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Helpful information about the basics of surveying

The History of the Spirit Level

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Spirit levels are used in surveying, carpentry, construction and other professions to identify whether a surface is horizontal or vertical. In their early days, they included curved, glass vials with continuous inner-diameter at each viewing point. The vials were partially filled with liquid – usually alcohol or a coloured sprit – leaving a bubble in the tube. The vials’ subtle upward curve caused the bubble to rest in the middle, at the highest point. The bubble reacted to any inclination by moving away from the center position. 


Alcohols such as ethanol are often used in spirit levels, due to their low surface tension and viscosity, which allows the bubble to move quickly through the tube and settle accurately, with minimal interference.

The bull’s eye level is a circular, flat-bottomed device with liquid underneath a convex glass face, with a circle at the center. Unlike a standard level, a bull’s eye level can be used to level a surface across a plane – rather than only in the direction of the tube.

The sprit level was invented by Melchisedech Thevenot, a wealthy amateur scientist and royal librarian to Louis XIV of France. Examination of correspondence between Thevenot and scientist Christiaan Huygens has revealed that the spirit level was invented at some point before February 2, 1661. Thevenot quickly released a description of his creation to others around Europe, including Vincenzo Viviani, in Florence, and Robert Hooke, in London. Exactly when use of the spirit level in land surveys and other projects became widespread remains unclear, with some arguing that they did not gain popularity until the 18th century, since it is from this period that the oldest surviving examples date. However, there are records of the Academie Royale des Sciences being advised to take “levels of the Thevenot type” on their expedition to Madagascar, in 1666.

The modern level with a single vial was invented by Henry Ziemann, in the 1920s and, in 1939, William B. Fell created the Fell All-Way precision level, in Rockford, Illinois. This bull’s eye level could be positioned on a machine bed and display tilt on the x-y axes, making it unnecessary to rotate the level 90 degrees. Compared to previous designs, Fell’s level was remarkably accurate, and set a new standard of .0005 inches per foot resolution.

Production of the Fell All-Way precision level ceased in around 1970, and was resumed in the ‘80s by Thomas Butler Technology, in Rockford, Illinois, before finally ending in the ‘90s. The spirit level remains an important tool for surveying and various other industries, and has been incorporated into the design of a number of other tools. The dumpy level, which is used to measure height differences over larger distances, often features an inbuilt spirit level. It was invented in 1832, by English civil engineer, William Gravatt. Commissioned to examine a railway route from London to Dover, he devised the dumpy level as a more mobile and easier-to-use alternative to the Y level.


 

The History of the Compass

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The most common type of compass has a magnetised needle which points to magnetic north and south by rotating to align with Earth’s magnetic field. Exactly when the principles underlying the magnetic compass were discovered remains unclear. There is evidence that the Ancient Greeks grasped magnetism, and as much as 2,000 years ago, Chinese scientists may have understood that it was possible to temporarily magnetise an iron bar by rubbing it against a lodestone, so it would point north and south.

The first compasses consisted of a magnetised needle attached to a piece of wood which floated in a container of water. As the needle settled, it would point in the direction of magnetic north and magnetic south. As scientists’ and engineers’ understanding of magnetism progressed, the needle of the compass was mounted on a card which displayed north, east, south and west. A spearhead accompanied by the letter ‘t’ (standing for the Latin name for the north wind, Tramontana) signified north. The compass card continued to develop, until all 32 directions were displayed. China may have developed compasses as early as the 11th century, closely followed by western Europe, in the 12th century. It is likely that these rudimentary compasses were used when more traditional means of determining direction, such as the moon or stars, were obscured.

By the 15th century, a discrepancy between the ‘north’ indicated by magnetic compasses and Earth’s actual geographic north had been identified. The difference between magnetic north and actual north became known as ‘variation’ or ‘magnetic declination’. It differs depending on the user’s proximity to Earth’s poles; variation is at its minimum on the equator, but is significantly greater near the south and north poles, where compasses must be adjusted to avoid misleading directions.

There is evidence of magnetic compasses having been used for building orientation in Denmark during the 12th century. A fourth of the country’s Romanesque churches are rotated by 5-15 degrees clockwise from east to west, which corresponds with the predominant variation at their time of construction – indicating that the use of magnetic compasses was already reasonably widespread during this period.

When shipbuilders started using iron and steel instead of wood, it was discovered that ships can affect the reading of their on-board compass – a phenomenon known as ‘deviation’. It became common to place iron balls or bars close to the compass to increase its accuracy. Deviation is also taken into account on aircraft, the metal of which can affect compass readings.

Not all compasses use Earth’s magnetism to indicate direction. The gyrocompass, which was invented in the 20th century, has a rotating gyroscope to follow Earth’s axis of rotation in order to point northwards. Variation is not an issue for the gyrocompass, which is widely used on aircraft and ships.

Although global positioning systems (GPS) have become increasingly popular in recent years, the compass remains an important tool in various industries, including surveying, and for a range of recreational activities.

 

The History of the Theodolite

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The theodolite is a precision instrument for measuring horizontal and vertical angles. Mounted on adjustable legs and with a swiveling telescope, the modern theodolite is used to record detailed measurements for triangulation in road construction and other civil engineering projects.

Before the invention of the theodolite, various tools such as graduated circles and semicircles, and geometric squares, were used to measure either vertical or horizontal angles. German writer Gregorius Reisch described a devise which was capable of measuring both angles simultaneously in Margarita Philosophica, which was published in 1512.

