Book Extract | India in Triangles: The Incredible Story of How India was Mapped and the Himalayas Measured by Shruthi Rao and Meera Iyer

The boy scratched his head. What were they doing? Were they building something? Were they digging a well? Or was that . . . a weapon of some sort? He decided it was safest to make himself scarce.

The boy had stumbled upon the beginnings of the Great Trigonometrical Survey of India, one of the largest and most impressive scientific feats in history.

The Great Trigonometrical Survey of India (GTS) was an ambitious project to carry out the survey of the Indian subcontinent with scientific precision. It began in 1802 and took nearly 100 years to complete.

WHAT WAS THE POINT OF THIS SURVEY?

By the late 1700s, the East India Company (EIC) had taken control over large parts of the Indian subcontinent. As the East India Company (EIC) took over more land, they realized there weren’t any accurate maps of these places.

So, when William Lambton, an English infantry officer with experience in surveying, proposed to the authorities that an accurate map of India could be made using a precise technique called triangulation, the EIC approved of it promptly.

WHAT DID THE GREAT TRIGONOMETRICAL SURVEY DO?

The Great Trigonometrical Survey (GTS) was the most advanced survey of its kind in the Indian subcontinent at the time—and the largest in the world! It mapped the entire subcontinent and even measured the Himalayan peaks, proving for the first time that Mt Everest is the tallest mountain in the world. Before this, people thought the highest mountain was somewhere in South America!

But that’s not all—the Survey made a huge impact on science and technology. It helped scientists figure out exactly how curved the Earth’s surface is. Sounds unbelievable… . How could a few triangles help map an entire land and even measure the Earth’s shape? …

THE AUDACITY OF THIS ENDEAVOUR

The Great Trigonometrical Survey (GTS) lasted nearly a hundred years and was led by several men, including William Lambton, George Everest and Andrew Waugh. But it wasn’t just them—thousands of people worked on this massive project.

They did it all using just a few simple (but heavy!) instruments, which they had to lug across mountains, forests and rivers. Their work involved thousands of calculations. Imagine doing them all without computers or even calculators!

The surveyors faced all kinds of challenges—malaria, diarrhoea, heavy monsoons, dense jungles, wild animals, injuries and even distrust from locals. Yet, they kept going, mapping the entire country from south to north.

MAPMAKING IN BRITISH INDIA

As the British gradually took control of India’s administration, they became desperate for maps. At first, these were needed for military purposes, but soon, the focus shifted to collecting revenue, especially taxes—or, to put it more bluntly, to loot the land. The first large-scale survey of India was conducted by James Rennell, who is often called the Father of Indian Geography. In 1782, he created the Map of Hindoostan using route surveys, a common method Britain used to map its colonies in the eighteenth century.

A route survey involved travelling along a road, river or track and measuring the distance and direction of the route taken.

Rennell measured distances using the perambulator—a wheel with a counter attached. As it was rolled along the ground, it recorded the number of revolutions, and from that, the distance could be calculated. Neat, huh! He used a compass to determine directions. At intervals, he checked the latitude and longitude of the place through astronomical observations using the positions of the stars, which is explained at the end of this chapter.

Then he put all these observations into a grid of latitudes and longitudes to make a map. Okay, but why into a grid, you ask. Remember the dog, the cat, the squirrel and your friend? Imagine they were standing very far apart and you had to tell your friend how to get to the squirrel. You might say something like, ‘Go close to the tree and then walk a bit.’ Or, ‘Walk up to the telephone pole and then go right ten paces.’

See how you need reference points to help someone find their way? That’s exactly what the grid of latitudes and longitudes does! Instead of using a tree or a telephone pole, you can pinpoint locations using a global grid system.

In fact, this grid system dates back to the third century BCE— that’s over 2,200 years ago! It was developed by Eratosthenes, one of those genius, super-smart old Greek scholars with a long name. He was a whiz at maths, physics, astronomy, poetry, geography and even music!

Anyway, back to Rennell. His route survey was quick and covered large distances efficiently. But there was a problem—it wasn’t very accurate. The instruments he used lacked precision, and his latitude and longitude measurements had errors. This meant his maps provided only a rough idea of places and distances.

A trigonometrical survey using triangulation, on the other hand, was a far more accurate way to create maps. And this precise method became the foundation of the Great Trigonometrical Survey.

