Triangulation station

A trigonometrical station in Hong Kong
The triangulation point on top of Mount Washington in New Hampshire
A geodetic survey marker in Wellington, New Zealand
Class 3 triangulation point in Shiroyama Park in Inagi, Tokyo
Trig beacon on the summit of Lion's Head in Cape Town
A triangulation pillar in Torre del Miguelete. Valencia (Spain)
A trig point near Wootton Wawen, Warwickshire, England
Trig point on Mam Tor, Derbyshire, England
Trigonometrical station, NSW, Australia.

Fixed surveying station, used in geodetic surveying and other surveying projects in its vicinity.

- Triangulation station

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Benchmark (surveying)

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.

An Ordnance Survey cut mark in the UK
Occasionally a non-vertical face, and a slightly different mark, was used
An Ordnance Survey flush bracket
C&GS benchmark disk in the United States
Bench mark at Saint Goussaud, Limousin, France, by Institut Géographique National
City of Toronto benchmark disk in Canada
Benchmark disk in Ottawa, Ontario, Canada
Vial Moll de Bosch i Alsina, near Port de Barcelona building, & pedestrian crossing Barcelona
Benchmark on the plinth of the statue of King Charles I in Trafalgar Square, London: the site of the medieval Charing Cross, and the point from which distances from London are calculated
Benchmark painted on an electricity pylon's foundation
Benchmark of the Prussian Survey Authority on a church in Brandenburg

Triangulation points, also known as trig points, are marks with a precisely established horizontal position.

Ordnance Survey

National mapping agency for Great Britain.

Grid square TF from the Ordnance Survey National Grid, shown at a scale of 1:250,000. The map shows the Wash and the North Sea, as well as places within the counties of Lincolnshire, Cambridgeshire and Norfolk
Part of an Ordnance Survey map, at the scale of one inch to the mile, from a New Popular Edition map published in 1946
Roy Military Survey of Scotland 1747–1755, Mansewood.
The original draftsman's drawings for the area around St Columb Major in Cornwall, made in 1810
Detail from 1901 Ordnance Survey map of the Imperial fortress colony of Bermuda (showing St. George's Town and St. George's Garrison), compiled from surveys carried out between 1897 and 1899 by Lieutenant Arthur Johnson Savage, Royal Engineers.
The former headquarters of the Ordnance Survey in London Road, Southampton (2005)
The cover of the 5th series OS map Chelmsford and Southend sheet 108. Art by Ellis Martin
Front cover of a one-inch to the mile New Popular Edition, from 1945
Detailed scan of a complete 7th series sheet
Former Ordnance Survey headquarters in Maybush, Southampton, used from 1969 until 2011
Headquarters in Adanac Park opened in 2011
Ordnance Survey maps on sale
Illustration of the Ordnance Survey National Grid coordinate system, with Royal Observatory Greenwich as an example
The Ordnance Survey maps of Great Britain use the Ordnance Survey National Grid

The new Director General, Major-General Malcolm MacLeod, started the retriangulation of Great Britain, an immense task involving the erection of concrete triangulation pillars ("trig points") on prominent hilltops as infallible positions for theodolites.

Cold Ashby

Village and civil parish in West Northamptonshire in England.

Eight exclaves of highly anomalous Cowley, all in Hillingdon, then in Middlesex.

The British Ordnance Survey's first trig point, the triangular post used by surveyors, was erected on 18 April 1936 near Cold Ashby.

Martin Hotine

The head of the Trigonometrical and Levelling Division of the Ordnance Survey responsible for the 26-year-long retriangulation of Great Britain (1936–1962) and was the first Director General of the Directorate of Overseas Surveys (1946–1955).

Hotine was responsible for the design of the triangulation pillars constructed during the Geodetic resurvey of Britain.

Slieve Donard

Highest mountain in Northern Ireland and the wider province of Ulster, with a height of 850 m. The highest of the Mourne Mountains, it is near the town of Newcastle on the eastern coast of County Down, overlooking the Irish Sea.

One of the summit cairns in 2009
The Mourne Wall and the stone tower at the summit
Slieve Donard seen from Port William, Scotland, across the Mull of Galloway (about 105 km)

They camped on the mountaintop from late July until late November that year and used the two cairns to make triangulation points, badly damaging the cairns.

County Durham

Ceremonial county in North East England.

Durham palatinate plaque
The historic boundaries of the county shown in John Speed's map of the county in his Theatre of the Empire of Great Britaine, c. undefined 1611. These boundaries remained in use for administrative purposes until the local government reforms starting in the 1960s. A depiction of the city of Durham is inset in the top right.
The entrance to Durham Castle, the bishops' palace until 1832 when it moved to Auckland Castle
High Force waterfall on the River Tees
Banner of Durham County Council since 1974, based on the council's coat of arms. This was used as County Durham's unofficial flag until an official flag was adopted in 2013.
Ceremonial county from 1974–1996
Population over time of the current remit of Durham County Council between 1801 and 2001
Flymos are made in Newton Aycliffe
Queen Elizabeth Sixth Form College, Darlington

The highest point (county top) of historic County Durham is the trig point (not the summit) of Burnhope Seat, height 746 m, between Weardale and Teesdale on the border with historic Cumberland in the far west of the county.

Black Hill (Peak District)

Highest hill in West Yorkshire, England.

Laddow rocks

The triangulation column ("trig point") and highest point on Black Hill (and the highest point in West Yorkshire) is on a small elevated mound, called Soldiers' Lump.

Brendon Hills

The Brendon Hills are a range of hills in west Somerset, England.

A map of the county in 1646, author unknown

The highest point of the range is Lype Hill at 1388 ft above sea level with a secondary summit several kilometres to the southeast at 1350 ft. Both points are marked by Ordnance Survey trig points and are located within enclosed farmland.

Cheriton Hill

Hill overlooking the English Channel near Folkestone in the south-east corner of Kent, England.

The Cheriton Channel Tunnel terminal from the Pilgrims' Way

The highest point is on a covered reservoir next to the trig point; the highest natural point is nearby, probably close to the road to the village of Paddlesworth, near a transmitter mast, but the relatively flat summit gives no real impression of being on top of a hill.

Baseline (surveying)

Line between two points on the earth's surface and the direction and distance between them.

This BLM map depicts the principal meridians and baselines used in the survey of the United States.

In a triangulation network, at least one baseline between two stations needs to be measured to calculate the size of the triangles by trigonometry.