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". . . that they may with certainty be recognised hereafter."

–Thomas Jefferson, Instructions to Meriwether Lewis, 20 June 1803

After the Expedition Jefferson noted that "it is only to latitudes that [their] map may be considered as tolerably correct, not as to its longitudes." This was overly critical: certainly the captains' longitude observations were less than perfect. Yet William Clark's ultimate map of the western portion of North America, based in part on those observations, nevertheless proved quite accurate in its longitudinal reckoning. In the final analysis, in spite of the extant errors, their longitudinal observations were better than could or should have been expected. The ultimate results of their "observations of latitude & longitude" bore that out.

Clark's final map was the culminating piece of geographical information emerging from the Expedition and resulting directly from Jefferson's injunction to preserve knowledge for the future. Completed in manuscript form by 1811, engraved and published with the Biddle History of the Expedition├ľ in 1814, it portrayed with artistry, skill, and accuracy the area through which the Expedition had passed. Course and distance information is apparent on that map and coordinate positions are accurate to within 5%, an accuracy level that would not be matched by many cartographers until the advent of mapping aided by aerial photography in the early 20th century.

Clark's map represented the ultimate triumph of the vertical perspective, the ability to synthesize mental and actual maps and to portray geographical features with precision in the context of the geographic coordinate system within which he operated.

That map, like the other geographical results of the Expedition, set the standard for other explorers to follow and, although America's first great territorial mapping project did not achieve all the scientific precision the President had wished for, it was enough. The mental maps had become actual maps and an American claim over territory from the Mississippi to the Pacific was secured.