Lewis and Clark's river and route surveys all were made using a magnetic compass. The bearings they recorded all were referenced to magnetic north. The Expedition's surveys, thus had to be corrected to True North between the places where they obtained coordinates for latitude and longitude. The survey correcting, however, could have been done without taking observations for magnetic declination simply by plotting the survey between two points of known latitude and longitude. The true bearing and distance between those two points compared with the resultant bearing and distance between those two points would tell what scale factor to apply to the survey and how much rotation was needed to make it fit the true coordinates Perhaps it was easier just to take the observations for magnetic declination, calculate the declination and apply that correction to the surveyed bearings. Even this, however, would not tell how much to expand or shrink the estimated distances to make them fit.
Lewis took three observations at Fort Mandan to determine the magnetic declination. At some convenient time after the sun's altitude usually was at least 10° above the horizon (to minimize the effects of refraction) Lewis would set out the artificial horizon. Meanwhile Clark would set up the 6-inch diameter surveying compass (circumferentor) attaching it to a staff or stick firmly implanted in the ground. Using a spirit level (call it a miniature carpenter's level) he would adjust the surveying compass so that it was level no matter what horizontal angle he measured. One of the men would be standing by with the chronometer to note the time. While Clark "tracked" the center of the sun through the slotted uprights of the surveying compass, Lewis took the sun's altitude. As he did so he would signal to Clark to read the compass needle and the man at the chronometer to record the time. When this sequence of operations had been repeated two more times the observation was complete.
Lewis never made any calculations for magnetic declination. He could have done so using the steps outlined in Patterson's Astronomy Notebook, either 1) calculating these three observations separately and then averaging the results or 2) he could have averaged the times, the bearings and the altitudes, then calculated the result. The latter operation is more expeditious, but the former sometimes shows where observational errors have been made.
Funded in part by a grant from the National Park Service, Challenge-Cost Share Program.