Retriangulation of Great Britain

The Crow Knoll triangulation station at Crompton Moor in the South Pennines. It is one of the concrete pillars erected by the Ordnance Survey during the retriangulation of Great Britain. It was possible (in clear weather) to see at least two other trig points from any one trig point.

Triangulation project carried out between 1935 and 1962 that sought to improve the accuracy of maps made of Great Britain.

- Retriangulation of Great Britain
The Crow Knoll triangulation station at Crompton Moor in the South Pennines. It is one of the concrete pillars erected by the Ordnance Survey during the retriangulation of Great Britain. It was possible (in clear weather) to see at least two other trig points from any one trig point.

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100km squares

Ordnance Survey National Grid

System of geographic grid references used in Great Britain, distinct from latitude and longitude.

System of geographic grid references used in Great Britain, distinct from latitude and longitude.

100km squares
Grid square TF. The map shows The Wash and the North Sea, as well as places within the counties of Lincolnshire, Cambridgeshire and Norfolk.
Illustration of the Ordnance Survey National Grid coordinate system, with Royal Observatory Greenwich as an example

The grid is based on the OSGB36 datum (Ordnance Survey Great Britain 1936, based on the Airy 1830 ellipsoid), and was introduced after the retriangulation of 1936–1962.

The principal triangulation mesh over Britain.

Principal Triangulation of Great Britain

The first high-precision triangulation survey of the whole of Great Britain , carried out between 1791 and 1853 under the auspices of the Board of Ordnance.

The first high-precision triangulation survey of the whole of Great Britain , carried out between 1791 and 1853 under the auspices of the Board of Ordnance.

The principal triangulation mesh over Britain.
The first Ramsden theodolite as used by Roy. (Destroyed by war damage in 1941.)
The second Ramsden theodolite as purchased by the Board of Ordnance. Now in the Science Museum, London
General Roy's baseline: map as about 1935

The survey stood the test of time for a century, until the Retriangulation of Great Britain between 1935 and 1962.

Ordnance Survey

National mapping agency for Great Britain.

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.

The beach at Dunnose

Dunnose, Isle of Wight

Cape on the Isle of Wight in the English Channel.

Cape on the Isle of Wight in the English Channel.

The beach at Dunnose

The Retriangulation of Great Britain that began in 1935 again took Dunnose as a base point.

Malcolm MacLeod (British Army officer)

Director General of the Ordnance Survey from 1935 to 1943.

Director General of the Ordnance Survey from 1935 to 1943.

In 1935 he started the retriangulation of Great Britain, an immense task which involved erecting concrete triangulation pillars (trig points) on prominent (often inaccessible) hilltops throughout Britain.

Martin Hotine

Brigadier Martin Hotine CMG CBE (17 June 1898 – 12 November 1968) was 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).

Triangulation of Kodiak Island in Alaska in 1929.

Triangulation (surveying)

Process of determining the location of a point by measuring only angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly as in trilateration.

Process of determining the location of a point by measuring only angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly as in trilateration.

Triangulation of Kodiak Island in Alaska in 1929.
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Liu Hui (c. 263), How to measure the height of a sea island. Illustration from an edition of 1726
Gemma Frisius's 1533 proposal to use triangulation for mapmaking
Nineteenth-century triangulation network for the triangulation of Rhineland-Hesse

Today, large-scale triangulation networks for positioning have largely been superseded by the global navigation satellite systems established since the 1980s, but many of the control points for the earlier surveys still survive as valued historical features in the landscape, such as the concrete triangulation pillars set up for retriangulation of Great Britain (1936–1962), or the triangulation points set up for the Struve Geodetic Arc (1816–1855), now scheduled as a UNESCO World Heritage Site.

The School crest

Southend High School for Boys

Selective secondary Grammar school situated along Prittlewell Chase in Prittlewell, in the north-west of Southend-on-Sea, England, south-west of the roundabout of the A127 and A1159.

Selective secondary Grammar school situated along Prittlewell Chase in Prittlewell, in the north-west of Southend-on-Sea, England, south-west of the roundabout of the A127 and A1159.

The School crest

Brigadier Martin Hotine CMG CBE, commanded the Royal Engineers from 1917 to 1918, responsible for the design of the UK's triangulation pillars and for the Retriangulation of Great Britain in the 1930s

The Countie Pallatine of Lancaster Described and Divided into Hundreds, 1610, a map of Lancashire engraved in around 1627 by John Speed. The map features a street plan of the county town, Lancaster, and side panels containing portraits of kings from the House of Lancaster and the House of York.

Weets Hill

Hill in the West Craven area of Pendle, Lancashire, England.

Hill in the West Craven area of Pendle, Lancashire, England.

The Countie Pallatine of Lancaster Described and Divided into Hundreds, 1610, a map of Lancashire engraved in around 1627 by John Speed. The map features a street plan of the county town, Lancaster, and side panels containing portraits of kings from the House of Lancaster and the House of York.

The summit has a Triangulation Pillar and commands views over much of east Lancashire, the north Ribble Valley, north Aire Valley and the Yorkshire Dales.

The wall ditch at Limestone Corner

Limestone Corner

Area of Hadrian's Wall at its most northerly point, in present-day northern England.

Area of Hadrian's Wall at its most northerly point, in present-day northern England.

The wall ditch at Limestone Corner
The layout of structures at Limestone Corner
Excavated stones left in and around the wall ditch
Site of Roman camp
Limestone Bank trig pillar

These pillars filled in gaps between the Primary Stations, erected during the Retriangulation of Great Britain.