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Will James Middle School
Billings, Montana
Mapping Instruments
of the Expedition
Meriwether Lewis and William Clark took along approximately 20 technical "mathematical instruments" for mapping. They also had two books, Kelly's Spherics and the Nautical Almanac, as well as several astronomical tables. These supplies totaled about $473.45.
Tai's drawing
Lewis ordered these supplies and studied with surveyor Andrew Ellicott and mathematician Robert Patterson. Clark was an experienced surveyor and ended up being the main mapmaker on the expedition.
The octant, also called Hadley's quadrant, and the sextant were used to find latitude. The parallel glass, along with talc, were used in the artificial horizon; the telescope was used for obtaining positions of the moon, stars and sun. The spirit level was used with the sextant and also in calculating altitude. To obtain their latitude readings, Lewis and Clark took around twelve sightings of the sun right before and after solar noon in order to measure the sun's height above the horizon. Pairs of sightings at equal altitudes, before and after noon, were used to compute the exact time the sun reached its highest point. From tables they carried they could then find their latitude.
Though Jefferson preferred the theodolite for obtaining longitude readings, the Expedition did not appear to take this instrument and instead chose the chronometer or timekeeper for longitude. This was their most expensive purchase. Longitude readings were taken and left to be calculated later back in St. Louis with other tables.
The two pole chain gave an accurate measurement of 33 feet and a measuring tape, rare back then, was also taken along. The log line reel and log ship were used to measure the rate and distance of boat travel. Compasses were used to locate north and the magnet was used to maintain them. Plotting instruments and the circular protractor were used to draw their maps.
Their method of mapping with these tools is somewhat complicated since they seemed to be gathering two different kinds of data: data for exact location in latitude/longitude coordinates and data for relative location using the route traversing method and dead reckoning.
Donald Jackson writes about their mapping methods:
...he (Clark) was working with two kinds of data: the tried and true collection of headings and distances, so faithfully kept by himself or the sergeant in command of the keelboat, and time-honored by mariners as the system called dead reckoning; and the highly suspect astronomical observations that he and Lewis had been making with the quadrant since the establishment of the Wood River camp a year earlier...Latitude was not a problem, being determined by a noon observation of the sun-or observations in midmorning and midafternoon-and a few simple calculations. The tough part was the longitude...Lewis and Clark would have been well advised to forget about longitude, except for the key readings they had been given in advance for St. Louis, Fort Mandan, and the mouth of the Columbia. Their most useful data proved to be their dead reckoning records, particularly those showing their westering, combined with their observations for latitude.
Source: Donald Jackson. Thomas Jefferson and the Stony Mountains: Exploring the West from Monticello. (Norman: University of Oklahoma Press, 1993), p. 173, 176.
Gary Moulton also writes:
For "courses and distances" Clark relied mainly on his compass readings, and occasionally, perhaps, he again trusted dead reckoning. It is unlikely that Lewis and Clark made allowance in their compass readings for variations in the earth's magnetic field. Clark set down the party's course of travel on the route maps on the basis of his compass traverse notes, which he placed at nearly every journal entry. He probably employed the route traverse method, taking bearings at each turn of the trail or bend in the river and plotting those shifts on his maps. The background grids on many maps were a useful guide in such plotting. For his mapping Clark was probably taking "back sightings" or "back azimuths," giving his bearing from north or south in degrees. After traveling forward some miles Clark would turn and take an azimuth reading from his previous point of sighting. This procedure was necessary because he could not always determine his next point of observation.5
Source: Gary E. Moulton. Atlas of the Lewis and Clark Expedition, Vol. 1. (Lincoln: University of Nebraska Press, 1983), p. 4.
This information was pretty complex for us to understand. To try and gain a better sense of the traversing method, we worked with Mr. Prchal, a former surveyor and our school's Tech Ed teacher, to build some instruments for determining angles and taking backsightings. We took these instruments outside and worked in teams to navigate the westbound and eastbound routes of the Expedition through Montana, simulating the traversing method in the process.
These are models of transits we constructed for our sightings.
We also looked at a trigometric equation Clark might have used for calculating distance and for measuring the height of landmarks, such as Pomp's Tower.
Sketches by Peter and Norma
The Expedition's instruments were all subject to error. However, Lewis and Clark were not just calculating their location with mathematical instruments. They were also looking for geographic information from the Indian tribes they met. The Indians, who gave them this information verbally and in the form of maps, had their own ways of making maps. One method was to draw an outline of rivers, channels, and other physical features with charcoal on animal hides. Another method the Indians frequently used was the method known as the "relief map." Using rocks and dirt, Indians constructed relief maps on the ground from information memorized in their minds. These maps were important to the expedition; Lewis and Clark wrote about them in their journals and re-drew them on paper. Clark at one time writes "this War Chief gave us a Chart in his Way of the Missourie." (Source: Ronda, James. " 'A Chart in His Way' Indian Cartography and the Lewis and Clark Expedition", Great Plains Quarterly, winter, 1984. University of Nebraska.)
These maps were very important to the expedition's mapmaking efforts and even to their survival. You can see many of these maps in Gary Moulton's Atlas of the Lewis and Clark Expedition.
After Clark obtained his bearings and his relative location from various methods and sources, he then drew maps using standard symbols. Here are a few examples of these symbols.
Here is an example of one of Clark's maps. This map shows their route from September 13-19, 1804.
Lewis and Clark successfully mapped their route and determined the latitude for most of the important points along the route. Considering the final map produced later in St. Louis, the Expedition produced an incredible map of their journey.
Lewis and Clark's Expedition · The Legacy Grows: Lewis and Clark's Garden · Student Projects (Teach Lewis and Clark) · About the Student Projects
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© 2001 Northwest Regional Educational Laboratory Date of Last Update: 2/25/2002 
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