Two hundred and fifty years of Survey of India
The contribution of surveyors from Survey of India is very important and a
major milestone achievement.
Today, any adventure or travel begins with collection of relevant maps. But that was not so 250 years ago. In 1757, after their victory in the Battle of Plassey, the English East India Company (EICO) was first granted a zamindari by Mir Jaffar and by 1765 the nominal Delhi emperor granted them the Dewani of the subah of Bengal.
Events were moving in a manner which ultimately transformed the EICO into a kingdom. And they knew almost nothing about their new acquisition.
This made it necessary for them to draw a map of their new possessions and to achieve that they appointed James Rennell as their first Surveyor General in 1767.
A map is a graphic representation of geographical features of an area of the Earth, drawn to scale and usually on a flat surface like paper or cloth. Globes are maps represented on the surface of a sphere.
A map is drawn on some scale. The scale refers to the distance between two objects, say temples on the map as compared to their position on the ground. It is usually convenient to express the scale by a representative fraction or proportion, as 1/50,000, [one metre to 50 kilometres] or (1cm to 500 metres). In other words, if the distance between two prominent temples or trees is 500 metres on the ground, it is shown as one centimetre on the map.
A good map also shows the position of an object in latitude and longitude. The determination of latitude and longitude was well established by this time. The idea that the Earth is an oblate spheroid (like an orange) was also known.
In other words, the radius along the equator was slightly more than the radius along the poles. But the accurate value of the equatorial radius on of Earth and the question, 'how much more than the polar radius', remained unanswered.
The determination of the distance between two points on Earth separated by one-degree latitude gives the surveyors some idea about the curva-
of the Earth in that region. For a perfect sphere, it should be the same everywhere. But for an oblate spheroid, this would be less near the equatorial latitudes than at the poles.
It is also possible to work out the radius of the Earth from this in both directions from these data.
Some measurements were made by the father and son team of Cassinis in France.
Surprisingly, the result of that experiment showed that the length of a meridian degree north of Paris to be 111,017 metres, or 265 metres shorter than one south of Paris (111,282 metres).
This suggested that the Earth is a prolate spheroid, i.e., one elongated at the poles like an egg, with the equatorial radius shorter than the polar radius. This was completely at odds with Newton's conclusions.
In order to settle the controversy caused by Newton's theoretical derivations and the measurements of Cassinis, the French Academy of Sciences sent two expeditions, one to Peru, South America, near equator in 1735 and another to Lapland in the Arctic, in 1736.
The Peru expedition was led by Bouger and La Condamine and the object was to measure the length of a meridian degree. The Lapland expedition led by Maupertuis was to make similar measurements.
Both parties determined the length of the arcs using the wellknown method of triangulation in surveying. The expedition to Lapland returned in 1737, and Maupertuis reported that the length of one degree of the meridian in Lapland was 111.95 km. The Peru result was 110.61 km. This proved that Newton was right. For Paris, somewhere in the middle latitude, the result was 111.21 km.
One of the most important work done by Col. Lambton followed by George Everest, surveyors from Great Trignometrical Survey (GTS) under Survey of India was the meature surement of an arc of meridian along 780 East longitude from Tirunalvelli in Tamil Nadu to Banog Hill near Mussoorie in Uttrakhand.
At that time, the work was justified as part of an attempt to provide an accurate base for systematic topographic and revenue surveys, but it was also part of an attempt to answer one of the thorniest scientific problems of the day, the determination of equatorial and polar radius of Earth.
The result of Lambton’s and later Everest’s work also confirmed that Newton was right. The radii determined by Everest and his followers were used in determining the shape of the Earth and they are still being used by Survey of India.
Although Global Positioning System (GPS) is much quicker than the methods used then by Survey of India, the mathematics used by GPS would not have been possible without knowing the dimensions of Earth and therefore the contribution of surveyors from Survey of India is very important and should be recognised as a major milestone achievement.
The 1870 Index of the Great Trigonometrical Survey of India.
World political map as a globe.
Political map of modern India.