Formal Navigation by Lewis & Clark

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Much of what Lewis and Clark had to do in terms of their own mapmaking was to ascertain the accuracy of Indian information by doing what we call today "ground truthing"—checking Indian data against their own visual observations—and "celestial reckoning," using the instruments they carried for the purpose of fulfilling Jefferson's instructions to "take [careful] observations of latitude & longitude." To fulfill these tasks they used instruments: spirit and telescopic levels, several compasses including a surveyor's compass or circumferentor with extra needles and even a magnet to "polarize" them, a sextant, a "Hadley's quadrant" or octant, rods and chains, telescopes, artificial horizons, drafting instruments, a very early version of a measuring tape, and a clock or chronometer. They also used books and tables giving the daily locations of sun, moon, and planets for use in computing geographical position after obtaining sightings of these "celestial objects." The two most important groups of items of the Lewis and Clark Expedition, if cost is the measure, were mapping instruments and gifts for native peoples: these were the tools of empire, necessary in establishing a claim to place and space and defending that claim through trade,

The Sextant

a sextant with all its parts labeled

On July 22, 1804, while at White Catfish Camp on the Missouri River, ten miles above the mouth of the Platte, Lewis took time to enter into his journal brief descriptions of the instruments he and Clark used in making celestial observations.

1st— a brass Sextant of 10 Inches radius, graduated to 15' which by the assistance of the nonius was devisible to 15"; and half of this sum by means of the micrometer could readily be distinguished, therefore—7.5" of an angle was perceptible with this instrument; she was also furnished with three eye-pieces, consisting of a hollow tube and two telescopes one of which last reversed the images of observ ed objects. finding on experiment that the reversing telescope when employed as the eye-piece gave me a more full and perfect image than either of the others, I have most generally imployed it in all the observations made with this instrument; when thus prepared I found from a series of observations that the quantity of her index error was 8' 45"—; this sum is therefore considered as the standing error of the instrument unless otherwise expressly mentioned. the altitudes of all objects, observed as well with this instrument as with the Octant were by mens of a reflecting surface; and those stated to have been taken with the sextant are the degrees, minutes, &c shewn by the graduated limb of the instrument at the time of observation and are of course the double altitudes of the objects observed.


The Octant

An octant with its parts labeled

On July 22, 1804, while at White Catfish Camp on the Missouri River, ten miles above the mouth of the Platte, Lewis took time to enter into his journal brief descriptions of the instruments he and Clark used in making celestial observations.

A common Octant of 14 Inches radius, graduated to 20', which by means of the nonius was devisible to 1', half of this sum, or 30" was perceptible by means of a micrometer. this instrument was prepared for both the fore and back observation; her error in the fore observation is 2°+, & in the back observation 2° 11' 40.3"+ at the time of our departure from the River Dubois untill the present moment, the sun's altitude at noon has been too great to be reached with my sextant, for this purpose I have therefore employed the Octant by the back observation.    the degrees ' & " [minutes and seconds], recorded for the sun's altitude by the back observation express only the angle given by the graduated limb of the instrument at the time of observation, and are the complyment of the double Altitude of the sun's observed limb; if therefore the angle recorded be taken from 180° the remainder will be the double altitude of the observed object, or that which would be given by the fore observation with a reflecting surface.


Both Lewis and Clark were reasonably proficient in the use of these instruments and for 28 months, as long as the Expedition was on the move, a part of the daily routine was the measurement of latitude and longitude and the calculation of course, time, and distance of travel. Even during those times when Lewis and Clark were fixed in location for lengthy periods of time, such as at Fort Mandan and Fort Clatsop, the "mathematical instruments" saw almost daily use, weather permitting.

In this, the captains followed Jefferson's instructions to the letter and throughout the Expedition made the two basic types of geographical observations their sponsor had requested: (1) daily measurements of local features, taken continually during a day's travel; and (2) the more abstract measurements of latitude and longitudinal position, usually made by astronomical observation when and where atmospheric conditions allowed, but most commonly at camp during the night. Their guides were the moon and the stars.

Global Positioning System

Lewis and Clark tested Indian data by astronomical and other readings obtained under conditions that would defeat most of us today: heat, cold, wet, dry, darkness, hunger. It often took three or four men to complete the necessary mechanical operations necessary to get even a moderately accurate figure for the elevation of the sun or moon above the horizon and the end results of their calculations were, by modern standards, not very good.

Today, someone with but a few minutes of training can use a hand-held GPS (Global Positioning System) device to download latitude and longitude information from an orbiting satellite, link that information to a portable computer that will show a map locating the coordinate position of the observer, accurate to within a few meters. And the cost of the modern equipment (including the computer), in 1804 dollars, is only a small fraction of the money spent by Lewis on his "mathematical instruments."

Since we already have reliable maps, identifying a location precisely on the landscape for us is merely a convenience that helps us find our way to a new and unfamiliar place. The handheld GPS device is to us, quite simply, a toy. For Lewis and Clark, who paid an enormous sum for a somewhat reliable watch, such a device would have been priceless.


This page is supported in part by a grant from the National Park Service Challenge-Cost Share Program.