A tribrach with an optical plummet (the black cylinder pointing to the left lower corner of the image). A tribrach is an attachment plate used to attach a

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 ...

instrument, for example a

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 ...

total station A total station (TS) or total station theodolite (TST) is an electronic/optical instrument used for surveying and building construction. It is an electronic transit theodolite integrated with electronic distance measurement (EDM) to measure ...

GNSS A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pre ...

antenna or

target Target may refer to: Physical items * Shooting target, used in marksmanship training and various shooting sports ** Bullseye (target), the goal one for which one aims in many of these sports ** Aiming point, in field artillery, fi ...

to a

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 ...

. A tribrach allows the

survey Survey may refer to: Statistics and human research * Statistical survey, a method for collecting quantitative information about items in a population * Survey (human research), including opinion polls Spatial measurement * Surveying, the techniq ...

instrument to be repeatedly placed in the same position over a surveying marker point with sub-

millimetre 330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter. The millimetre (American and British English spelling differences#-re, -er, ...

precision Precision, precise or precisely may refer to: Science, and technology, and mathematics Mathematics and computing (general) * Accuracy and precision, measurement deviation from true value and its scatter * Significant figures, the number of digit ...

, by loosening and re-tightening a lock to adjust the instrument base in a horizontal plane.

Components of a tribrach

The device consists of two triangular metal plates connected at their corners by leveling thumbscrews, a

bubble level A spirit level, bubble level, or simply a level, is an instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb). Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, ot ...

, a locking

mechanism Mechanism may refer to: * Mechanism (engineering), rigid bodies connected by joints in order to accomplish a desired force and/or motion transmission *Mechanism (biology), explaining how a feature is created *Mechanism (philosophy), a theory that ...

and often an

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

plummet. The centre of the bottom plate of the tribrach has a 5/8-11 UNC thread bolt hole which is used to attach the tribrach to the

. The top plate has three small holes or slots spaced 120˚ apart above leveling thumbscrews, and a locking mechanism which enables a survey instrument or target to be accurately placed on the tribrach and locked in place. The

plummet has either a bullseye or

cross hair A reticle, or reticule also known as a graticule, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the screen of an oscilloscope ...

sight for positioning the instrument over a

marker or ground control point. The older tribrachs required a

plumb line 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 ...

to allow them to be positioned vertically over a point and took longer to set up. Some modern

s have an integral

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

plummet built in.

Using a tribrach

The process is iterative between horizontal position and top plate level, because they are interactive. The following general process is followed: * The tribrach is attached to the

and placed over the marker. * Looking through the

plummet pick up two of the

legs and position the cross hairs over the marker. Tread all three legs into the grass,

bitumen Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...

, concrete. * Look through the

plummet and using the thumbscrews put the cross hairs back over the marker. * Adjust the level of the tribrach by slackening the

screws and sliding the legs. Re-center the cross hairs using the thumb screws, then fine-tune the level using the

legs. Once the tribrach is nearly centered over the marker, level the tribrach the last little bit using the thumbscrews. * Slacken the tribrach from the

head and slide it the last 4-5mm so that it is centered over the marker and is level. If the target over which the instrument is to be placed is roughly at the same elevation as the tripod feet, the preceding technique is applicable. If there is a considerable difference between the elevation of the target and the elevation of the marker and the instrument bottom plate, such as when the target is in a deep hole, or on a

Bilby tower A Bilby tower is a type of steel surveying, survey tower used by the United States Coast and Geodetic Survey from 1927 to 1984. It is named after Jasper S. Bilby who designed it in 1926. In 1927, Herbert Hoover, then the Secretary of Commerce, c ...

, the target will appear to move laterally by a large distance in the optical plummet as the tribrach level is adjusted. Tribrachs are

instruments and should be checked regularly to ensure they do not become a source of error.

References

* Gerard L'E. Turner, ''Nineteenth Century Scientific Instruments'', Sotheby Publications, 1983, * Gerard L'E. Turner, ''Antique Scientific Instruments'', Blandford Press Ltd. 1980, * {{ cite patent , country = US , number = 5501018 , title = Tribrach device , pubdate = March 26, 1996 , inventor = Sonny J. Porter

External links

Tribrachs and other accessories
to Wild

s. Surveying instruments