The date was 10 April 1802. The sun beat down mercilessly, as it often did during summer in Madras. Even in the shade, the temperature had already climbed to 30°C. But the heat and humidity did not seem to bother the men who were scurrying about the Madras Racecourse. At first glance, their hats and trousers might have suggested they were spectators waiting for a horse race. Yet their serious expressions told a different story. Besides, no race would be held this early in the morning.
Major William Lambton glanced absently at the empty stands behind him before turning his attention back to the setup in front of him. To a casual observer, it would have looked curious. Very curious. A row of tripods stood in a straight line, each bearing on it a long wooden box. A sturdy steel chain extended from one of them. Lambton bent down, examining a silk thread and the plumb bob hanging from it. He watched with satisfaction as it dangled precisely over a hollow bamboo marker embedded in the ground.
The entire apparatus was sheltered under a tent.
He ran through a checklist in his mind. Tripods aligned – check. Plumb line – check. Thermometers in the coffers – check. Tent securely pitched to block the harsh sunlight – check. Rods, telescopes, poles and other instruments set for sighting – check.
Everything was ready. He nodded. It was time.
Lambton had been preparing for this day for weeks. He had scouted the countryside near Madras for many days, deciding on the best spot to begin his “mathematical and geographical survey” as he called it. It wouldn’t be until 1818 – 16 years later – that this project would officially be named the Great Trigonometrical Survey (GTS), the name we know it by today.
Another crucial instrument was on its way from England, but rather than sit around waiting for it, Lambton had decided to start work on the Madras Baseline. He chose the Madras Racecourse to set up the baseline. Why here particularly? Because it was close to St Thomas Mount, which sat on the 13th parallel, the same latitude as Bangalore. Lambton was already familiar with the Bangalore region, which would be useful when he extended his triangulation from coast to coast, going from Madras to Bangalore and onwards to Mangalore along this latitude. Moreover, Bangalore lay almost perfectly on India’s central north-south meridian, the very line he intended to measure.
Even as he planned the Madras Baseline, Lambton was already thinking ahead to the larger triangulation project.
It took Lambton and his team nearly a month and a half to complete the baseline. It began at the Madras Racecourse in Guindy, ran south through Velachery Lake, past 100 Feet Road in Ram Nagar, and stretched all the way to Sholinganallur, just east of a hillock in Perumbakkam. As in his Bangalore trial baseline, the entire 12.2-kilometre length was measured meticulously using a 100-foot-long steel chain.
But there was no time for celebration. The work had only just begun.
With a second 100-foot steel chain now in his possession, Lambton spent several days comparing the two – testing for differences in length and how each expanded with temperature. Next came determining the exact elevation of the baseline above sea level, a task he carried out using basic trigonometry.
Once everything was measured to his satisfaction, Lambton and his team were ready for the next phase. But the new theodolite from England had taken an unexpected detour to Mauritius. It wasn’t until September that it finally arrived, and they could get down to the business of triangulation.
Lambton first climbed St Thomas Mount to observe the angles formed between the hill and the two ends of the baseline. Of course, he wasn’t alone – several other surveyors accompanied him, and twenty-four groaning men hauled up the heavy theodolite, while porters carried tents, equipment and other surveying tools. Next, he ascended Perumbakkam Hill and repeated the process. Now, he had four crucial points:
The north end of the baseline
The south end of the baseline
St Thomas Mount
Perumbakkam Hill
Since this distance would be the basis for further calculations all the way to Bangalore, Lambton wanted to ensure it was measured with utmost precision. Having four triangles to calculate this distance allowed him to cross-check for errors. For instance, when he calculated the distance between the two hills using two different imaginary triangles –
1. St Thomas Mount – North end of baseline – Perumbakkam Hill
Perumbakkam Hill – South end of baseline – St Thomas Mount
– he found that the results differed by just three centimetres. Given that the distance between the two hills was 13 kilometres, this was an exceptionally small error.
The last process in the surveying methodology was to observe stars so as to pinpoint the locations of various places.
Excerpted with permission from India in Triangles: The Incredible Story of How India was Mapped and the Himalayas Measured, Shruthi Rao and Meera Iyer, Puffin India.
