“A map showing the Great Bend of the Missouri.” 1798.

David Thompson (1770-1857) was born in London and, at the age of fourteen, was apprenticed to the Hudson’s Bay Company as a clerk in their offices in Canada. From 1789 to 1790 Philip Turnor, a compiler of the Nautical Almanac, taught Thompson surveying and how to make astronomical observations. The Hudson’s Bay Company appointed Turnor Inland Surveyor in 1778 and from 1778 to 1787 he mapped a large portion of the shoreline of the Hudson Bay and some rivers to the west. Thompson was to have gone on Turnor’s expedition in 1790 to determine the exact location of and the best route to Lake Athabasca. A leg injury, however, forced Thompson off the expedition and his place was taken by Peter Fidler, who later succeeded Turnor as Inland Surveyor. Thompson continued to work as a surveyor for the Hudson’s Bay Company until 1797 when he left to join the North West Company. He eventually became a partner in that 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. From 1816 to 1826 he headed a 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 1797-98, Thompson surveyed and mapped the North West Company’s trade route from Lake Superior to Lake Winnipeg. He returned to Lake Superior via the Assiniboine River, the Mandan villages, the Red River, and the headwaters of the Mississippi. During this trip Thompson discovered Turtle Lake, which is 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 was an important resource for the Lewis and Clark Expedition. Following Albert Gallatin’s instructions, Nicholas King incorporated Thompson’s representation of the upper portion of the Missouri into the new map he produced for the expedition. Meriwether Lewis may have traced Thompson’s map of the Great Bend of the Missouri from materials in the possession of Edward Thornton, the British charg‚ d’affaires in Washington. Lewis and Clark carried this tracing on their trip to the Pacific Ocean.

The tracing on display belongs to the Library of Congress. 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.”

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