Surveying Instruments
Surveying is the apportionment of land by measuring and mapping. It is employed to determine boundaries and property lines, and to plan construction projects. Any civilization that had any degree of sophistication in construction methods required surveys to ensure that work came out according to plan.
Surveying is thought to have originated in ancient Egypt as early as 2700 B.C., with the construction of the Great Pyramid of Khufu at Giza, though the first recorded evidence of boundary surveying dates from 1400 B.C. in the Nile River valley. The classic surveyors were the Romans. In order to forge an Empire that stretched from the Scottish border to the Persian Gulf, a large system of roads, bridges, aqueducts, and canals was built, binding the country economically and militarily. Surveying was a major part of Roman public works projects. It also was used to divide the land among the citizens. Roman land surveying was referred to as centuriation after the century, which was a common rectangular unit of land area. These land parcels can still be seen in aerial photographs taken over France and other parts of Europe, the work of the Roman agrimensores, or "measurers of land." The property lines were usually marked by stone walls and boundary markers.
With the advent of new methods of trigonometry and calculus, new surveying instruments emerged. The theodolite was invented in the sixteenth century. Its precise origin is unclear, but one version was invented by English mathematician Leonard Digges in 1571, who gave it its name. A great theodolite was invented by Jesse Ramsden more than 200 years later in 1787, and its use led to the establishment of the British Ordnance Survey. Made of a telescope mounted on a compass or of a quadrant plus a circle and a compass, the theodolite is used to measure horizontal and vertical angles. The modern theodolite is usually equipped with a micrometer, which gives magnified readings up to 1/3600°, or one second of arc. The micrometer is derived from the vernier scale, which was invented by French engineer and soldier Pierre Vernier (1584-1638) in 1631 to measure in fractions.
The transit is a theodolite capable of measuring horizontal and vertical angles, as well as prolonging a straight line or determining a level sight line. A telescope atop a tripod assembly is clamped in position to measure either horizontal or vertical angles. The transit employs a plumb bob hanging from the center of the tripod to mark the exact location of the surveyor.
The practice of triangulation was introduced by Gemma Frisius in 1533. By measuring the distance of two sides of a triangle in a ground survey, the third side and the triangle's area can be calculated. Triangulation was aided by the inventions of the prismatic astrolabe and the heliotrope. The latter was invented by German mathematician Johann Gauss (1777-1855), who is considered the father of geodesy, the science of earth measurement. Both instruments used a beam of sunlight to signal the positions of distant participants in a land survey.
Other survey instruments include the surveyor's compass, which is used for less precise surveying, the surveyor's level used to measure heights of points above sea level or above local base points, and metal tapes, first introduced by English mathematician Edmund Gunter in 1620, used for shorter measurements.
In the late twentieth century, surveying has been aided greatly by remote sensing. Photogrammetry employs aerial photography to survey large areas for topographic mapping and land assessment purposes. Artificial satellites and global positioning techniques enable surveryors to make quick and accurate measurements of the distance between two points, and global positioning is also used in mapping utility lines and monitoring shipping.
Laser technology has increased the precision of survey sightings. Laser-based speed and distance measurement instruments, or laser rangefinders, are used in a wide variety of applications ranging from traffic speed enforcement to forestry, from surveying and mapping to golfing and hunting, and from docking ships to docking the Space Shuttle. Lasers have even been used to measure the distance between the Earth and the moon. During the first landing on the moon, Apollo 11 astronauts set up a laser reflector that is still in operation. By measuring how long it takes a laser pulse to return to Earth (about 2.5 seconds), the Earth-moon distance has been measured to an astonishing accuracy of about 1 in (2.5 cm).
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