Last week I wrote about the great geospatial initiatives being taken globally to help inform, understand and support decisions to combat Covid 19. But the undoubted reliance we place on geospatial knowledge today has its roots in history, in great expeditions pushing science to its limit. And today is the anniversary of the start of one of the greatest – the precise survey of India called the Great Trigonometrical Survey. It started on this day, 10th April, in 1802.
Today we take maps for granted, and techniques that create them from surveyors on the ground through to satellite imagery, crowdsourcing and the emerging use of AI. GNSS including GPS gives us precise position anywhere on the earth. But creating precise position and quality mapping 200 years ago was a very different story, reflected in the history of great organisations such as UK’s Military Survey and the Survey of India.
On 10th April 1802, UK Army Officer Major Lambton commenced a project to precisely survey India, using a technique called triangulation. Little did he anticipate it would last 70 years. For the uninitiated, a triangulation is based upon basic school trigonometry. Know the length of one side of a triangle and two of its angles, and you can calculate the length of all 3 sides. One triangle leads to the next and eventually a robust network of lines of triangles spanned east to west and north to south in India. This gave coherent precise positions across India from which accurate maps could be created.
Unlike school trigonometry, survey triangles are not measured on flat pieces of paper with protractors and grubby blunt pencils, but with theodolites on the curved earth’s surface, in jungles, on mountain tops, in rain and in places where surveyors are unwelcome. Mountain tops are not prevalent in all of India, and great towers had to be built in lieu. All calculations were by hand, and incredibly complex, taking account the many variables of atmospherics as well as the shape of the earth’s surface. And as with so much great science, always with limited funds.
The observatory at Madras was chosen as the ‘known start position’. It was comparatively easy to determine the latitude of the Observatory with considerable accuracy, but it was not nearly so easy to ascertain the true longitude. It was measured, as well as it could be 200 years ago, in part by observations of eclipses of Jupiter’s moons.
10th April is remembered as the start day for the measurement of first length of the side of the first triangle, called the base line and 7.5 miles long near Madras. Measured with steel chains the importance of its accuracy is clear, the smallest of errors in azimuth (angle from north), distance and temperature would be magnified as the calculations for subsequent triangles were undertaken.
Likewise, the accuracy of the theodolites to measure the major angles was paramount – the largest of which weighed around 500 kg. These were the equivalent of GPS technology of today. The surveying, the maths, the logistics and the human endeavour of this great survey started this day 218 years ago. Its impact is still felt today.
Major Lambton died in 1823 at the age of 70, still surveying, and was succeeded by George Everest. George Everest is, of course, the man that Mount Everest is named after by the Royal Geographical Society. But he did not survey it or compute the height. Those credits went in 1856 to the Bengal Engineers Officer and Surveyor General of India, Andrew Waugh, and the Indian computer Radhanath Sikdar.
But George Everest was largely responsible for surveying the most testing arc of the Survey. Called ‘The Great Arc’ it extended from the southernmost point of India north to the Himalayas, about 1,500 miles and taking 35 years to complete. His determination was absolute. To quote Lieutenant General Strahan, Royal Engineers, who lectured on the Survey of India in 1902: “the unhealthiness of the country, which at last brought on (in Everest) a severe attack of jungle fever, causing partial paralysis; still he persevered but he had to be lowered into and hoisted out of his observing seat”.
The whole triangulation of India took 70 years and is known as The Great Trigonometrical Survey. The scale of the survey is truly appreciated on the 1870 Index Map. It’s accuracy is best noted in that at the end of ‘an arc’, comprising a line of adjoining triangles spanning the country, a new baseline is measured and the difference between theory and practice is seen. Then the entire arc was recalculated, by hand, to reduce errors. On one such measurement towards the end of the Great Trigonometrical Survey, after an arc of 480 miles the calculated and actual distances of the ‘finish baseline’ were only half an inch different.
Height is always more difficult to get right, but Sikdar’s calculations 164 years ago placed the height of Everest at 29,002 feet. Today, with far more precise survey and computational techniques, it’s height is calculated to be only 27 feet (9 metres) higher than the 1864 figure. A remarkable achievement. (There is an alleged story here, Waugh and Sikdar heighted Everest at 29,000 feet precisely but Waugh didn’t want it to look like an approximated round number, so added two feet to the summit height).
Serving in Afghanistan some 13 years ago, I learned of a historical local saying: “First come the hunters, then the surveyors, and finally the Army. So kill the surveyors”. There is no doubt that the importance of accurate mapping for military purposes, particularly for the artillery, helped drive the science behind this great survey and explain why survey has so often been led by the military in history. Much more importantly the Great Trigonometrical Survey paved the way for road, canal and railway construction, trade and settlement. It contributed to Anglo-Indian relationships, good and bad, over the two centuries since. But ultimately is was a human feat by Indian and British people, a feat that led to great maps that have opened the doors for decisions that impact the whole of humanity.
The Survey of India, now 253 years old, is older than Britain’s Ordnance Survey. It is still based at Dehradun, at the north end of The Great Arc. It serves the nation with maps, data and knowledge, all of which are helping India manage Covid 19 and its consequences. I am lucky enough to have visited and have seen its excellent museum with some great artefacts from the age of this Great Trigonometrical Survey.
Also Watch: The Great Arc: Dramatic Tale of How India was Mapped and Everest was Named
