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The Anglo-French Survey (1784–1790) was the geodetic survey to measure the relative position of the
Royal Greenwich Observatory The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in G ...
and the
Paris Observatory The Paris Observatory (, ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world. Its historic building is on the Left Ban ...
via
triangulation In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points. Applications In surveying Specifically in surveying, triangulation involves only angle m ...
. The English operations, executed by William Roy, consisted of the measurements of bases at
Hounslow Heath Hounslow Heath is a local nature reserve in the London Borough of Hounslow and at a point borders London Borough of Richmond upon Thames, Richmond upon Thames. The public open space, which covers , is all that remains of the historic Hounslow He ...
(1784) and
Romney Marsh Romney Marsh is a sparsely populated wetland area in the counties of Kent and East Sussex in the south-east of England. It covers about . The Marsh has been in use for centuries, though its inhabitants commonly suffered from malaria until the ...
(1787), the measurements of the angles of the triangles (1787–1788) and finally the calculation of all the triangles (1788–1790). The survey is very significant as the first precise survey within Britain, and the forerunner of the work of the
Ordnance Survey The Ordnance Survey (OS) is the national mapping agency for Great Britain. The agency's name indicates its original military purpose (see Artillery, ordnance and surveying), which was to map Scotland in the wake of the Jacobite rising of ...
which was founded in 1791, one year after Roy's death.


Cassini's memoir

Late in life, when he was 57, Roy was granted the opportunity to establish his lasting reputation in the world of
geodesy Geodesy or geodetics is the science of measuring and representing the Figure of the Earth, geometry, Gravity of Earth, gravity, and Earth's rotation, spatial orientation of the Earth in Relative change, temporally varying Three-dimensional spac ...
. The opening came from a completely unexpected direction. In 1783 the director of the
Paris Observatory The Paris Observatory (, ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world. Its historic building is on the Left Ban ...
, Cassini de Thury, addressed a memoir Concerning the Latitude and Longitude of Greenwich. to the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
in London, in which he expressed grave reservations about the latitude and longitude measurements undertaken at the
Royal Greenwich Observatory The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in G ...
. He suggested that the correct values might be found by combining the Paris Observatory figures with a precise trigonometric survey between the two observatories. (The French surveys had already been carried out in the course of the preparation of the map of France.) This criticism was roundly rejected by
Nevil Maskelyne Nevil Maskelyne (; 6 October 1732 – 9 February 1811) was the fifth British Astronomer Royal. He held the office from 1765 to 1811. He was the first person to scientifically measure the mass of the planet Earth. He created '' The Nautical Al ...
who was convinced of the accuracy of the Greenwich measurements but, at the same time, he realised that Cassini's memoir provided a means of promoting government funding for a survey which would be valuable in its own right. Approval was granted and Sir
Joseph Banks Sir Joseph Banks, 1st Baronet, (19 June 1820) was an English Natural history, naturalist, botanist, and patron of the natural sciences. Banks made his name on the European and American voyages of scientific exploration, 1766 natural-history ...
, president of the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
, proposed that Roy should lead the project. Roy gladly accepted and set matters in motion by submitting to the Crown a grossly underestimated budget for manpower (by far the largest element) and new precision instruments to be constructed by
Jesse Ramsden Jesse Ramsden Fellow of the Royal Society, FRS FRSE (6 October 1735 – 5 November 1800) was a British mathematician, astronomy, astronomical and scientific instrument maker. His reputation was built on the engraving and design of dividing engine ...
. The whole project is described in Roy's three large articles in the ''Philosophical Transactions of the Royal Society of London'' in 1785, 1787 and 1790. There are shorter accounts of the project in the ''History of the Royal Engineers'' by Porter and in every history of the Ordnance Survey, particularly the book by Seymour and also that by Owen and Pilbeam.


