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A triangulation station, also known as a trigonometrical point, and sometimes informally as a trig, is a fixed surveying station, used in geodetic surveying and other surveying projects in its vicinity.
The station is usually set up by a government with known coordinates and elevation published. Numerous stations are installed on summits for purposes of visibility and prominance. A graven metal plate on the top of a pillar may provide a mounting point for a theodolite or reflector, often using some form of kinematic coupling to ensure reproducible positioning.
Trigonometrical stations form networks of triangulation. Positions of all land boundaries, roads, railways, bridges and other infrastructure can be accurately located by the network, a task essential to the construction of modern infrastructure. Apart from the known stations set up by government, some temporary trigonometrical stations are set up near construction sites for monitoring the precision and progress of construction.
Some trigonometrical stations use the Global Positioning System for convenience. Its accuracy factors in ionospheric and tropospheric propagation delay errors.
Although stations are no longer required for many modern surveying purposes, they remain useful to hikers and even aviators as navigational aids. Particular small triangles on maps (◬) mark summits, for example.
The nomenclature for triangulation stations varies regionally: they are generally known as trigonometrical stations or triangulation stations in North America, trig points in the United Kingdom, trig pillars in Ireland, trig stations or trig points in Australia and New Zealand, [1] and trig beacons in South Africa.
In the 1820s, much of New South Wales was unsurveyed territory to the European arrivals. To aid the mapping of the country, the science of trigonometic surveying was introduced by Major Thomas Mitchell who had been brought out to the colony as Assistant Surveyor General of New South Wales. The freestanding peak of Mount Jellore was selected as the first trigonometric summit for his triangulation survey of the countryside. In 1828 Mitchell headed south from Sydney with a small party and camped at the base of the basalt, making daily excursions to the top. While his convict crew cleared the summit of trees, Mitchell plotted and measured distant peaks and sketched the skyline.
A national geodetic survey and adjustment carried out in the early 1970s in Australia has left a legacy of trig stations, many consisting of a ground mark with a black quadripod (pyramid frame) supporting a visible disc above the ground mark. [2]
Many trigonometrical stations were placed on hilltops around Hong Kong. They strongly resemble those used in other former British colonial territories such as Australia, consisting of a white column topped with a black band.
In Japan, there are five classes of triangulation stations (三角点, sankakuten, lit. 'three corner points'):
As of August 2023 [update] there are 5,765 trig stations in New Zealand. They are placed on top of hills and are usually black and white. [5]
South Africa has a network of approximately 28,000 trig beacons, established by the Chief Directorate: National Geo-spatial Information (historically known as the Trigonometrical Survey). [6] These beacons are typically white-painted concrete pillars supporting black metal plates in a cross shape, installed on mountains, hills or tall buildings.
In Spain there are 11,000 triangulation stations, concrete structures which typically consist of a cylinder 120 cm high and 30 cm in diameter over a concrete cubic base.
They were erected by the Instituto Geográfico Nacional, usually painted in white, and can be marked with a metallic label with the warning: "The destruction of this sign is punishable by law."
In the United Kingdom, trig points are typically concrete pillars and were erected by the Ordnance Survey.
The process of placing trig points on top of prominent hills and mountains began in 1935 to assist in the accurate retriangulation of Great Britain. The Ordnance Survey's first trig point was erected on 18 April 1936 near Cold Ashby, Northamptonshire. In low-lying or flat areas some trig points are only a few metres above sea level and one is even at −1 m (near Little Ouse, Cambridgeshire, TL61718 89787). [7] When all the trig points were in place, it was possible in clear weather to see at least two other trig points from any one trig point, but subsequent vegetation growth means that this is not necessarily still the case. Careful measurements of the angles between the lines-of-sight of the other trig points then allowed the construction of a system of triangles which could then be referenced back to a single baseline to construct a highly accurate measurement system that covered the entire country.
