English-French trigonometric survey

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Planned triangulation

The English-French Trigonometric Survey ( English Anglo-French Survey ) was from 1784 to 1790 carried out basic surveying to determine the exact location of the Royal Observatory in relation to the Paris Observatory .

It was the first triangulation survey in Great Britain , which was also linked to the French survey network. They laid the foundations for the subsequent trigonometric survey of Great Britain and Ireland and the establishment of the Ordnance Survey .

prehistory

King Louis XIV had informed the members of his Académie des Sciences, founded in 1666, that he wanted more precise maps of his empire . Then Jean Picard began with trigonometric measurements, which continued under the head of the Paris observatory Giovanni Domenico Cassini and under Louis XV. were completed by his son Jacques Cassini and grandson César François Cassini de Thury . As a result, the latter was able to publish maps in 1744 and 1747, which represented the whole of France including Dunkirk in the part of Flanders, which at that time belonged to France, in the correct position and formed the basis for the publication of the Carte de Cassini , which then began . During these measurements, the Paris meridian was also determined, the prime meridian still widespread at the time .

Cassini de Thury had the French in the years 1761 and 1762 net of Strasbourg by triangulation with the Vienna meridian arc connected.

In Great Britain there was the first government plan in 1763 to measure the island by triangulation and to integrate the Map of Scotland created by William Roy between 1747 and 1755 into the triangular grid, but the project was not carried out.

Cassini de Thury's memorandum

In 1783, César François Cassini de Thury, who was also a member of the London Royal Society , wrote a memorandum in which he questioned the correctness of the coordinates of the Royal Greenwich Observatory and suggested that a triangulation be carried out from the French network across the English Channel to London to determine the exact location of the Royal Greenwich Observatory beyond any doubt.

The memorandum was given to King George III. who forwarded it to Joseph Banks , President of the Royal Society, in October 1783 . He asked Charles Blagden , who had met Cassini de Thury and his son on his trip to Paris a few months earlier, for an opinion. Blagden was rather reserved; his opinion of the son was far more positive than that of the father. His suggestion would certainly have to be carried out, but by his own people who would be just as capable and had just as good instruments. Banks made a similar statement to the king, who then commissioned the Royal Society with the project and granted 2000 pounds, especially for a new theodolite . This ensured that the surveyor General William Roy was commissioned to carry out the survey, who had already spoken out in favor of a geodetic survey of Great Britain several times in the past, but had never received the financial means for it. In May 1784, Banks wrote to Cassini de Thury that the king had ordered the project; a baseline would be measured near London, but the triangulation could only begin later, as new instruments would be needed for it. William Roy was entrusted with the execution of the project. Cassini de Thury replied in June that people should be sent in the meantime to check the suitability of the survey points on the French coast. Cassini de Thury died in September 1784. Nevil Maskelyne , director of the Royal Greenwich Observatory, only officially learned of the memorandum after his death.

The Measurement

Ramsden's theodolite delivered in 1787

William Roy began the survey in the summer of 1784 by measuring a baseline in Hounslow Heath , at that time an almost deserted plain in west London. The measurement, which took a total of 4½ months, was started with the pine laths customary at the time, but repeated after some time with glass tubes, which had a lower coefficient of expansion than metal and with which therefore much more accurate results could be achieved. They were placed exactly horizontally behind one another on adjustable racks and their temperature was continuously measured. For this baseline, Roy was awarded the Copley Medal by the Royal Society the following year .

For the next three years they waited for Jesse Ramsden and his 40-man workshop to deliver the theodolite ordered . It was supposed to be the best instrument of its time, but Ramsden must have misjudged the difficulties involved in making it. At that time the glassworks in Cork , Ireland and Birmingham had a monopoly on the flint glass they needed .

Roy used the time to find suitable survey points and to critically understand the surveying of France by Cassini de Thury. In the report to the Royal Society he also dealt with the earth figure , the great scientific question of his time, and with various meridian arcs .

Meanwhile, the Comte de Cassini had taken over from his late father.

In the summer of 1787, Roy had only a few triangulations from his baseline. In September he met with Cassini, Pierre Méchain and Adrien-Marie Legendre in Dover to agree on the details, especially the times of the nocturnal flares, without which observations over long distances would not have been possible. Because of the advanced season, both sides now concentrated on surveying the coast and across the English Channel. A second baseline was measured in the Romney Marsh between October and December . A few triangles could still be measured by the end of the year, then the weather became permanently too bad. In 1788 the missing triangulations were carried out and a connection to St Paul's Cathedral was measured.

Final report

In his final report from 1790, Roy explained the coordinates of the two observatories and the data of the individual triangulations. He also noted the coordinates of numerous locations in the hope that they would be useful for a future land survey.

On the advice of his doctors, he spent the winter before the report was completed in Portugal. After his return he was no longer able to get the report ready for printing. He died in London on July 1, 1790. His report was then published by Isaac Dalby, who had already organized the trigonometric calculations.

Trigonometric surveying of Great Britain and Ireland began the following year.

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Triangulations carried out in 1787 and 1788

literature

Web links

Commons : English-French Trigonometric Surveying  - collection of images, videos and audio files

Individual evidence

  1. ^ A b William Roy: An Account of the Measurement of a Base on Hounslow-Heath, p. 387
  2. ^ Mémoire sur la jonction de Douvres à Londres. In: Concerning the Latitude and Longitude of the Royal Observatory at Greenwich; With Remarks on a Memorial of the Late M. Cassini de Thury. By the Rev. Nevil Maskelyne, DDFRS and Astronomer Royal. In: Philosophical Transactions of the Royal Society of London (1776-1886).
  3. ↑ Danielle ME Fauque: An Englishman abroad: Charles Blagden's visit to Paris in 1783. In: Notes and Records of the Royal Society. Volume 62, 2008, pp. 373-390, doi: 10.1098 / rsnr.2008.0041
  4. ^ Jean-Pierre Martin, Anita McConnell: Joining the Observatories of Paris and Greenwich.
  5. ↑ Today the runways at Heathrow Airport cross the baseline. It was 8.35 km long and began at the corner of Northern Perimeter Road-Nene Road ( ) and ended in Hampton at Roy Grove ( ). The beginning and end of the baseline are marked by cannon barrels sunk vertically into the earth.World iconWorld icon
  6. ^ Charles Wolf: Histoire de l'Observatoire de Paris, de sa fondation à 1793 . Gauthier-Villars, Paris 1902, p. 298 ( digitized on Gallica ).
  7. ^ William Roy: An Account of the Mode Proposed to be Followed in Determining the Relative Situation of the Royal Observatories of Greenwich and Paris.
  8. ^ William Roy: An Account of the Trigonometrical Operation, Whereby the Distance between the Meridians of the Royal Observatories of Greenwich and Paris Has Been Determined . With notes by Isaac Dalby. In: Philosophical Transactions . tape 80 , 1790, pp. 111–270, 591–614 , doi : 10.1098 / rstl.1790.0015 ( digitized from royalsocietypublishing.org [PDF; 45.8 MB ]).