TRIGONOMETRICAL SURVEYS

Triangulation is also important in geodesy, the science of measuring the Earth’s shape….

Despite its accuracy, triangulation has its challenges. It is slow and requires careful planning. Bad weather, like rain or haze, can disrupt the process. The calculations involved are complex and cumbersome. It also relies on simple yet extremely delicate instruments and needs a team of highly skilled professionals, making it expensive.

Trigonometrical surveys were being recognised in the late 1700s as the most accurate, modern and scientific method of mapping. Britain had already started on a trigonometrical survey on their own lands in the 1790s. Now India needed one.

And the person who would convince the higher-ups to invest in this immense project was a gentle, unlikely man.

William Lambton was born in . . . well, to tell you the truth, no one really knows. In fact, we know very little about the man who started the humongous country-wide science project that became the GTS. The reclusive Lambton did not like talking about himself. We can guess his birth year thanks only to a king’s whim. At a dinner hosted by the Raja of Coorg (now Kodagu) in 1803, guests were asked to declare when they were born. Everyone did so—except Lambton. Later, he asked a friend, ‘What would you have said if I had claimed to be fifty?’ The friend did some quick calculations: the party was in 1803, which meant Lambton was born in 1753. And that is all we know about his age.

He was born in Durham . . . or was it Yorkshire? But he stayed in Darlington, right? Well, we know he was born and studied somewhere in England. His family was probably not very rich. And we know for sure he loved mathematics.

He also loved astronomy, so much so that it once made him act very foolishly. When he was in his early twenties (or maybe thirties!) and serving in Canada as a soldier in the English army, he looked at a solar eclipse through the telescope of a theodolite. Uh oh! The result? A permanently damaged left eye.

Incidentally, he wasn’t the first scientist to do something foolish during an eclipse. Isaac Newton had once tried to observe an eclipse with a mirror and ended up blind for three days. This wasn’t the only thing he and Newton had in common, as we’ll see shortly.

In the 1790s, Lambton’s regiment fought in the fourth Anglo-Mysore war between the British and Tipu Sultan. In fact, Lambton’s stargazing played a small role in that war. Late one night in April 1799, Lambton and a few other soldiers were led by General David Baird to scout an areca plantation near Srirangapatna where Tipu’s rocket men were believed to have set up a small defensive post. After scouring the area and finding neither rocket men nor any other men, Baird decided it was time to return to their camp. He set off and the other soldiers followed. But Lambton felt something was amiss. He looked up, found the constellation of the Great Bear, found the Pole Star and realised they were marching North, right towards Tipu’s encampment!

When he told General Baird, the senior officer responded that he knew well enough what he was about without having to consult the stars, thank you very much. Hardly had he spoken when they ran into an advance party of Tipu’s army. The General hastily caught a firefly, held it up to his compass and realised Lambton was right: they were indeed heading North. The soldiers did an about turn and marched back to the safety of their own camp. Exactly a month later, General Baird led the successful British assault against Tipu Sultan in which he was defeated and killed.

Lambton’s star turn may have changed the course of history, but he was not one to talk about it. He was happiest when behind a telescope or solving equations or working on his theodolite. And while he was usually awkward at parties, he was friendly, genial and compassionate with his co-workers. Not surprisingly, his colleagues and juniors were all fiercely fond of him.

Lambton had a curiosity about all things scientific. He was especially interested in geodesy, which is a field of science devoted to figuring out the shape of the Earth (who knew there was such a thing!). Lambton had developed this fascination for geodesy while in his previous job surveying in Canada. He followed the latest developments in this field very closely. Of course, he was also interested, very interested, in making a map.

Lambton realised that using triangulation, he could do both of these at once.

With triangulation, his survey wouldn’t just be a map; it would also be a measurement. It wouldn’t just be a feat of geography but also of geodesy.

THE PILOT PROJECT IN BANGALORE

While waiting for his equipment to arrive, Lambton—never one to let the grass grow under his feet—decided to carry out an initial survey near Bangalore. A pilot, if you will.

Bangalore was an ideal starting point. It sat near the centre of the peninsula, almost exactly along the central spine of the country. This was the north-south line—the Great Arc—that Lambton planned to triangulate along, measuring it meticulously to take a crack at determining the exact shape of the Earth. But that was all to come later. First, he needed to set up a trial baseline.

