition during his lifetime. The narrative of his discoveries was not published until 1916.

In 1797-1798, Thompson surveyed and mapped the North West Company’s trade route from Lake Superior to Lake Winnipeg, traveling via the Assiniboine River, the Mandan villages, the Red River, and the headwaters of the Mississippi on his return. During this odyssey Thompson discovered Turtle Lake, one of the sources of the Mississippi River, and he accurately determined the latitude and longitude of the Great Bend of the Missouri River near the Mandan villages.

Thompson’s map proved to be an important resource for the Lewis and Clark Expedition. Nicholas King incorporated Thompson’s representation of the upper portion of the Missouri into the map he prepared for the expedition. Meriwether Lewis may have traced Thompson’s map from materials in the possession of Edward Thornton, the British chargé d’affaires in Washington. Lewis and Clark carried this tracing on their journey to the Pacific Ocean.

A notation on the front of the map in Jefferson’s hand reads: “Bend of the Missouri, Long. 101° 25' Lat. 47° 32' by Mr. Thomson astronomer to the N.W. Company in 1798.” Another notation on the back of the tracing reads: “A sketch of the North Bend of the Missouri. This belongs to Capn. Lewis.” Thompson’s original map is housed in the Geography and Map Division of the Library of Congress.

Aaron Arrowsmith. “A Map Exhibiting all the New Discoveries in the Interior Parts of North America.” 1795, with additions to 1811. The Tracy W. McGregor Library of American History

Aaron Arrowsmith was considered the finest mapmaker of his day. He produced his first map of North America in 1795 from data collected from the archives of the Hudson’s Bay Company. Arrowsmith’s 1795 map shows a vestige of the “Great River of the West” and the Missouri River appears as a river fragment unconnected to either the single ridge of the “Stony Mountains” or the Mississippi River. Arrowsmith notes that the “Stony Mountains” are “3250 Feet High Above the Level of their Base and according to the Indian account is five Ridges in some parts.”

The 1802 revision of the map of North America, shown here, delineates the complete length of the Missouri River. Although the revised map still shows a single ridge of mountains in the west, a note placed near the southern sources of the Missouri states: “Hereabout the Mountains divide into several low Ridges.” This note, which was based on the reports of Fidler, Mackenzie, and Thompson, was more encouraging to Jefferson and Lewis than the note about the Stony Mountains on the 1795 map, which, unfortunately, turned out to be more accurate. Arrowsmith’s map situates the Great Lake River on the western slopes of the mountain range and connects this river to the Columbia River with a dotted line. Since another note claims that this river can be descended to the sea in eight days, the Arrowsmith map supported the erroneous belief in a convenient route to the Pacific Ocean.

Arrowsmith’s map was probably the most important map used in planning the Lewis and Clark Expedition. Nicholas King consulted both the 1795 and 1802 versions as he prepared his map for the expedition. Lewis and Clark, in fact, carried the 1802 Arrowsmith map along on their journey. Thomas Jefferson owned the 1802 map of North America, which was the most comprehensive map of the West available at the time.

The map shown here is the lower half of “A Map Exhibiting all the New Discoveries” from an edition labeled “1795, with additions to 1811.” This half of the map is identical to the 1802 edition.