Lewis & Clark’s Planning Challenges

On most days, the expedition covered less than 30 miles. Even with their sextant, an octant, a surveying compass, and an accurate chronograph which they had, their daily track could be within the margin of error even if, on each night, they took three sightings. Their surveying compass enabled them to differentiate between true and magnetic north. In the latitudes where they were traveling, the difference between true and magnetic north could range from five degrees near St. Louis to 200 at the mouth of the Columbia River.

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