In most of the UK, trig points are truncated square concrete (occasionally stone) pyramids or obelisks tapering towards the top. On the top a brass plate with three arms and a central depression is fixed, known as a "spider": it is used to mount and centre a theodolite used to take angular measurements to neighbouring trig points. A benchmark is usually set on the side, marked with the letters "O S B M" (Ordnance Survey Bench Mark) and the reference number of the trig point on a plaque called a "flush plate". Within and below the visible trig point, there are concealed reference marks whose National Grid References are precisely known. The standard trig point design is credited to Brigadier Martin Hotine (1898–1968), head of the Trigonometrical and Levelling Division of the Ordnance Survey. [8] Many of them are now disappearing from the countryside as their function has largely been superseded by aerial photography and digital mapping using lasers and GPS. To quote from a page at the OS site: "Like an iceberg, there is more of trig pillar below the surface than above it." [9] From the same source: "Today the receivers that make up the OS Net network are coordinated to an accuracy of just 3 mm over the entire length of Great Britain."
The United States National Geodetic Survey (NGS) and predecessor agencies manages the National Spatial Reference System (NSRS), which includes permanent survey marks for horizontal position (latitude and longitude), height, or gravity. Some marks have information for both horizontal position and height. Some marks were established by NGS. Others were established by other organizations, such as state highway departments, but are included in the database that makes up the NSRS. Information about marks is available to the public online. The number of points in the NSRS is over 1,500,000. [10]
The image is the spire of the Fair Haven, Vermont First Baptist Church, a horizontal survey mark (triangulation station), and is described in the NGS National Spatial Reference System under the permanent ID OD1373.
Geodesy or geodetics is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems. Geodesy is an earth science and many consider the study of Earth's shape and gravity to be central to that science. It is also a discipline of applied mathematics.
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. These points are usually on the surface of the Earth, and they are often used to establish maps and boundaries for ownership, locations, such as the designated positions of structural components for construction or the surface location of subsurface features, or other purposes required by government or civil law, such as property sales.
In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points.
The National Geodetic Survey (NGS) is a United States federal agency based in Washington, D.C. that defines and manages a national coordinate system, providing the foundation for transportation and communication, mapping and charting, and a large number of science and engineering applications. Since its founding in 1970, it has been part of the National Oceanic and Atmospheric Administration (NOAA), a division within the Department of Commerce.
Major-General William Roy was a Scottish military engineer, surveyor, and antiquarian. He was an innovator who applied new scientific discoveries and newly emerging technologies to the accurate geodetic mapping of Great Britain. His masterpiece is usually referred to as Roy's Map of Scotland.
A geodetic datum or geodetic system is a global datum reference or reference frame for unambiguously representing the position of locations on Earth by means of either geodetic coordinates or geocentric coordinates. Datums are crucial to any technology or technique based on spatial location, including geodesy, navigation, surveying, geographic information systems, remote sensing, and cartography. A horizontal datum is used to measure a horizontal position, across the Earth's surface, in latitude and longitude or another related coordinate system. A vertical datum is used to measure the elevation or depth relative to a standard origin, such as mean sea level (MSL). A three-dimensional datum enables the expression of both horizontal and vertical position components in a unified form. The concept can be generalized for other celestial bodies as in planetary datums.
The Meades Ranch Triangulation Station is a survey marker in Osborne County in the state of Kansas in the Midwestern United States. The marker was initially placed in 1891. From 1901, it was the reference location for establishing a system of horizontal measurement in the United States, known as geodetic datum. In 1913, the datum was adopted across all of North America, and the system revised and formalized as the North American Datum of 1927 (NAD27). A similar reference for vertical measurement was established in 1929 as the National Geodetic Vertical Datum of 1929. The NAD27 was later supplanted by the North American Datum of 1983 (NAD83), which was formally adopted by the United States in 1989 and Canada in 1990; the new system moved the reference point to a point in the Earth's core, and the Meades Ranch marker lost its special significance to the geodetic datum system.
Cold Ashby is a village and civil parish in West Northamptonshire in England. At the time of the 2001 census, the parish's population was 255 people, increasing to 278 at the 2011 census.