The Hounslow Heath base, 1784

The first task of any survey is to establish a baseline and, after a search by Roy and three other members of the Royal Society on 16 April, they fixed upon the heart of what was mainly still
common land Common land is collective land (sometimes only open to those whose nation governs the land) in which all persons have certain common rights, such as to allow their livestock to graze upon it, to collect wood, or to cut turf for fuel. A person ...
, the western swathe of Hounslow Heath. This was a near-uniform descent from AOD between King's Arbour (memorialised by a replica cannon outside of the Northern Perimeter Road of
Heathrow Airport Heathrow Airport , also colloquially known as London Heathrow Airport and named ''London Airport'' until 1966, is the primary and largest international airport serving London, the capital and most populous city of England and the United Kingdo ...
) and a point in the fields of Hampton Poor-house, five miles and one thousand feet to the south-east. The direction was first chosen to suit the lie of the land, but it was seen to almost coincide with the telescopic view toward the steeple of All Saints' Church in
Banstead Banstead is a town in the borough of Reigate and Banstead in Surrey, England. It is south of Sutton, London, Sutton, south-west of Croydon, north of Reigate, south-east of Kingston-upon-Thames, and south of Central London. On the North Dow ...
, about 12 miles away, and it was therefore made to coincide exactly with that direction: it is shown as a blue line on the map above. The ground was cleared of bushes and a preliminary measurement of the line was carried out with a 100 ft. steel chain of 100 links prepared by
Jesse Ramsden Jesse Ramsden Fellow of the Royal Society, FRS FRSE (6 October 1735 – 5 November 1800) was a British mathematician, astronomy, astronomical and scientific instrument maker. His reputation was built on the engraving and design of dividing engine ...
. It was the intention to measure more accurately with a set of three calibrated deal rods manufactured as 20 ft. in length. Three were supported on trestles and the ends aligned to an accuracy of a thousandth part of an inch. The first rod was then carried to the end of the third, an operation to be repeated 1,370 times. Unfortunately the deal rods had to be abandoned because of their susceptibility to lengthen and shorten in wet weather, and they were replaced by one-inch-thick rods made of glass. The rods were not affected by humidity but it was key to correct for thermal expansion. There was also a correction to sea level since the calculation of the survey was referred to that height. The final measurement gives the length of the base as 27404.01 ft. The measurement of the baseline to such a high standard of precision was a remarkable achievement and in recognition the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
awarded Roy the
Copley Medal The Copley Medal is the most prestigious award of the Royal Society of the United Kingdom, conferred "for sustained, outstanding achievements in any field of science". The award alternates between the physical sciences or mathematics and the bio ...
in 1785. In 1784 the ends of the baseline were marked by the central axes of two vertical wooden pipes over which the
theodolite A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and ...
was centred for measurements from the base: the pipes could also support flagstaffs as markers when the base was sighted from other stations (or one end from another). After the resurvey in 1791 the pipes were replaced by cannons which are still in place although it is certain that the cannons have been disturbed and slightly moved over the intervening years. The modern locations of the southeast end points is in Roy Grove, Hampton. The northwest end at
Heathrow Airport Heathrow Airport , also colloquially known as London Heathrow Airport and named ''London Airport'' until 1966, is the primary and largest international airport serving London, the capital and most populous city of England and the United Kingdo ...
is situated on the northern perimeter road. Plaques adjacent to the cannons read as follows:
THIS TABLET WAS AFFIXED IN 1926 TO COMMEMORATE THE 200TH
ANNIVERSARY OF THE BIRTH OF MAJOR GENERAL WILLIAM ROY F.R.S.,
BORN 4TH MAY 1726 – DIED 1ST JULY 1790.
HE CONCEIVED THE IDEA OF CARRYING OUT THE TRIANGULATION OF THIS
COUNTRY AND OF CONSTRUCTING A COMPLETE AND ACCURATE MAP, AND
THEREBY LAID THE FOUNDATION OF THE ORDNANCE SURVEY.
THIS GUN MARKS THE S.E. TERMINAL OF THE BASE WHICH WAS MEASURED IN 1784
UNDER THE SUPERVISION OF GENERAL ROY, AS PART OF THE
OBSERVATIONS FOR DETERMINING THE RELATIVE POSITIONS OF THE
GREENWICH AND PARIS OBSERVATORIES – THIS MEASUREMENT WAS
RENDERED POSSIBLE BY THE MUNIFICENCE OF H.M. KING GEORGE III, WHO
INSPECTED THE WORK ON 21ST AUGUST 1784.
THE BASE WAS MEASURED AGAIN IN 1791 BY CAPTAIN MUDGE, AS THE
COMMENCEMENT OF THE PRINCIPAL TRIANGULATION OF GREAT BRITAIN.
LENGTH OF BASE – REDUCED TO M.S.L.
• AS MEASURED BY ROY – 27404.01 FEET
• AS MEASURED BY MUDGE – 27404.24 FEET
• DETERMINED BY CLARKE IN 1858 IN TERMS OF THE ORDNANCE SURVEY
STANDARD 01 – 27406.19 FEET