In 1571, English surveyor and mathematician Leonard Digges used the term ‘theodolitus’ in Pantometria, his book on measurement, to describe a surveying instrument with a circular ring divided into 360 degrees, and a pivoting alidade with sight vanes at each end. This rudimentary theodolite became popular among surveyors in England, and in 1791, writer and mathematical instrument maker George Adams described the instrument as a ‘common theodolet’, using ‘theodolite’ only for telescopic instruments with vertical arcs and horizontal circles.

The first theodolite capable of measuring horizontal angles with geodetic accuracy was made in London by mathematician and scientific instrument maker Jesse Ramsden, in the late 1700s. He created the Great Theodolite for the Royal Society, which had decided to link the Royal Observatory in Greenwich with the Observatory in Paris, by means of triangulation. Ramsden designed a highly accurate dividing engine for his theodolite, which incorporated a horizontal circle which was three feet in diameter and weighed approximately 200 pounds. Ramsden’s Great Theodolite is now housed at the Greenwich Museum, in London.

Use of the telescopic theodolite became widespread among English surveyors but in America, the preference for the cheaper and more robust surveyor’s compass and surveyor’s transit endured. In the 18th century’s commonest design, the telescope was mounted on the theodolite’s vertical arc. In the 1840s, the transit theodolite was introduced in London, featuring a transit-mounted telescope, with a vertical circle on one edge. During the late 1800s, American inventor and physicist Edward Samuel Ritchie created a water-based theodolite, which the U.S Navy used to record the first precision surveys of the Gulf and Atlantic coast’s harbours.

The next major step in the development of the theodolite took place in Switzerland, in the 1920s, when inventor and designer Heinrich Wild introduced the optical theodolite, with a number of improvements on previous models, including an auxiliary telescope which made it possible to read either circle without leaving the station.

Theodolites remain an important tool in surveying. Various specialised models are available, including the photo-theodolite, which incorporates a camera and theodolite, mounted on one tripod, and is used for a range of projects, including the production of maps. Today, the reading of a theodolite’s vertical and horizontal circles is generally carried out with a rotary encoder, while CCD sensors have been added to the telescope’s focal plane, making an even greater degree of surveying precision possible.

 

The History of the Tape Measure

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The tape measure, or measuring tape, has a colourful history which goes back much further than its first patents. An Englishman, Charles White, was found guilty of stealing a measuring tape and transported to Australia as punishment in 1839, and the Romans used marked strips of leather as rudimentary measuring devices.

In 1842, in Sheffield, England, Steelmaker James Chesterman used an inventive heat-treating process to redevelop the wire he had designed for crinoline skirts into a strong, steel measuring device. Chesterman’s ‘steel band measuring chains’ quickly became popular among surveyors. Comparing his product to the bulkier metal chains which surveyors had traditionally used, he said his measuring chain had “equal strength, greater correctness, is easier to clean and to coil and uncoil, and is very much lighter and more compact”.

In 1868, Alvin J Fellows, of New Haven, Connecticut, received a patent for his spring tape measure, the design of which remains common today. Although Fellows conceded in his patent application that his design included springs which wound up the tape “in the usual way”, his design included an innovative clip which locked the tape in one position, so it didn’t move until the clip was released.

In 1871, Long Island’s Justus Roe and Sons launched the production of cheap, steel tape measures, which were patented ‘Roe’s Electric Reel’ – even though there was nothing electric about them. They were a big hit, and the company soon developed an etched steel-ribbon tape measure. Despite the availability of affordable tape measures, it wasn’t until the early 20th century that they overtook traditional carpenter’s folding wooden rulers as the most popular collapsible measuring device.

During the late 19th century, Jasper Freemon Meek and H.D Beach, a pair of entrepreneurs in the U.S, started using their printing press to display adverts on various everyday objects, such as book bags, pencils, yard sticks and tape measures. In the following decades, it became increasingly common to house tapes measures in celluloid containers, which often featured intricate designs and are in high demand among collectors today.

 

What is a Digital Terrain Model (DTM)?

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A digital terrain model (DTM) is a graphic representation of a location’s surface. DTMs provides detailed data on the bare earth which is manipulated by various computer programs for a range of surveying projects.

The data generated by DTMs is important in the creation of 3D renderings in surveying. The 3D models which are rendered from DTM data are a versatile and powerful tool for Ogilvie Geomatics’ surveyors, and are particularly useful for terrain-visualisation, reduction of gravity measurements and rectification of satellite photos.   

DTMs often include digital spatial elevation data which generally appears on a rectangular grid. Unlike a digital surface model (DSM), digital terrain models involve the digital removal of surface features, such as trees and buildings, which is essential for accurate and up-to-date surveys of the underlying terrain. DTMs usually involve raster data, which represents the surveyed surface as a regular grid of cells, with a space resolution of 50 to 500 meters.

Digital terrain modelling is a key tool for geographic information systems. Data can be stored in a variety of ways, from an irregularly spaced set of points, to a set of contour vectors or a rectangular grid of equally spaced points. The accuracy of DTMs depends on a range of factors, such as the quality of the source data and the model resolution. It can be difficult to precisely locate objects using digital terrain modelling by referencing topographic features, but DTMs can be enhanced by overlaying raster data over the terrain model.

Ogilvie Geomatics use digital terrain modelling for a wide range of projects, including specialist land surveys, route surveys for road access, beach monitoring and coastal erosion surveys and hydrographical surveys

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