A circumferentor, or surveyor's compass, is an instrument used in
surveying
Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...
to measure
horizontal angles. It was superseded by the
theodolite
A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and i ...
in the early 19th century.
A circumferentor consists of a circular
brass
Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other with ...
box containing a magnetic needle, which moves freely over a brass circle, or
compass
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
divided into 360
degrees.
The needle is protected by a glass covering.
A pair of
sights
A sight is an aiming device used to assist in visually aligning ranged weapons, surveying instruments or optical illumination equipments with the intended target. Sights can be a simple set or system of physical markers that have to be aligne ...
is located on the North-South axis of the compass.
Circumferentors were typically mounted on
tripod
A tripod is a portable three-legged frame or stand, used as a platform for supporting the weight and maintaining the stability of some other object. The three-legged (triangular stance) design provides good stability against gravitational loads ...
s and rotated on
ball-and-socket joint
The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...
s.
Circumferentors were made throughout Europe, including in England, France, Italy, and Holland. By the early 19th century, Europeans preferred theodolites to circumferentors. However, the circumferentor remained in common use in
mines and in wooded or uncleared areas, such as in America.
Usage
Measuring angles
To measure an angle with a circumferentor, such as angle EKG (Figure 1), place the instrument at K, with the
fleur-de-lis
The fleur-de-lis, also spelled fleur-de-lys (plural ''fleurs-de-lis'' or ''fleurs-de-lys''), is a lily (in French, and mean 'flower' and 'lily' respectively) that is used as a decorative design or symbol.
The fleur-de-lis has been used in the ...
in the card towards you. Then direct the sights, until through them you see E; and note the degree pointed at by the south end of the needle, such as 296°. Then, turn the instrument around, with the fleur-de-lis still towards you, and direct the sights to G; note the degree at which the south end of the needle point, such as 182°. Finally, subtract the lesser number, 182, from the greater, 296°; the remainder, 114°, is the number of degrees in the angle EKG.
If the remainder is more than 180 degrees, it must be subtracted from 360 degrees.
Surveying a region
To take the plot of a field, forest, park, etc., with a circumferentor, consider region ABCDEFGHK in Figure 2, an area to be surveyed.
#Placing the instrument at A, the fleur-de-lis towards you, direct the sights to B; where suppose the south end of the needle cuts 191°; and the ditch, wall, or hedge, measuring with a
Gunter's chain
Gunter's chain (also known as Gunter’s measurement) is a distance measuring device used for surveying. It was designed and introduced in 1620 by English clergyman and mathematician Edmund Gunter (1581–1626). It enabled plots of land to be a ...
, contains 10 chains, 75 links.
#Placing the instrument at B, direct the sights as before to C; the south end of the needle, ''e.g.'' will cut 279°; and the line BC contains 6 chains and 83 links.
Then move the instrument to C; turn the sights to measure D, and measure CD as before. In the same manner, proceed to D, E, F, G, H, and lastly to K; still noting the degrees of every bearing, or angle, and the distances of every side. This will result in a table of the following form:
From this table, the field is to be plotted, or protracted.
Alternative plotting method:
An alternative way to plot the area in Figure 2 is to use several angles and only a few measurements and calculate their positions.
This could be done by starting at the center point in Figure 2 which is not labeled, but which will be referred to as "Center." Assume each point can be seen from each other point. From the "center" point, sight and record the angle to each point using the sights as described above. Then move to, and measure the distance to, one of the other points referenced, such as point B. At point B, measure the angles to all the other points. Then, move to an additional point such as point F. Again, measure the distance from the center to the point chosen (F). At that point, measure and record the angles to each of the other points as was done at point B. Chose a scale (a ratio between the size of the area to be plotted and the size of the paper on which you will draw the plot) that will allow the plot to fit on your paper and plot the angles and distances.
The advantage of this method over the first one above is that there are fewer distance measurements and any errors in angles or distances will not be cumulative; that is, if you use the first survey method, any angle that is slightly off will distort the remainder of the plot. The second method can also be used when it is not possible to measure some of the distances, for example, if there is a water barrier between two of the points. Also, if there are any inaccuracies in the measurements, they will be revealed in the plot because the points plotted from various angles will not coincide.
Additional considerations include the number of times the circumferentor must be set up and aligned. With the first method, the instrument must be set up at each point with a compass. With the second method, the initial set up is at "center." After that, for example at point B, the instrument can be set up by aligning the sight with the reciprocal of the angle between "center" and B. Thus, any local change in the magnetic field that would affect the compass would be nullified.
Surveyor's double prism
A double prism is a device to measure right angles, consisting of two five sided prisms stacked on top of each other and a
plumb-bob
A plumb bob, plumb bob level, or plummet, is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical reference line, or plumb-line. It is a precursor to the spirit level and used to establish a vertic ...
. It is used to stake out
right angle
In geometry and trigonometry, a right angle is an angle of exactly 90 Degree (angle), degrees or radians corresponding to a quarter turn (geometry), turn. If a Line (mathematics)#Ray, ray is placed so that its endpoint is on a line and the ad ...
s, for example on a
construction
Construction is a general term meaning the art and science to form objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Press 2009 and com ...
site.
See also
*
Graphometer
The graphometer, semicircle or semicircumferentor is a surveying instrument used for angle measurements. It consists of a semicircular limb divided into 180 degrees and sometimes subdivided into minutes. The limb is subtended by the diameter with ...
*
Alidade
An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site ...
References
*{{1728, title=Circumferentor
*"Circumferentor". ''Oxford English Dictionary''.
Oxford University Press
Oxford University Press (OUP) is the university press of the University of Oxford. It is the largest university press in the world, and its printing history dates back to the 1480s. Having been officially granted the legal right to print books ...
. 2nd edition. 1989.
Surveying instruments
Angle measuring instruments