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Survey Legend

William Clark

The mission of the Lewis and Clark expedition of 1804-1806 was to explore the recent Louisiana Purchase by "ascending the Missouri River, crossing the 'highlands' and following the best water communication to the Pacific Ocean." Meriwether Lewis, the personal secretary to President Thomas Jefferson, spent two years planning the expedition. William Clark, another experienced outdoorsman, was a surveyor and mapmaker. The "Corps of Discovery" shoved off on May 14, 1804 from Wood River, Illinois, after taking a reading of 38 deg. 55 min. 19.6 sec. North latitude. The party consisting of 45 men, traveled primarily by keel-boat and canoe and made about 10 to 20 miles per day. On July 31, 1804, they took a meridian altitude and calculated the Latitude of their position to be 41 deg. 18 min. 1.5 sec. North. Latitude was much easier for them to measure than Longitude, as they had no accurate timepiece, which is needed for determining Longitude. However, they were able to estimate the distance traveled each day with considerable accuracy and, combined with compass and latitude, allowed them to make good maps. On January 5, 1805, at 12:57:54 to 02:39:10 AM, at their winter quarters near Mandan, North Dakota, there was a total eclipse of the moon which allowed Clark to reset his timepiece and subsequently measure Longitude. On May 20, 1805, Lewis and Clark measured the Latitude of the mouth of the Muscleshell River to be 47 deg. 00 min. 24.6 sec. North. In reality, it is 47 deg. 30 min. North. Their observations were consistently about 1/2 degree too far South.Lewis "August 28th, 1805: Having now crossed more than once the country which separates the head waters of the Missouri from those of the Columbia, we can designate the easiest and most expeditious route for a portage: it is as follows. From the forks of the Jefferson River N60deg.W, five miles to the point of a hill on the right: then S80deg.W, ten miles to a spot where the creek is ten yards wide, and the highlands approach within 200 yards; Southwest five miles to a narrow part of the bottom; then turning S70deg.W, two miles to a creek on the right: thence S80deg.W, three miles to a rocky pointClark opposite to a thicket of pines on the left: from that place West three miles to the gap where is the fountain of the Missouri; on leaving this fountain S80deg.W, six miles across the dividing ridge, to a run from the right passing several small streams; N80deg.W, four miles over hilly ground to the East fork of Lewis' (Lemhi) River, which is here forty yards wide." The party resided at a wet Fort Clatsop, Oregon, December 7, 1805 to March 23, 1806. Lewis and Clark had hoped to return by ship, however no ships were sighted and they began their return on March 23, 1806. They arrived in St. Louis to the surprise of everyone who thought the party had perished.
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