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

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 . . . 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.

New Northwest

Clark's Final Map, 1814

Large map of the lands from the Louisiana Purchase

Map Division, Library of Congress

William Clark began this map at Fort Clatsop during the winter of 1806. He continued to revise it for several years after his return, drawing upon new information from Zebulon Pike as well as from John Colter and George Drouillard, both veterans of the Expedition. The size of the original is 25-5/8 by 11-7/8 inches. Clark presented a more accurate picture of the Rocky Mountains than anyone had previously imagined, as may be seen in a comparison with the pre-expedition map drawn by Nicholas King.


Clark's 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.

Mental Maps and the Modern Microcosm

Lewis and Clark measured the land they passed across, pace by pace, landmark by landmark, mile by mile. They estimated, calculated, compared, recalculated, and proceeded on, assembling their mental maps. Imagine all the ways in which their basic perceptions of time and distance and direction differed from those we might experience while traveling: how the captains' sense of time and distance must have gotten all out of whack every now and then, just like ours does if we travel between familiar points but at speeds different from those we're accustomed to.

How different must be the perceptions of time and distance of an astronaut orbiting the earth several times, or the modern air traveler flying from New York to Australia in about the same time required for those several earth orbits, from those of Lewis and Clark struggling a few dozen miles up the Missouri River during the period required for several shuttle orbits or a jet flight halfway around the world.

Imagine the difference in ease of the calculations we obtain instantaneously from a handheld GPS device, those that an astronaut obtains after a supercomputer processes billions of pieces of information, and those that Lewis and Clark made in bad weather, at night, by firelight, with a crow quill pen in an elkskin-bound book. Think what a feat it was for Lewis and Clark just to keep track of the right day of the week (don't you ever lose track while you're on vacation?).

The mental maps the captains acquired so laboriously quickly became real ones, penned on paper and then engraved on copper plate. How close their mental maps and their actual maps must have been! Although the real maps available to us are supremely accurate, our mental maps are not. We cross landscapes not by the windings of rivers but by the straight lines of interstate highways. We follow not the local curvature of the terrain but the global curvature of the earth, again in apparent straight lines that mark the great circle airline routes between the country's and the world's airline hubs.

The sense of place and space with which we come away from our travels is quite different from that of Lewis and Clark: the smell of gasoline and diesel fumes rather than that of wood smoke; the haze of city and suburban pollution rather than the clarity of the High Plains at sunrise; the accumulation of airport destinations as experiences rather than the numerous nightly camps along the great Missouri; a fast-food "happy meal" instead of buffalo hump rib.

Even along the Lewis and Clark Trail, the modern traveler's experience cannot be like that of the Corps of Discovery: a few days in canoes through the Missouri's White Cliffs country or a few days on horseback in the Bitterroot Mountains cannot add up to the incremental experiences and mental maps of 28 months from St. Louis to the Pacific and back again.