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Triangulation Baseline |
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King George the
Third was interested in many things scientific and practical. As
well as his observatory in Kew he kept Merino sheep in the surrounding
park, ready to improve the stock in the country. Not for nothing was
he called farmer George.
When in 1783, in time of peace, the French Government suggested that the triangulation survey of France should be extended across the Straits of Dover, the King took a close interest in the project. George III agreed to defray the costs of the instruments required to make the measurements. A base line was required to begin the process. Hounslow Heath was selected and the line was first measured in 1784 by Major General William Roy, FRS. |
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North Western end of the Baseline |
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These two pictures show the
ends of the Baseline - the North Western end just by the entrance tunnel of
Heathrow airport. The South eastern end is to be found in a Hampton
housing estate although at the time of the measurement the Hampton end was
next to the town workhouse. Each end point is marked by an old cannon.
There are plaques set up in 1927, in both locations, to commemorate the 200th anniversary of Roy's birth, which record the distance between the two points as 27,404.01 feet; about 5.2 miles. This base line was used, in conjunction with another base which Roy measured in Romney Marsh, to complete the measurements of points between Dover and Calais and as a consequence establish the exact relative positions of the Observatories of Paris and Greenwich. The initial suggestion, for the cross channel triangulation, came from Jean Dominique Cassini who became Director of the Observatory of Paris in 1784. The Cassini family had four generations of service to the French State for mapping the country. This project gave rise, directly, to the UK Ordnance Survey which started in 1791 from a new baseline in Salisbury Plain. |
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South Eastern end of the Baseline. |
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The Hounslow Heath baseline was measured on three
occasions. The first result was 27,404.01 feet and the second
measurement was taken after Roy's death in 1791 as 27,404.24 feet.
Nearly 70 years later, in 1858, the length of the baseline was found to be
27,406.19 feet. When Roy was involved with the triangulation
measurements between France and England, King George III had agreed to
defray the cost of the instruments required for accurate
measurements. A new theodolite was finally delivered on 31st of July
1787. Roy was then measuring the Romney Marsh baseline which came
out at 28,535.7 feet. The preparations were complete.
On the 23rd of September 1787, the French surveyor Jean Dominique Cassini arrived at Dover around mid-day accompanied by M Mechain and M Legendre. The group agreed the processes of measurment and then crossed the Channel to Boulogne on the 25th. Measurements were begun on the 29th. Stations on the French coast were Dunkirk, Calais, Blanc Nez and Mont Lambert near Boulogne. Dover and several other observation points were used on the English coast. The distance between Dover Castle and the spire of the church of Notre Dame in Calais was found to be just over 26 miles. The estimate of error being seven feet. The official date of the constitution of the Ordnance Survey is July 10th 1791. The main work undertaken was a one-inch to the mile map of the United Kingdom. A new baseline was set up in the early 19th century on Salisbury Plain which continued to be the used right up to the late 1900's. This writer can remember the surveyors working in Shropshire in the late 1940's - a bench mark was cut into the side of our house at that time. All the work that was used to measure the exact points on the earth's surface is now taken over by the Global Positioning Systems. The hand held GPS detectors can be purchased for about the same amount as a portable telephone. This is real progress.
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