European Mapping Traditions
Lewis and Clark followed the European tradition of mapping land with astronomical readings and a grid system based on magnetic north, longitude, and latitude. They had learned this system of two-dimensional interpretation of three-dimensional space from surveyors and cartographers of England and the eastern United States. They knew that by carefully measuring where they were compared to the sun, stars, horizon, and the direction from which they had come, they could make a map of territory previously unknown to them. They were also familiar with the globe and other American and European maps of various parts of the world through time. As they traveled through the west, they added to their map mile by mile based on physical measurements. Lewis made extensive celestial readings to later compare with the Nautical Almanac (which, in the end, he did not use). Lewis and Clark both used the dead reckoning method.
Maps to Prepare Them for the Trip
Jefferson, Lewis, and Clark received help from many distinguished mapmakers of their day. Albert Gallatin, Secretary of the Treasury, had Nicholas King make a map of the west for Lewis and Clark. King used maps from Alexander MacKenzie (north central areas), James Coo, and George Vancouver (far west and Pacific coast), Andrew Ellicott (Mississippi Valley), and Aaron Arrowsmith (big bend of the Missouri area).
While they were wintering in Illinois, Lewis and Clark received maps and information from several important sources. One source was Andre Soulard, the surveyor for the Spanish Governor of Louisiana. He referred them to a 1794 map of Upper Louisiana which may have been owned by one of the Chouteaus (St. Louis businessmen) and taken from Victor Collot’s A Journey in North America. Lewis was given permission to have this map copied.
Another source was James Mackay, who visited Clark at Camp DuBois. They discussed John Evan’s maps of the Missouri and northern areas that Jefferson had sent to Lewis. John Hay gave Lewis valuable information derived from his experience as a trader with the Osage tribe. Lewis also interviewed many traders and merchants in St. Louis.
Three main points were definitely located on maps of 1804 — St. Louis, from where the journey would start, the Mandan villages, which traders had mapped, and the mouth of the Columbia River, which was known to maritime traders. The spaces in the middle were known by word of mouth and conjecture. For example, the dotted line of the western path of the Missouri came from a Blackfoot Chief named Ackomokki via Peter Fiddler, who had never seen the territory. Lewis and Clark planned to measure these spaces and fill in the gaps on these maps.
See information and images of maps that helped Lewis and Clark prepare for the expedition at http://www.lib.virginia.edu/speccol/exhibits/lewis_clark/planning.html
See the European and Native American Mapping Traditions Lesson Plan in