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Lewis and Clark set out up the Missouri with a considerable amount of information they had gathered locally, plus a number of different maps, including one compiled in March of 1803 by the Washington, D.C. cartographer and surveyor, Nicholas King, at the direction of Secretary of the Treasury Albert Gallatin. King's instructions were to compile "a blank map" of North America from 88° to 126° west longitude and from 30° to 55° north latitude – roughly from the mouth of the Ohio River to the Pacific Ocean, and from New Orleans to somewhat north of Lake Winnipeg. He was to combine the contents of earlier explorers' maps, including those by Cook, Vancouver, Arrowsmith, Thompson and Mackenzie.
King's map represented all that was known or conjectured about the features of the whole area, and showed that the geography of the land between the Mandan and Hidatsa Indians' homeland on the middle Missouri, and the lower Columbia River, was indeed a blank in the minds of Euro Americans.
Comparisons of King's conjectural sketch with Clark's chart from 1805 and the version of Clark's concept which was published with Nicholas Biddle's 1814 paraphrase of the journals, show the magnitude of Clark's contribution to the understanding of the geography of the Northwest. Never again would such a great cartographic leap be made by one man.