A baseline of 8 miles situated near Madras—that's the point where it began.
On April 10, 1802, the British Army officer William Lambton surveyed a level area lying between St. Thomas Mount and Perumbauk Hill and started taking the measurements with a chain, as mentioned in the history of the Great Trigonometrical Survey. Beginning with that one line, the team went on to extend the chain of measurements right across the length of the Indian subcontinent. The operation took almost forty years, cost the lives of several people, and resulted in a reference system which is still carried in some of the present-day cadastral records in Pakistan, India, Bangladesh, Nepal and Myanmar, even if the individuals reading them are unaware of the fact.
What was the Great Trigonometrical Survey?
The Great Trigonometrical Survey, often referred to as the GTS, was Lambton's initiative, consisting of an effort to measure and triangulate the whole of the subcontinent with scientific accuracy for the East India Company. He called his first chain of triangles the Great Arc of the Meridian, a line extending northward from Cape Comorin, which is at the southern end of India. As stated in a thorough account of the early years of the survey, Lambton had carried out the triangulation over the peninsula to the Malabar Coast by 1806, having used a second baseline that had been measured near Bangalore in 1804.
How could you measure a subcontinent when there were no satellites and no reliable maps of the interior at that time?
How do you carry out triangulation of a subcontinent by hand?
The concept of triangulation is simple: you have to measure a baseline accurately and then use the angles seen from each end of the baseline to determine the position of a third point without actually measuring the distance to it. If you join together enough triangles you can cover huge distances from just one measured baseline.
The operation was by no means simple. As set out in an account of the survey's instruments, Lambton's team employed a theodolite weighing approximately 50 kg for measuring both horizontal and vertical angles, the instrument being fitted with micrometers, microscopes and spirit levels in order to ensure accuracy. They used a zenith sector, which was basically a telescope mounted on a long tube, to determine positions by reference to the stars and to calculate the length of a degree of latitude. The baselines were measured directly using a chain: one source states that the first baseline was 7.5 miles, having been measured over a period of 58 days.
The entire journey was made by hand through the jungle, the flat country and the hilly areas, all of which had never been properly mapped. There were no vehicles appropriate for the terrain, no radios, and the only way to verify a result was to measure it once again.
From Lambton to Everest
Lambton carried out the survey northwards for over twenty years after the first triangulation of the peninsula, and he died while carrying out this work in 1823, being buried at Hinganghat in central India, as his biography states. George Everest, who was his assistant, then became superintendent and extended the Great Arc by roughly 2,400 km from the southern end of India to Nepal; this task lasted from 1806 to 1841, according to Everest's own biography. He was appointed Surveyor General of India in 1830. The broader GTS project, under his successors, went on until the 1840s and 1850s.
The Royal Geographical Society named Peak XV, which had recently been identified as the highest mountain in the world, Mount Everest long after Everest had retired; however, he had never seen the peak himself. The name was given in recognition of the survey system that had been established through his many years of triangulation, not as a result of having made a personal discovery.
The Number That Outlived the Empire
The fact that the arc of the meridian had been measured so accurately served a second purpose besides that of producing maps: it enabled the surveyors to calculate the shape of the Earth itself, at least the portion of the Earth lying directly beneath the Great Arc. This calculation resulted in the Everest ellipsoid of 1830, a mathematical representation of the curvature of the Earth which was tailored specifically to the Indian subcontinent.
This section remains relevant since the Everest 1830 datum was refined into other datums such as Kalianpur 1880 and subsequent updates, and thus became the geodetic basis for maps throughout British India and its successor countries. It is still employed as the reference ellipsoid for older topographic and cadastral maps in Pakistan, India, Bangladesh, Nepal, Myanmar and Sri Lanka. Nowadays, GPS and modern surveying operations use WGS 84, a global ellipsoid that has been fitted to the entire Earth and not merely to South Asia; that is why transformation parameters between Everest 1830 and WGS 84 still exist and are still applied each time an old paper record has to be compared with a modern GNSS survey.
What This Still Means on a Modern Survey Control Network
Ourselves we came across a similar question in connection with the Warsak Wind Farm project, during which our responsibility was to install concrete monuments serving as survey control points and to set up permanent reference benchmarks for the Tractebel–ILF joint venture, under the PEDO project financed by the World Bank. The work was examined and found to be in accordance with the technical specifications, a completion certificate therefore being awarded.
The tools have changed entirely—GNSS receivers having replaced the zenith sector and a total station taking the place of a 50 kg theodolite. Nevertheless, the fundamental question that a control network seeks to answer is the same one that Lambton and Everest were addressing when they used chains and triangles. Each coordinate on a project must be traceable to a defined reference point, and that reference must be both permanent and precise enough so that someone can go back to it years later and obtain the same result. Our topographic survey work still begins with that very same basic principle, no matter what datum the project specifies.
It is in this connection that Everest 1830 turns into a practical issue rather than remaining a historical footnote; a cadastral boundary drawn on an old map using the Everest datum does not coincide with the position indicated by its WGS 84 coordinates, and resolving the discrepancy is standard work carried out today by those providing GIS and spatial data services in the area.
A Chain, Then a Satellite
Now, instead of taking decades to do by hand, satellites are able to achieve the same result in just a few minutes. Our recent article on Landsat 1 tells the other side of that story: how the world has moved from ground-based triangulation to imaging the entire Earth from orbit. Yet in certain areas, the ellipsoid that was produced by a baseline measured by chain near Madras is still used as a reference against which modern surveys have to be checked.
The instruments will continue to change but the question that the control network is meant to answer won't.
