The visit to this starting point of
“The Great Trigonometrical Survey of India” at St. Thomas Mount was part of the “Charnockite
Monument and the starting point of the Great Trigonometrical
Survey of India” on 26th
September 2026. I sincerely thank Mrs. Uma Jairam for
taking me to these monuments.
FIRST STATION OF GREAT TRIGONOMETRICAL SURVEY (1802)
On April 10, 1802, the Great Trigonometrical Survey began at St. Thomas Mount
with the measurement of a 7.5-mile baseline. Col. William Lambton laid this
baseline from St. Thomas Mount to Perumbakkam (near Sholinganallur) to
determine the length of a degree of latitude of the Indian subcontinent. This
pioneering effort laid the foundation for mapping India with unmatched
precision. Sir George Everest later extended this work over 50 years, building a
2,400 km chain of triangulations across the country - the famous "Great
Indian Arc of the Meridian". This GT Survey not only provided the basis
for accurate mapping but also contributed to understanding the magnitude and
figure of the sub-continent. St. Thomas Mount remains a historic milestone in
India's scientific and geopolitical history. To mark its significance, the
Department of Posts issued a special stamp series commemorating 200 years of
the Great Arc.
ஏப்ரல் 10, 1802 அன்று,
இந்தியாவின் பெரிய முக்கோணவியல் கணக்கெடுப்பு செயின்ட் தாமஸ்
மலையில் 7.5 மைல் அடித்தளத்தை அளவிடுவதன் மூலம் மெரிடியனின்
இந்தியவளைகோடு'. இந்த கணக்கெடுப்பு துல்லியமான
வரைபடத்திற்கான அடிப்படையை வழங்கியது மட்டுமல்லாமல்,இந்திய
துணைக்கண்டத்தின் அளவு மற்றும் உருவத்தைப் புரிந்து கொள்ளவும் பங்களித்தது.
செயின்ட் தாமஸ் மலை இந்தியாவின் அறிவியல் மற்றும் புவிசார் அரசியல் வரலாற்றில் ஒரு
வரலாற்று மைல்கல்லாக உள்ளது. அதன் முக்கியத்துவத்தைக் குறிக்கும் வகையில், அஞ்சல்துறை கிரேட் ஆர்க்கின் 200 ஆண்டுகளை நினைவு
கூரும் சிறப்பு தபால் தலை வெளியிட்டது. தொடங்கியது. இந்திய துணைக்கண்டத்தின் ஒரு
டிகிரி அட்சரேகையின் நீளத்தை தீர்மானிக்க கர்னல் வில்லியம் லாம்டன் செயின்ட் தாமஸ்
மலையிலிருந்து பெரும்பாக்கம் (சோழிங்கநல்லூர் அருகே) வரை இந்த அடித்தளத்தை
அமைத்தார். இந்த முதற்கட்ட முயற்சி இந்தியாவை ஒப்பிட முடியாத துல்லியத்துடன்
வரைபடமாக்குவதற்கான அடித்தளத்தை அமைத்தது. சர் ஜார்ஜ் எவரெஸ்ட் பின்னர் இந்த பணியை
50 ஆண்டுகளுக்கும் மேலாக நீட்டித்தது,
நாடு முழுவதும் 2,400 கி.மீ. நீளமுள்ள
மாபெரும் முக்கோணத் தொடரை உருவாக்கினார்- அதுவே பிரபலமான மெரிடியனின்
இந்தியவளைகோடு'. இந்த கணக்கெடுப்பு துல்லியமான
வரைபடத்திற்கான அடிப்படையை வழங்கியது மட்டுமல்லாமல், இந்திய
துணைக்கண்டத்தின் அளவு மற்றும் உருவத்தைப் புரிந்து கொள்ளவும் பங்களித்தது.
செயின்ட் தாமஸ் மலை இந்தியாவின் அறிவியல் மற்றும் புவிசார் அரசியல் வரலாற்றில் ஒரு
வரலாற்று மைல்கல்லாக உள்ளது. அதன் முக்கியத்துவத்தைக் குறிக்கும் வகையில், அஞ்சல்துறை கிரேட் ஆர்க்கின் 200 ஆண்டுகளை நினைவு
கூரும் சிறப்பு தபால் தலை வெளியிட்டது.
COL. WILLIAM LAMBTON
Col. William Lambton was born in 1753 in England and died on 30th January 1833, in a camp at Hinganghat in Maharashtra. A Genius who planned the Great Trigonometrical Survey, he launched his fieldwork from this place, St. Thomas Mount, on 10th April 1802. As part of the Bicentennial Celebration of “The Great Arc”. This bronze statue of Col. William Lambton was unveiled by Dr. P. Nag, Surveyor General of India, on 17th September 2003.