The monsoon rains had drenched the landscape in the first week of October 1800 but the Sun was warm the day Lambton set out to begin work. There were about thirty-five men in the group—Lambton himself, a handful of European surveyors, an interpreter, some guards, and perhaps a carpenter. Most of the party were Indian porters, trudging several kilometres under the weight of all the equipment.

The 100 feet steel chain was the star of the show for setting up a baseline. Besides this all-important item, the porters carried iron stands, wooden boxes, telescopes of various sizes, wooden poles, rods, planks, screws, nails, hammers and an assortment of other tools.

Finally, they arrived at the spot Lambton had carefully chosen. He had already surveyed the area around Bangalore, selecting terrain that was reasonably flat—ideal for establishing a baseline. At the same time, the location needed to offer clear sightlines through the theodolite, whether to hillocks or other easily visible landmarks.

This was also Lambton’s opportunity to fine-tune his methods. Since this baseline would form the foundation for countless triangles and complex calculations, it had to be absolutely straight and correctly measured. After all, he had maps of the entire country in mind—he could hardly go about that properly if his baseline was askew.

The trial for what was to become one of the largest scientific endeavours of the century began on 14 October 1800, near Banaswadi in Bangalore.

The first step in the operations sounded straightforward: draw a straight line. The steel chain would be laid out along this line.

To draw a straight line, surveyors usually use a set of simple tools with a very strange name: boning rods. Here’s how it works. Stick one of these T-shaped tools in the ground with the top bar of the T at a convenient height above the ground. Place another boning rod at the end of the line. Place a third one somewhere in between. Now look along the tops of the boning rods and move them around slightly till they are all in a straight line. This method is so simple and effective, it is still used today.

Lambton was going to measure the distance along this line, so he had to be absolutely certain that it was completely, totally and utterly straight. Naturally, his method was slightly more exacting. He placed a telescope on a tripod at the start point and used that for sighting. He used narrow wooden pieces with long keyhole-like slits in them—these were called vanes—and attached them to the boning rods, then sighted through the telescope so that all the vanes were aligned. There must have been much calling out for small adjustments before everything was lined up to Lambton’s satisfaction: ‘Move the rod . . . just . . . a little to the left. A little lower. Back up and to the right . . . ’

Once he was satisfied that he had a truly straight line, the steel chain could be placed along it. But rather than risk damaging the steel chain by keeping it on the ground, the surveyors kept it inside five specially made wooden boxes, each about as long as a bus, all lined up end to end. Specially made iron stands were driven into the ground along the straight line, and the wooden boxes were placed on them. If the iron stands were in a proper straight line, the boxes would align perfectly, and the chain inside them would be absolutely straight too.

The surveyors were such sticklers for accuracy that once the chain was set up, they also kept a thermometer inside each of the five wooden boxes that the chain nestled in. Can you guess why? You know that most substances, whether solid, liquid or gas, expand when heated and contract when cooled. Solids like the steel chain that the surveyors used expand and contract too, and that meant it could sometimes be slightly more or slightly less than 100 feet long. The whole arrangement of stands, boxes and chain was kept under a tent to keep the Sun out. But even so, there would of course be fluctuations in temperature. By measuring the temperature, the surveyors could correct for any such expansion or contraction so that the distances the chain measured were accurate.

Lambton wanted a baseline that was about 12 kilometres long. But their steel chain was only 100 feet or about 30 metres long. So, when about 30 metres had been measured, the exact spot where the steel chain ended was marked on the ground. Then the chain had to be moved forward by 100 feet. This is just like how when you try to measure the length of a room with a small ruler, you have to keep moving it forward after each measurement. But moving this chain was no easy matter. The chain was treated like a baby—with great care. Porters delicately lifted the chain out of the boxes and gingerly laid it on the ground. Others set up the stands in their new positions, set up the wooden boxes on them and then the chain was lifted and kept inside the boxes.

And so they went, proceeding 100 feet or about 30 metres at a time. Some days, that was all the surveyors could manage. Other days, they blitzed through 90, 120, sometimes even 180 feet. In between, they encountered deep trenches, rocky ground, roads and villages. Lambton dealt with all these obstacles, revising his methods slightly to deal with these hurdles.