Waiting for Ramsden

Once the baseline had been measured Roy was keen to press on with the triangulation as soon as possible but he was thwarted by Ramsden's failure to produce the new theodolite. This led to a certain amount of acrimony and Roy went so far as to accuse Ramsden of being remiss and dilatory – in public and in his next report in the ''Philosophical Transactions''. The 1787 report is witness to how Roy occupied himself whilst waiting for the theodolite. The first section deals with the proposed route of the survey. Greater accuracy is achieved if the triangles are kept regular in shape, and the figure shows that most are close to equilateral, with departures only where the triangles step down in size toward the bases and where they have to stretch over the
English Channel The English Channel, also known as the Channel, is an arm of the Atlantic Ocean that separates Southern England from northern France. It links to the southern part of the North Sea by the Strait of Dover at its northeastern end. It is the busi ...
. Typical sides are about 20 miles but the lines which cross the Channel are up to 45 miles in length. The second part of the report is a thorough examination of the results of Cassini's survey of France between Paris and
Dunkirk Dunkirk ( ; ; ; Picard language, Picard: ''Dunkèke''; ; or ) is a major port city in the Departments of France, department of Nord (French department), Nord in northern France. It lies from the Belgium, Belgian border. It has the third-larg ...
. The third part includes a comparison of seven models for the
Figure of the Earth In geodesy, the figure of the Earth is the size and shape used to model planet Earth. The kind of figure depends on application, including the precision needed for the model. A spherical Earth is a well-known historical approximation that is ...
, deduced from both meridian arcs and pendulum experiments, and Roy goes on to propose further meridian arcs in India and Russia, as well as an arc of longitude at the equator. Finally, he makes a plea for the continuation of the survey to the rest of Great Britain.


Borda's circle

In addition to the construction delay, the Ramsen theodolite was relatively large and clumsy. Once the French team join in earnest, Cesar-François Cassini de Thury brought in
Pierre Méchain Pierre François André Méchain (; 16 August 1744 – 20 September 1804) was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep-sky objects and comets. Life Pierre Méchain was bo ...
and he brought in the repeating circle, an instrument for geodetic surveying, developed from the reflecting circle by Étienne Lenoir in 1784. Invented by
Jean-Charles de Borda Jean-Charles, chevalier de Borda (4 May 1733 – 19 February 1799) was a French mathematician, physicist, and Navy officer. Biography Borda was born in the city of Dax to Jean‐Antoine de Borda and Jeanne‐Marie Thérèse de Lacroix. In 17 ...
it was as capable of accurate measurement as Ramsden theodolite but much smaller. The repeating circle aimed at avoiding observational errors. The principle was to multiply the observations of the same angle, enough to no longer find any significant difference between several consecutive measurements made on different parts of the circle. It would later be used to measure the
meridian arc In geodesy and navigation, a meridian arc is the curve (geometry), curve between two points near the Earth's surface having the same longitude. The term may refer either to a arc (geometry), segment of the meridian (geography), meridian, or to its ...
from
Dunkirk Dunkirk ( ; ; ; Picard language, Picard: ''Dunkèke''; ; or ) is a major port city in the Departments of France, department of Nord (French department), Nord in northern France. It lies from the Belgium, Belgian border. It has the third-larg ...
to
Barcelona Barcelona ( ; ; ) is a city on the northeastern coast of Spain. It is the capital and largest city of the autonomous community of Catalonia, as well as the second-most populous municipality of Spain. With a population of 1.6 million within c ...
by Jean Baptiste Delambre and
Pierre Méchain Pierre François André Méchain (; 16 August 1744 – 20 September 1804) was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep-sky objects and comets. Life Pierre Méchain was bo ...
.


