
Library of Congress, Geography and Maps Division
In 1803, President Thomas Jefferson charged Meriwether Lewis with the great mission of finding a direct water route to the Pacific Ocean. He would send the Corps of Discovery into unmapped territory, armed with limited knowledge of what lay ahead.
It would be up to Captain William Clark to map the territory as they made their way west. It is hard to imagine today what it would be like to venture out without access to our smartphone navigation system or a road atlas. How often would you get lost? Would you be able to create your own map?
It would be up to Captain William Clark to map the territory as they made their way west. It is hard to imagine today what it would be like to venture out without access to our smartphone navigation system or a road atlas. How often would you get lost? Would you be able to create your own map?
William Clark was off by a mere 40 miles in his calculation of how far they had traveled from Camp River Dubois to the Pacific Ocean.
The challenge was very real to Lewis and Clark – and the seriousness of it is evidenced by their preparation. As part of the supplies purchased for the expedition, the following mathematical instruments were purchased:
- Surveyor's compass
- Hand compass
- 1 Hadley's quadrant
- 1 Telescope
- 3 Thermometers
- 2 Sextants
- 1 Set of plotting instruments
- 1 Chronometer
- 1 Portable microscope
- 1 Tape measure

NPS Photo/R. Kephart
Clark usually made his calculations from the boat, since the Expedition was constantly on the move. But he would also employ other tools such as the surveyor’s chains when on land.
Every few days, Clark would take the notes from all of his observations and measurements and compare them to Lewis’ notes to draw maps of the river and landmarks to scale. The accuracy of his maps is widely admired, he was off by a mere 40 miles in his calculation of how far they had traveled from Camp River Dubois to the Pacific Ocean.
Every few days, Clark would take the notes from all of his observations and measurements and compare them to Lewis’ notes to draw maps of the river and landmarks to scale. The accuracy of his maps is widely admired, he was off by a mere 40 miles in his calculation of how far they had traveled from Camp River Dubois to the Pacific Ocean.

NPS Photo
Discover more:
- Lewis and Clark National Historic Trail Park Atlas
- Jefferson's Instructions to Meriwether Lewis (from Monticello)
- Expedition's Scientific Equipment (from the Montana Bureau of Mines and Geology)