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Which diameter theodolite was used in Indian triangulation survey ?
91.4 cm
36”
91.4 mm
Both A and B
Both A and B
The Great Trigonometrical Survey of India utilized heavy-duty precision theodolites for primary triangulation. A 36-inch (91.4 cm) diameter theodolite was specifically employed by William Lamb G.C. Everest to ensure extreme angular accuracy over long geodetic distances.
The Great Trigonometrical Survey of India utilized heavy-duty precision theodolites for primary triangulation. A 36-inch (91.4 cm) diameter theodolite was specifically employed by William Lamb G.C. Everest to ensure extreme angular accuracy over long geodetic distances.
1 inch=2.54 cm
36 inches=36×2.54=91.44 cm≈91.4 cm
The large diameter of the theodolite horizontal limb allows for a more precise graduation of the scale, minimizing reading errors. By using a larger radius, the angular subdivision represents a larger physical arc length, which significantly reduces the effect of errors during vernier or micrometer readings.
The 36-inch theodolite is famously associated with the instrument used by Everest.
Such large instruments were essential for minimizing angular observation errors in first-order triangulation.
Due to their massive weight, these instruments required specialized transport and sturdy observation towers.
Higher graduation accuracy due to larger circumference
Reduced angular error for long-range geodetic measurements
Extremely heavy and difficult to transport in rugged terrain
Sensitive to atmospheric refraction and temperature variations
Primary Triangulation of India
Geodetic Surveying
The Great Trigonometrical Survey (GTS) was a monumental project to map the Indian subcontinent.
Option C is incorrect because 91.4 mm is approximately 3.6 inches, which is far too small for the historical primary triangulation instruments.
D is correct — The 36-inch theodolite is equivalent to 91.4 cm, and both represent the specifications of the primary instruments used in the Great Trigonometrical Survey of India.
In surveying exams, always check the units carefully; 36 inches is a standard historical reference for the Everest theodolites in the context of the GTS.