William Clark: A Master Cartographer

Map of Lewis and Clark's track across the western portion of North America
A map of Lewis and Clark's track, across the western portion of North America from the Mississippi to the Pacific Ocean: by order of the executive of the United States in 1804, 5 & 6. Published by Bradford and Inskeep, 1814.

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?

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
The chronometer alone cost 10% of their total supplies budget, the single most expensive piece of equipment. But it was a tool they could not leave behind because it was used, along with a sextant, to determine latitude and longitude.
Sextant, pocket compass, telescope
Sextant, pocket compass, and telescope.

NPS Photo/R. Kephart

The daily practice of making celestial observations and complicated mathematical calculations to measure the angle of the sun from the horizon was tedious and required skill, which both Lewis and Clark learned to do. Eventually Clark took this on as his primary role. He also became a master at dead reckoning, the process of estimating your position by using your last known point then using your rate of speed and how long you have traveled to calculate distance.

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.

Three individuals face away, observing a river valley from an overlook
A contemporary look at the landscape from Lewis Lookout, located a short distance from Dillon, Montana.

NPS Photo

William Clark’s legacy to the study of cartography and his contributions to the success of the expedition are celebrated to this day, inspiring visitors to follow in the footsteps of the Corps of Discovery.

Lewis & Clark National Historic Trail

Last updated: September 28, 2023