Navigation and Mapping

President Thomas Jefferson instructed Meriwether Lewis and Wiiliam Clark to take detailed readings of the rivers and land as they worked their way west. It was important work for future explorations. Very little was known of the territory the American government had just purchased. Lewis and Clark’s duties on the expedition included the creation of a navigational record of the trip up the Missouri and down the Columbia. Lewis and Clark were exploring new American territory for the President, who, among others, believed that there was an all-water route to the Pacific Ocean. They thought that after reaching the source of the Missouri, they would find the source of the westward-flowing rivers nearby, although evidence of this consisted was anecdotal from travelers and Native Americans.

Lewis learned to take celestial readings, calculate distances, angles, and altitudes. He and Clark knew how to do dead reckoning on land and water. Lewis' journal shows that he was practicing his navigational skills as he sailed down the Ohio River and up the Mississippi River. While in Illinois for the winter of 1803-04, they honed their navigational and surveying skills, and gathered map information from French, Spanish, and Native American sources.

Navigational Devices
Lewis had learned surveying and navigation procedures from astronomer Andrew Ellicott and mathematician Robert Patterson. The latter supplied him with an astronomy manual with thirty pages of instructions and examples. Lewis made notes in it as he practiced his skills. Clark also knew basic surveying techniques using the tools of the era.

Lewis spent $217 for mathematical instruments. This purchase was almost ten per cent of his original government budget of $2,500. The instruments included a chronometer for keeping the time to determine longitude, a circumferentor or compass for finding directions, a sextant and an octant for finding the height of celestial bodies from the horizon, and artificial horizons for use when they were in the mountains. Lewis’ journal entry of July 22, 1804 Go to the Journals' Home Page (University of Nebraska) and enter this date into Search

On December 2, 1803, Lewis made notes on his use of the circumpherenter and chronometer. He measured the “Azamuth of pole start 7° 47’ 00” at 8h 11m 45s P.M. Using the Moon and Aldeberan * W" Aldebaran is the brightest star of the Taurus constellation . He repeatedly located the star in the sky that night. He would be able to compare those readings with those in his English copy of the Nautical Almanac to find his location. (See the December 12, 1803 entry in the online journal.)

Although many people take instruments such as the clock and the compass for granted today, in 1803 they were still experimental and less than accurate. Clark preferred to estimate short distances by a method called dead reckoning. There was one method of dead reckoning for overland travel by foot and another for water travel.

Dead Reckoning on Foot
Dead reckoning is a method of measuring distances that is based on an individual’s average walking pace on land or on the average speed of a boat. To determine average land speed, a person can use a watch with a second hand and a measuring tape. The person notes the time on the watch, walks to the target spot, marks the time upon arrival, and measures the distance traveled with a tape. For example, if a person walks for 15 seconds and has covered 100 feet, the speed is 6.67 feet per second (400 feet per minute, 4.54 mph). Once the average speed is known, the person can estimate the distance traveled by keeping track of time elapsed between landmarks. For mapping purposes, Clark would also keep track of direction changes with a compass. Every change in direction would equal a landmark and a cue to write down the time.

Dead Reckoning From a Boat
Dead reckoning from a boat is done with the help a compass, a watch, and a device called a log line,. To begin, a person notes the exact time and then he makes a compass reading to find the direction of a landmark in sight ahead. When the boat starts moving, the person throws a weighted piece of wood overboard. This weight is attached to a line that is marked with knots 42 inches apart (and loops at 21 inches apart). As the boat sails away from the weight, its line and its knots pass over the rail. By counting how many knots passed by in 30 seconds, the person can determine the speed of the boat in knots (nautical miles).

This process was repeated for each landmark sighted along the river. Notes were made for each section measured, the directions traveled, and the time elapsed. From the written record, it was possible to make a map of the river to scale.

William Clark estimated that the Corps had traveled 4,162 miles from Camp Wood to the Pacific Ocean. His estimate was close, considering he had to use the methods and equipment available at the time.

Sample Reckoning by William Clark on the Mississippi River: (November 11, 1803 entry) Clark mentioned in his field notes that they were setting out on a smoky (misty) day, which might cause his measurements to be inaccurate. Nonetheless, he took measurements every one to three miles over a ten-hour period as the river turned in the stretch from Saline Creek to St. Genevieve.

Find out more about the mathematical instruments at This Website provides a history of the sextant and other navigational devices by Peter Iflan. It is well-illustrated and clear. This pdf from the Canadian firm Spatial Geolink Limited features a project that brings modern digital and satellite mapping techniques to the Lewis and Clark Expedition history. They provide links to Web sites that feature maps, trials, and natural features of the Lewis and Clark Expedition. Some of the sites require special browsers to view the maps.

See the Native American and European Mapping Traditions Lesson Plan. PDF format