The triangulation, 1787

Ramsden's theodolite was eventually delivered in the summer of 1787, and all haste was made to complete as much as possible before winter. The first triangles were from the ends of the base to Hanger Hill Tower in
Ealing Ealing () is a district in west London (sub-region), west London, England, west of Charing Cross in the London Borough of Ealing. It is the administrative centre of the borough and is identified as a major metropolitan centre in the London Pl ...
and St Ann's Hill in
Chertsey Chertsey is a town in the Borough of Runnymede, Surrey, England, southwest of central London. It grew up around Chertsey Abbey, founded in AD 666 by Earconwald, St Erkenwald, and gained a municipal charter, market charter from Henry I of Engla ...
, with an additional sight to
Windsor Castle Windsor Castle is a List of British royal residences, royal residence at Windsor, Berkshire, Windsor in the English county of Berkshire, about west of central London. It is strongly associated with the Kingdom of England, English and succee ...
from the NW end only. Thus two angles were measured at the SE end of the base and four angles at the NW end. The theodolite was then moved to Hanger Hill where the ends of the base line were observed from the other direction and another three new sight lines established to the
Royal Observatory, Greenwich The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in Gre ...
,
Upper Norwood Upper Norwood is an area of south London, England, within the London Boroughs of London Borough of Bromley, Bromley, London Borough of Croydon, Croydon, London Borough of Lambeth, Lambeth and London Borough of Southwark, Southwark. It is north ...
and Hundred Acres (
Banstead Banstead is a town in the borough of Reigate and Banstead in Surrey, England. It is south of Sutton, London, Sutton, south-west of Croydon, north of Reigate, south-east of Kingston-upon-Thames, and south of Central London. On the North Dow ...
). In this way the mesh of triangles was extended down towards the coast where sights could be made on some of the French stations that had been measured in the course of Cassini's triangulation. The figure below shows the actual triangulation, the relief making the structure more understandable. Note the second base at
Romney Marsh Romney Marsh is a sparsely populated wetland area in the counties of Kent and East Sussex in the south-east of England. It covers about . The Marsh has been in use for centuries, though its inhabitants commonly suffered from malaria until the ...
. By comparing the measured length with the length as calculated through the triangulation mesh it was possible to assess the accuracy of the framework. The process seems deceptively simple but it demanded much in the way of perseverance and organisation. Once the theodolite was on a station, signal men were sent to all the other stations that were to be observed from that point, typically five or six stations but as many as 10 in the case of Fairlight Head in
East Sussex East Sussex is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Kent to the north-east, West Sussex to the west, Surrey to the north-west, and the English Channel to the south. The largest settlement ...
. For short distances a simple flagstaff would suffice but for the longer sights it was necessary to use lights at night. The brightest lights, so called ''white lights,'' were obtained by burning an incendiary mixture that lasted for a short time only. The cross-channel use of these lights entailed careful timing arrangements which could take into account the vagaries of the weather and pocket watches. At many stations, for the theodolite or signals, it was necessary to raise the instrument on a portable tower over 30 ft. high. The tower had two components: an inner frame supported the instrument and the outer supported the observers, thus minimising the disturbance to the instrument.


The final report

The final report of 1790 presents figures for the distance between Paris and Greenwich as well as the precise latitude, longitude and height of the British triangulation stations. Throughout the survey Roy took every opportunity to fix the position of as many secondary landmarks as possible in the hope that they would be used as a basis for future topographic surveys from which new maps could be prepared for the counties of
Middlesex Middlesex (; abbreviation: Middx) is a Historic counties of England, former county in South East England, now mainly within Greater London. Its boundaries largely followed three rivers: the River Thames, Thames in the south, the River Lea, Le ...
,
Surrey Surrey () is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Greater London to the northeast, Kent to the east, East Sussex, East and West Sussex to the south, and Hampshire and Berkshire to the wes ...
,
Kent Kent is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Essex across the Thames Estuary to the north, the Strait of Dover to the south-east, East Sussex to the south-west, Surrey to the west, and Gr ...
and
Sussex Sussex (Help:IPA/English, /ˈsʌsɪks/; from the Old English ''Sūþseaxe''; lit. 'South Saxons'; 'Sussex') is an area within South East England that was historically a kingdom of Sussex, kingdom and, later, a Historic counties of England, ...
. In fact these counties were re-surveyed during the course of the Principal Triangulation of Great Britain which commenced in 1791, one year after he died. At the time of his death he was correcting the final proofs of his report and the work was brought to a conclusion by Isaac Dalby, a senior civilian employee of the Board of Ordnance who had organised the calculations of the triangles.


See also

*
Arc measurement Arc measurement, sometimes called degree measurement (), is the astrogeodetic technique of determining the radius of Earth and, by Circumference#Circle, extension, Earth's circumference, its circumference. More specifically, it seeks to determine ...
* Principal Triangulation of Great Britain *
Ordnance Survey The Ordnance Survey (OS) is the national mapping agency for Great Britain. The agency's name indicates its original military purpose (see Artillery, ordnance and surveying), which was to map Scotland in the wake of the Jacobite rising of ...
*
Surveying Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Plane (mathematics), two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of Point (geom ...
*
Triangulation In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points. Applications In surveying Specifically in surveying, triangulation involves only angle m ...


Notes


Bibliography

* * * * Freely available online at th
Ordnance Survew, Owen and Pilbeam

Volume 1Volume 2
* Freely available online at th

* * Plates follow text * {{DEFAULTSORT:Anglo-French Survey (1784-1790) Ordnance Survey Geodetic surveys