The length of the GTS’s first ever baseline was 7.4 miles or 11.9 kilometres. Its northern end was in Banaswadi in Bangalore, and southern end was in a small rocky area near Haralur. You could walk that distance in less than three hours. But it took the surveyors fifty-seven days to measure this baseline! For Lambton, accuracy was far more important than time.

This was the kind of accuracy the surveyors aimed for at every step of the survey, from start to finish, for decades.

A TRIAL OF TRIANGULATION

Setting up the baseline was only the first part of the pilot. The second part was trigonometry.

Like we’ve already seen, a trigonometrical survey is based on imaginary triangles. Lambton had a baseline that was one side of a triangle. He now needed places that he could sight through the theodolite, which would form the third points of imaginary triangles with the baseline. Using triangulation, he could then calculate how far these points were from the baseline. He had a choice of convenient spots to choose from: Bangalore’s many rocky outcrops, hillocks and the tall gopuras or entrance towers of some of its older temples.

He picked a spot that he called Muntapum. This was one of a handful of small watchtowers built in the sixteenth century on rocky outcrops around the city. The ‘Muntapum’ that Lambton chose for his first sighting was a watchtower in the northern part of Bengaluru. It was a little less than eight kilometres from the northern end of the baseline. Sighting from the northern and southern ends of the baseline he had just established to the ‘Muntapam’ tower, and then, as a sort of double-check, also from the tower to the two ends of the baseline, he was able to calculate the distances from the two ends of the baseline to the tower.

To put these places on a map, he also had to determine their latitudes and longitudes. He figured out the latitude and longitude of the tower by using astronomical observations (we’ve seen how you can use stars to figure this out).

Once he knew the distance between the ‘Muntapam’ and the northern and southern ends of the baseline, Lambton and his crew could use these known distances to draw more imaginary triangles by sighting to other places. Then, he could calculate the distance to and exact positions of those places. For example, they calculated that Bettahalasur, Savanadurga and Shivasandra hills were about 20, 30 and 100 kilometres away respectively, from the ‘Muntapam’ tower.

Once the baseline had been carefully laid down, the sighting of other places was quick. The surveyors didn’t have to move 30 metres at a time like they did when measuring the baseline. They could just sight convenient targets through the theodolite (which admittedly was a bear to lug around).

Over the next year and a half, Lambton ranged the land, triangulating from the initial baseline in Bangalore by sighting from hillock to temple to, sometimes, prominent trees. He extended his survey to Srirangapatna, Sira and other places till he had a web of triangles extending more than 200 kilometres around Bangalore. All important towns, hills and temples were plotted on a map.

With this initial survey, Lambton demonstrated how efficiently he could create an accurate map. The trial baseline in Bangalore, it would appear, had definitely been successful.

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Shruthi Rao and Meera Iyer, India in Triangles: The Incredible Story of How India was Mapped and the Himalayas Measured Puffin India/Penguin Random House India, 2025. Pb. Pp.144

The Great Trigonometrical Survey (GTS), an ambitious mission in the nineteenth century to map India using triangulation―a brilliant method that helped them measure every inch with accuracy. Along the way, they calculated the height of the world’s tallest mountain and revealed the true shape of the earth. The survey lasted nearly a hundred years and was led by several men, including William Lambton, George Everest and Andrew Waugh. But it wasn’t just them—thousands of people worked on this massive project.

India in Triangles is a fascinating account of the survey in India. The authors are able to share an important piece of history without dumbing down any information, even though the intended target audience is for young adults. This is the kind of book that will work supremely well in the crossover market for its readability, accessibility to information, listing and acquisition of the heavy equipment used in the mapping and of course, the stories involving the key people. It is a slim volume that is easily read like a thrilling adventure story. The fascination with which these pioneers chose to map the subcontinent does not dim with time.

Shruthi Rao has a master's degree in energy engineering, and worked in the IT industry before she started writing. She is the author of multiple books such as 10 Indian Women Who Were the First to Do What They Did ,20 Indians Who Changed the World, Manya Learns to Roar, among others. She lives in the San Francisco Bay Area, and loves books, desserts, trees, benches, science and long walks.

Meera Iyer loves listening to, researching and writing about stories of people and places, buildings and streets. She volunteers with the Indian National Trust for Art and Cultural

Heritage (INTACH) to help protect and celebrate Bengaluru's history and heritage. She loves coffee, dark chocolate and potsherds.