Snaefell – is the highest mountain and the only summit above 2,000 feet (610 m) on the Isle of Man, at 2,037 feet (620.9 m) above sea level. The summit is crowned by a railway station, cafe and several communications masts.
The Retriangulation of Great Britain was a triangulation project carried out between 1935 and 1962 that sought to improve the accuracy of maps of Great Britain. Data gathered from the retriangulation replaced data gathered during the Principal Triangulation of Great Britain, which had been performed between 1783 and 1851.
The Principal Triangulation of Britain was the first high-precision triangulation survey of the whole of Great Britain and Ireland, carried out between 1791 and 1853 under the auspices of the Board of Ordnance. The aim of the survey was to establish precise geographical coordinates of almost 300 significant landmarks which could be used as the fixed points of local topographic surveys from which maps could be drawn. In addition there was a purely scientific aim in providing precise data for geodetic calculations such as the determination of the length of meridian arcs and the figure of the Earth. Such a survey had been proposed by William Roy (1726–1790) on his completion of the Anglo-French Survey but it was only after his death that the Board of Ordnance initiated the trigonometric survey, motivated by military considerations in a time of a threatened French invasion. Most of the work was carried out under the direction of Isaac Dalby, William Mudge and Thomas Frederick Colby, but the final synthesis and report (1858) was the work of Alexander Ross Clarke. The survey stood the test of time for a century, until the Retriangulation of Great Britain between 1935 and 1962.
The term benchmark, bench mark, or survey benchmark originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle iron could be placed to form a "bench" for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future. These marks were usually indicated with a chiseled arrow – specifically a broad arrow – below the horizontal line. A benchmark is a type of survey marker.
A geodetic control network is a network, often of triangles, that are measured precisely by techniques of control surveying, such as terrestrial surveying or satellite geodesy. It is also known as a geodetic network, reference network, control point network, or simply control network.
Benchmarking, also known as benchmark hunting, is a hobby activity in which participants find benchmarks. The term "benchmark" is used only to refer to survey markers that designate a certain elevation, but hobbyists often use the term benchmarks to include triangulation stations or other reference marks. Like geocaching, the activity has become popular since 1995, propelled by the availability of online data on the location of survey marks and by the rise of hobbyist-oriented websites.
The North American Datum (NAD) is the horizontal datum now used to define the geodetic network in North America. A datum is a formal description of the shape of the Earth along with an "anchor" point for the coordinate system. In surveying, cartography, and land-use planning, two North American Datums are in use for making lateral or "horizontal" measurements: the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). Both are geodetic reference systems based on slightly different assumptions and measurements.
Survey markers, also called survey marks, survey monuments, or geodetic marks, are objects placed to mark key survey points on the Earth's surface. They are used in geodetic and land surveying. A benchmark is a type of survey marker that indicates elevation. Horizontal position markers used for triangulation are also known as triangulation stations. Benchmarking is the hobby of "hunting" for these marks.
The North American Vertical Datum of 1988 is the vertical datum for orthometric heights established for vertical control surveying in the United States based upon the General Adjustment of the North American Datum of 1988.
The National Spatial Reference System (NSRS), managed by the National Geodetic Survey (NGS), is a coordinate system that includes latitude, longitude, elevation, and other values. The NSRS consists of a National Shoreline, the NOAA CORS Network, a network of permanently marked points, and a set of models that describe dynamic geophysical processes affecting spatial measurements. The system is based on the datums NAD 83 and NAVD 88.
Townsville Astronomical Trigonometrical Station is a heritage-listed triangulation station at Stanton Hill, North Ward, City of Townsville, Queensland, Australia. It was designed by Robert Hoggan and built in 1891. It is also known as Townsville Observatory. It was added to the Queensland Heritage Register on 21 October 1992.
In surveying, triangulation is the process of determining the location of a point by measuring only angles to it from known points at either end of a fixed baseline by using trigonometry, rather than measuring distances to the point directly as in trilateration. The point can then be fixed as the third point of a triangle with one known side and two known angles.
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