Course from the Perogue & Distances &c
Proceeding up the south side of the Missouri on 17 June, Clark conducted a route survey2 of the falls, rapids, cascades and ravines from the lower portage camp to the upper falls (Black Eagle) and beyond, using a circumferentor (surveyor's compass) to determine bearings or directions, and a two-pole chain as a measuring unit. His five-man survey party included two chainmen and perhaps a stakeman, a rear flagman, and a record-keeper with a notebook and a pencil.
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The surveyor's chain was called Gunter's, after its inventor, the English mathematician and astronomer Edmund Gunter (1581-1626).3 The length of the chain is 33 feet, or two poles (also called rods or perches) of 16Ω feet each. It consists of 50 links separated from one another by three rings. The length of a link, from the center of one connecting ring to the next, is 7.92 inches. Tally tags having one, two, three or four notches divide the chain into five sections, for convenience in measuring distances of less than a full chain. Gunter's chain served as the basic surveying instrument for three hundred years, until it was replaced in the early 20th century by the steel tape and later in the same century by the Global Positioning System.4
Clark used the circumferentor to "shoot" a line from the beginning of his survey to the first visible landmark, or object, the mouth of Portage Creek. He might have sent a stakeman to the landmark with a pole, such as an espontoon, to hold erect as a reference point that the circumferentor sights could be aimed on. (Clark paused here to measure the width of the creek with the circumferentor. For that procedure, see "Heights and Distances, & Problem V.") While the hinder chainman held one end of the chain at the starting point, or station, the fore chainman carried the other end of the chain straight toward the first object. When the chain was fully extended and pulled taught, the hinder chainman signalled the fore chainman to move to his right or left until the chain was precisely on the line between the station and the object. The fore chainman then dropped a plummet (plumb bob), or else a rock, from the handle of the chain, and inserted in the ground at that point one of the ten iron marking pegs he carried.5
The two chainmen then walked straight toward the first object until the hinder man arrived at the first peg, stopping there until the chain was pulled tight, the alignment was checked by Clark with the circumferentor, and a peg marking the fore end of the chain was driven into the ground. The hinder man pulled up the first peg, the two chainmen moved ahead, and the process was repeated until the hinder man had all ten pegs in hand, indicating they had measured ten chains, or twenty poles (330 feet), in a perfectly straight line along the compass bearing Clark had established with the circumferentor. The crew continued similarly until they reach the first object .in this case the mouth of Portage Creek, or a marker of some sort held there to guide them.
All gradients were measured horizontally, rather than on the angle of the grade. To do this, the hinder chainman dropped a plummet from the handle of the chain, and raised his end until the chain and the plummet line formed a right angle. The fore chainman lowered his end if necessary. If the grade was too steep to be measured with the full two-pole chain, they measured the distance in links between the hinder man's end, as high as he could hold the chain, to the point where it intersected the grade. They then finished out the chain length horizontally from that station point.
There is no identifiable object at the close of any of the next six courses, so it may be presumed Clark sent the stakeman upstream to the next bend. After the stake was observed, and its compass bearing recorded, the whole procedure was repeated, and so on for each course. A rear flagman may have remained at the beginning of each course to serve as a reference point until the next object was reached.
In the brief example cited above, the crew has repeated the chaining steps 968 times on eight different compass bearings, totalling a little over six miles. The complete survey reached 4,747 poles in 22 courses, totalling 14æ miles and 27 poles (445Ω feet). Clark estimated the total descent of the riverbed in that distance at 360 feet, 2æ inches.
The same process was employed to survey the route for the portage around the falls, which consisted of six courses in 17æ miles plus 46 poles (759 feet).
with assistance from Donald Ebbutt,
past president, Surveyors Historical Society,
member, Montana Assocation of
Registered Land Surveyors
1) The foregoing is the beginning of one of two different sets of survey notes by Clark for the falls of the Missouri. The next course takes him directly to the river. Moulton, 4:310-17, 417n.
2. There were three types of surveys common in Lewis and Clark's time. The oldest, dating from the beginnings of civilization, was the land survey, by which property lines were fixed and land areas calculated, to facilitate the identification of real property for ownership, sale, or purchase. The cadastral survey fixed the boundaries of municipalities, as well as state and federal jurisdictions, chiefly as the bases for taxation. The route survey was used for siting and constructing roads – and later, railroads, highways, irrigation ditches, levees and transmission lines. During the nineteenth century, topographic, hydrographic, and mine survey techniques evolved, followed in the twentieth century by aerial surveys.
3. Gunter made significant contributions to trigonometry, and perfected the quadrant. http://www.groups.dcs.stand.ac.uk/~history/Mathematicians/Gunter.html (accessed 3 October 2003).
4. Gunter's chain, with its parcel of awkwardly divisible standards of measurement – chain, pole, yard, foot, and inch – also embedded the English statute mile into the very soil and substance of the United States as a nation, despite Thomas Jefferson's arduous but futile efforts to establish the metric system. Andro Linklater, Measuring America: How an Untamed Wilderness shaped the United States and Fulfilled the Promise of Democracy (New York: Walker & Company, 2002), 102-116.
5. The process is described in detail in Robert Gibson, The Theory and Practice of Surveying; Containing All the Instructions Requesite for the Skilful Pratice of this Art. (New-York: Evert Duyckinck, 1821), 145-62. Collection of Donald Ebbutt.
Funded in part by a grant from the Montana Cultural Trust