An ACCOUNT of the
TRIGONOMETRICAL OPERATIONS in crossing the PENINSULA of INDIA, and connecting
FORT ST. GEORGE with MANGALORE.
BY CAPT. WILLIAM LAMBTON.
Communicated by THE
HONORABLE WILLIAM PETRIE, Esq. Governor of FORT ST. GEORGE.
GENERAL ACCOUNT.
In the year 1801 I had the honor of
communicating to the Asiatic Society my intention of extending a geographical
survey across the peninsula of India, with a view to ascertain certain
positions on the Coromandel and Malabar coasts, and to fix the latitudes and
longitudes of all the principal places, in the interior country, within the
extent of the operations for connecting the two seas. My labours commenced in
the Carnatic, in 1803, in measuring a small arc on the meridian and on its
perpendicular, an account of which has been published in the 8th Vol. of the
Asiatic Researches. The triangles, from which those arcs were deduced,
constitute a part of the general survey under my superintendence, now extended
from sea to sea, taking in upwards of two degrees of latitude. A series of
principal triangles has also been carried down in a meridional direction, from
which has been deduced an arc of three degrees and upwards in amplitude, giving
the length of the degree, on the meridian, in lat. 11° 59 55, equal 60494
fathoms, and that from a great number of observations of different fixed stars.
As I expect that the detailed particulars of that are will appear before the
public in another place, it will be sufficient barely to mention it here, as
being the scale from which the latitudes of places are computed.*
A full account of this survey being intended for
a separate publication at some future period, when more materials will be
collected, I have chosen for the subject of the present paper, that part of it
which I think will be the most interesting; viz. the triangular operations in
connecting the two seas, and the method by which the difference of longitude
has been determined in my progress from east to west: and that it may be better
adapted to the general reader, who, perhaps, may have neither time nor inclination
to enter into minute detail, I shall previously state, in a concise form, the
manner in which these extensive operations have been carried over the great
mountains, forming the eastern and western……
The
Great Trigonometrical Survey of India started on 10 April 1802 with the
measurement of a baseline near Madras at St. Thomas Mount. The Great
Trigonometrical Survey was a project which aimed to measure the entire Indian
subcontinent with scientific precision. Col. Lambton selected the flat plains
with St. Thomas Mount at the north end and Perumbauk hill at the southern end.
The baseline was 7.5 miles (12.1 km) long. This 12.1 km-long horizontal distance at about sea level grew into the
shape of the Great Indian Arc of the Meridian, a gigantic geometric web of
‘triangulations’ roughly along the 78-degree longitude across the entire length
of the subcontinent, covering a distance of about 2,400 km from Kanyakumari to
the foothills of Mount Everest.
The
whole project of mapping, started in 1802 and finished in 1871, at the foothills of Himalayas/ Mount Everest, took almost 50 years, with the effort of Col.
William Lambton, George Everest, and Andrew Waugh, and finished at the foothills of the Himalayas. In memory of George Everest, the mountain was named “Everest”.
THE
METHOD OF TRIANGULATION
The
technique of ‘triangulation’ involves identifying three mutually visible
reference points, usually prominent hills or buildings, as the corners of a
triangle. Knowing the exact distance between two of these points, and then
measuring the angles made at each by the respective lines of sight with the
third reference point, the distance and position of the third point can be
deduced by simple trigonometry. One of the newly determined sides of this
triangle would now serve as the baseline for a second triangle that will
include a new reference point whose position would, in turn, be established by
the same procedure. Another triangle is thus completed, which serves as the basis
for yet another new triangle and so on. A chain of triangles thus results.
The
equipment involved was a 36-inch theodolite which weighed over half a tonne, to
be placed at vantage points - from the tops of temple gopurams to 30-metre-high
bamboo structures - from where the crucial measurements of angles were made.
In
the process of measurement, the theodolite had fallen down from a height of 217
feet, the Thanjavur Brihadeeswarar Temple vimanam. He repaired it himself with help from the military workshop
at Tiruchi. Apart from the theodolite damage, he had burnt out the
retina of his left eye while looking at a solar eclipse without necessary
filters.
THE
IDENTIFICATION OF STARTING POINT.
A
conical pyramid on a short structure on the terrace of the St Thomas Mount
Church would perhaps indicate the place from where the first plumb line was
dropped for the Great Trigonometrical Survey.
