Mapping the River in Illinois

On November 15, 1803, Lewis and Clark made their first chronicled tries at finding the longitude and latitude of their location at the mouth of the Ohio River. Lewis noted that he took equal altitudes of the sun in the morning and afternoon, that the altitude given by the sextant was 39° 50’ 00”. He corrected his altitudes and found his chronometer to be too slow.

Clark also wrote about measurements at the confluence of the Ohio and Mississippi Rivers. He referred to distances measured in poles, or rods, a surveyor’s unit of 16.5 feet in length (5.029 m). He measured the confluence by taking compass measurements (azimuth readings) from North and distances of poles to measure the width of the river and the size of islands. He used landmarks such as a forked tree, a cave, and a stand of willows as points for his dead reckoning. Clark also made a sketch in his journal of the confluence of the two rivers and islands they passed (above). Lewils and Clark noted the shifting sand bars and changing channels that made mapping even more difficult.

View a map of the Confluence of the Ohio and Mississippi rivers by Clark, 1803 at the Beinecke Library at Yale.

Here is an example of readings Lewis made on November 21, 1803. It illustrates how data were kept on compass readings, chronometer readings, and estimated distances (by dead reckoning) for use later for drawing the maps.

Courses of this day Novr. 21st 1803 (Lewis Journal entry)


Estim. Time



N. 50 W.


1 1/4

passed Is. Lad.

Do. Do.


3 3/4

Do. Is. Lad. 3 miles long(2)

S. 20 E.


1 1/4

to sand bar Stad. (3)




Small Is. Lard, Qtr.

S. 72 E.

1 "

" 1/2

Islds. continues on stad.

N. 62 W.

" 30


Still in grand bend

N. 52

2. 25

1 3/4

To an isld. on Lard. Side where we halted for the night

Total Distance




This chart was accompanied by a narrative that noted what landforms they saw on the shore, the shapes of the islands, and other details such as the flora and fauna seen in the area.

As they worked their way up the Mississippi River, they made daily measurements, sketches, and compass and sextant reading of their route. They were practicing for one of the most important tasks of the expedition. They were also providing more detailed information on the settled borders of the Illinois Country.

Read more about Mapping traditions (European and Native American) and Navigational Devices in the Resources section.

See a Mapping Activity. (pdf)