How did Lewis and Clark measure distances on their historic "Voyage of Discovery" from 1804 to 1806? William Clark was the primary cartographer of the expedition, according to Journal editor Dr. Gary Moulton "working with crude and unreliable instruments and with no apparent training," yet doing "a masterful job." Clark was primarily concerned with the "direction of travel from point to point, the number of miles covered between the points, and the daily mileage accumulation."
Volume One of The Journals of the Lewis and Clark Expedition, Lincoln: University of Nebraska Press, 1983, edited by Gary E. Moulton, is an oversize atlas which reproduces all the maps drawn during the expedition. This book is a fine source of geographical information as it was known to the explorers.
Moulton theorizes that Clark relied on compass readings for his "courses." His compass traverse notes are at the beginning of nearly every journal entry. According to Moulton, Clark "probably employed the route traverse method, taking bearings at each turn of the trail or bend in the river and plotting those shifts on his maps.... For his mapping Clark was probably taking 'back sightings' or 'back azimuths,' giving his bearing from north or south in degrees. After traveling forward some miles, Clark would turn and take an azimuth reading from his previous point of sighting. This procedure was necessary because he could not always determine his next point of observation. In converting back azimuths to forward readings, Clark may have become confused occasionally, and some of his readings may seem turned around because they reflect the direction he was looking rather than the direction that he had traveled."
Allen, John Logan. Passage Through the Garden: Lewis and Clark and the Image of the American Northwest. Urbana: University of Illinois Press, 1975.
Jackson, Donald, ed. Letters of the Lewis and Clark Expedition, with Related Documents, 1783-1854. Urbana: University of Illinois Press, 1962.
Moulton, Gary E., ed. Volume One of The Journals of the Lewis and Clark Expedition, Lincoln: University of Nebraska Press, 1983.
Wheat, Carl I. Mapping the Trans-Mississippi West, 1540-1861: Volume 2 - From Lewis and Clark to Fremont, 1804-1845. San Francisco, 1958.