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The earliest surveying instrument: The Plumb Bob
The observation that a heavy object hanging from a string causes the string to hang perpendicular to the earth is
By 2600 BC, we know the Egyptians had taken this concept and created the earliest surveying instruments: the
plumb board, the A-Level, T-Level and plumb square. This was the first use of the plumb bob, against a wood frame
that paralleled the surface being measured. The worker could then make a more precise visual judgment as to the
trueness of plumb or horizontal level. These earliest bobs were stone and their shape, although often egg-like, really
didn't matter. These simplest of tools remained virtually unchanged for the next 4400+ years.
The invention of the spirit level, and the beginnings of the Industrial Revolution that allowed the level to be
manufactured, both accurately and cheaply, began the demise of the ancient plumb tools. For establishing plumb
and true horizontal the level is simply a better tool. It is quicker and easier to use and just as accurate. But there is
one thing the level can't do easily, and that is to transfer an exact point from one height to another. The plumb bob is still an indispensable tool in modern
Early surveying units:
* The link (7.92 inches).
* The fathom (5.5 feet).
* The rod/ perch (3 fathoms or 16.5 feet).
* The chain (66 feet).
* The furlong or furrowlong (660 feet)
* The mile (5280 feet or 1760 yards).
* The league (3.125 miles or 16500 feet...also 198000 inches, 25000 links, 3000 fathoms, 1000 rods/ perches,
250 chains, 25 furlongs or furrowlongs).
Early standards of length were based on body measurements. The cubit was the distance from elbow to finger
tip, while the foot, palm and finger units are self-explanatory.
Among the earliest length measures was the foot, which understandably varied from district to district, and occurred in two common sizes. The first is the foot
of 246 to 252 mm based on a man’s unshod foot. The second foot measures 330 to 335 mm and is based on hand measurements.
Other units derive from the Roman, Saxons, Angles and Jutes who each invaded England at some time. The rod, furlong and acre are all of Saxon origin.
The mile is a compromise between the French derived Old British mile and the Roman milliarius.
The origin of the ROD as a unit of measurement
A traditional unit of distance equal to 5.5 yards (16 feet 6 inches or exactly 5.0292 meters). The rod and the furlong were the basic distance units
used by the Anglo-Saxon residents of England before the Norman conquest of 1066. The Saxons generally called this unit the gyrd, a word which
comes down to us as the name of a different unit, the yard. "Rod" is another Saxon word which meant just what it means today: a straight stick.
The Normans preferred to call the gyrd a pole or a perch (a word of French origin, meaning a pole; see perche). The length of the rod was well
established at least as early as the eighth century. It may have originated as the length of an ox-goad, a pole used to control a team of 8 oxen (4
yokes). Scholars are not sure how the rod was related to shorter units. It may have been considered equal to 20 "natural" feet (actual foot lengths;
see foot), or it may have been measured "by hand" as 30 shaftments. In any case, when the modern foot became established in the twelfth
century, the royal government did not want to change the length of the rod, since that length was the basis of land measurement, land records,
and taxes. Therefore the rod was redefined to equal 16.5 of the new feet. This length was called the "king's perch" at least as early as the time of
King Richard the Lionheart (1198). Although rods and perches of other lengths were used locally in Britain, the king's perch eventually prevailed.
The relationship between the rod and the other English distance units was confirmed again by the Parliamentary statute of 1592, which defined
the statute mile to be either 320 rods or 1760 yards, thus forcing the rod to equal exactly 5.5 yards or 16.5 feet.
© Russ Rowlett and the University of North Carolina at Chapel Hill
The Magnetic Compass
The magnetic compass is one of the most important instruments in the history of surveying. The compass was probably
invented by the Chinese during the Qin dynasty (221-206 B.C.). Chinese fortune tellers used lodestones (a mineral composed
of an iron oxide which aligns itself in a north-south direction) to construct their fortune telling boards.
Eventually someone noticed that the lodestones were better at pointing out real directions, leading to the first compasses. They
designed the compass on a square slab which had markings for the cardinal points and the constellations. The pointing needle
was a lodestone spoon-shaped device, with a handle that would always point south. Magnetized needles used as direction
pointers instead of the spoon-shaped lodestones appeared in the 8th century AD, again in China, and between 850 and 1050
they seem to have become common as navigational devices on ships. The first person recorded to have used the compass as
a navigational aid was Zheng He (1371-1435), from the Yunnan province in China, who made seven ocean voyages between
1405 and 1433. Source: Inventors with Mary Bellis
Shown: Early B. Rittenhouse Surveyor's Compass
Continue to Part 2
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