page_title: Anglo-French Survey (1784–1790)
section_title: The triangulation, 1787
hierarchical_section_title: Anglo-French Survey (1784–1790) / The triangulation, 1787
caption_attribution_description: English: The triangulation mesh of the Anglo-French survey 1784-1790
context_page_description: The Anglo-French Survey (1784–1790) was the survey to measure the relative situation of Greenwich Observatory and the Paris Observatory. The English operations, executed by William Roy, consisted of the measurements of bases at Hounslow Heath and Romney Marsh, the measurements of the angles of the triangles and finally the calculation of all the triangles. The survey is very significant as the first precise survey within Britain, and the forerunner of the work of the Ordnance Survey which was founded in 1791, one year after Roy's death.
context_section_description: 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 and St Ann's Hill in Chertsey, with an additional  sight to Windsor Castle 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 Greenwich Observatory, Upper Norwood and Hundred Acres (Banstead).  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. 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. 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.
caption: The actual triangulation  mesh.
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