From doodles in Clark’s journal to copies of maps given to the expedition by American Indians to celestial observation, William Clark drew, compiled or collected the maps that detailed one of the greatest expeditions in American history.
More than 140 maps were made during the Lewis and Clark expedition and more than 30 were collected from American Indians, fur trappers and traders.
Clark had begun mapping on the Virginia frontier while he was in the Army. His older brother was also a land surveyor, and Clark learned many of his skills from his brother.
“Most of the distances came from Clark’s head,” said James Harlan, senior research specialist and program director of geography at MU, who is also an expert in the fields of historical and regional geography. “He was really good at this and came really close on distance. He could look across a river and guess that it was a half mile.”
This did not mean they were always perfect.
“The maps Lewis and Clark made were not to scale, so that alone makes them inaccurate,” Harlan said.
Meriwether Lewis also learned about field surveying and other map-making skills. He was instructed on the proper use of instruments to survey and map the routes used by explorers.
Even with training the maps they produced had flaws.
“Celestial observation produced bad measurements,” Harlan said. “They were trying to determine their exact latitude and longitude using the sun and stars, and it wasn’t accurate.
“Their chronometer, or clock, stopped two or three times during the expedition,” Harlan said. “They had to reset it then by guessing, making it inaccurate.”
The broken chronometer was often used to determine longitude, while latitude was determined by measuring the angle of the moon and a star with an instrument called an octant.
A circumferentor, or plain surveyor’s compass, was used to determine direction, and distance was usually estimated in miles from one point to another.
Many of the men were trained in guessing distances and had been doing so since childhood. More accurate measurements were made with tools such as a “two-pole chain,” which was 33 feet of iron links. A “log-line reel” was used for measuring speed and distance while on water.
But even this had its limitations.
“The log-line reel isn’t as accurate as many believed it to be,” Harlan said. “They were not traveling in a straight line, but were trying to measure around curves and bends of rivers.”
During the journey Clark made four types of maps: large-scale traverse maps, small page-size maps sketched in his journals, copies of maps prepared by traders and American Indians and composite maps of the West.
The compass-traverse maps were the main maps the expedition produced and show the route that was traveled each day. Geographical longitude and latitude, however faulty, was determined by celestial observation at selected points on the trip.
Field sheets were what Clark used to plot the routes taken each day. They had a background grid of one-inch squares.
Journal maps were small-scale page-size maps drawn in the daily journal entries. Falls, narrows and rapids mark some of Clark’s more detailed drawings, while others were just sketches of larger areas.
“Lewis and Clark named very few things,” Harlan says. “The French had already been running up and down the river and had named many of the streams and landmarks.”
The map of the Great Bend of the Missouri River in the area of the junction of the Missouri and Knife rivers in present day North Dakota is one of the few surviving maps drawn by Lewis. It is actually a copy of a map drawn by astronomer and surveyor David Thompson in 1798.