The Principal Triangulation of Britain was a triangulation

Triangulation

In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

 project carried out between 1783 and about 1853 at the instigation of the Director of the Ordnance Survey

Ordnance Survey

Ordnance Survey , an executive agency and non-ministerial government department of the Government of the United Kingdom, is the national mapping agency for Great Britain, producing maps of Great Britain , and one of the world's largest producers of maps.The name reflects its creation together with...

 General William Roy

William Roy

Major-General William Roy FRS 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....

 (1726–1790).

In 1782, General Roy had commissioned the building of the Ramsden theodolite

Ramsden theodolite

The Ramsden theodolite is a large theodolite that was specially constructed for use in the first Ordnance Survey of Southern Britain. It was also known as the Great or 36 inch theodolite....

 from leading instrument maker Jesse Ramsden

Jesse Ramsden

Jesse Ramsden FRSE was an English astronomical and scientific instrument maker.Ramsden was born at Salterhebble, Halifax, West Riding of Yorkshire, England. After serving his apprenticeship with a cloth-worker in Halifax, he went in 1755 to London, where in 1758 he was apprenticed to a...

. The Ramsden theodolite

Theodolite

A theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are mainly used for surveying applications, and have been adapted for specialized purposes in fields like metrology and rocket launch technology...

 for the first time divided angular scales accurately to within a second of arc. General Roy and his team used it to accurately triangulate the distance between the London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 and Paris

Paris

Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

 observatories. The baseline for this triangulation was established in 1784 between Hampton

Hampton, London

Hampton is a suburban area, centred on an old village on the north bank of the River Thames, in the London Borough of Richmond upon Thames in England. Formerly it was in the county of Middlesex, which was formerly also its postal county. The population is about 9,500...

 Poor House and King's Arbour on Hounslow Heath

Hounslow Heath

Hounslow Heath is a public open space and local nature reserve to the west of Hounslow, a London borough. It now covers about , the residue of the historic Hounslow Heath that covered over .-History:...

, a distance of just over 27,400 feet. The triangulation was completed in 1787.

The baseline derived during that work, together with the new theodolite, served as the basis for the planning and execution of the subsequent work on the Principal Triangulation. Around 1791, shortly after his death, Roy's team began the field work, using the specially built Ramsden theodolite. In 1794 a seven mile long baseline of verification was measured on Salisbury Plain

Salisbury Plain

Salisbury Plain is a chalk plateau in central southern England covering . It is part of the Southern England Chalk Formation and largely lies within the county of Wiltshire, with a little in Hampshire. The plain is famous for its rich archaeology, including Stonehenge, one of England's best known...

.

Eventually the triangulation extended to cover the whole of the British Isles, after it was decided in 1824 that a 6-inch-to-the-mile (1:10,560) map

Map

A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....

 of Ireland

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

 was necessary for accurate land taxing.

The Principal Triangulation was subsequently superseded by the Retriangulation of Great Britain

Retriangulation of Great Britain

The retriangulation of Great Britain was a triangulation project which involved erecting concrete pillars on prominent hilltops throughout Great Britain...

 some 150 years later.

Baseline measurement

The original base-line across Hounslow Heath was measured with deal rods and iron bars to be 27,404 ft. When it was re-measured with 1,370 placements of glass tubes as 27,406 ft. This was corrected for temperature and mean sea-level, and the value and achieved accuracy of 1 inch in 27,404 ft is 3 parts per million.

Corrections

During subsequent triangulation errors due to atmospheric diffraction, deflection of plumbobs, temperature, the spherical nature of the earth (meaning there was more than 180 degrees in a triangle) were all allowed for.

External links

• PDF file including history and map of the Irish part and its links to Britain
• Information and Maps on many aspects of Triangulation
  Triangulation
  In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...
  
  (& Levelling) in Great Britain