1770–1857, Canadian geographer, fur trader, and explorer, b. London, England. In 1784 he came to Fort Churchill, Canada, as an apprentice of the Hudson's Bay Company, and until 1797 he was a fur trader of Hudson Bay and in the Athabasca country to the west. Although he had little scientific training, he developed great skill in geodetic and astronomical observations, and after 1797, when he joined the North West Company, he methodically located points in W Canada and made surveys of astonishing exactitude. In 1797–98 he traveled far S to the Mandan villages on the Missouri and then surveyed the headwaters of the Mississippi River. His most notable exploring expeditions were those across the Rocky Mts. and on the Columbia River. In 1807 he crossed the Howse Pass to the source of the Columbia River and traveled its length; he then explored the Kootenai, Pend Oreille, and Clark Fork river basins. In 1810, prevented by the Piegan from using Howse Pass, he went north to the head of the Athabasca River and across the mountains and explored all of the Columbia River system. He then went to Montreal, where he made (1812–14) a large and invaluable map of W Canada for the North West Company, long the best map of the region. Thompson, however, received little open recognition except an appointment (1816–26) to the commission for surveying the U.S.-Canadian boundary. It was not until the 20th cent. that his importance as a geographer was recognized.
- See his narrative (ed. by J. B. Tyrrell, 1916, repr. 1968);.
- biography by J. K. Smith (1971).