38 



III. 

 GREAT TRIGONOMETRICAL SURVEY OF INDIA. 



For more than a century and a half it has been generally 

 recognised that a trigonometrical survey forms the most correct 

 basis for mapping a country. The selection of sites for the careful 

 measurement of base lines, from which one or more series of triangles 

 are projected over the expanse of country to be mapped, and the 

 closing of the operations on to a second base line, the final measure- 

 ment of which forms a check on the accuracy of the operations — this 

 process spreads in every direction a network of precisely defined points 

 within which it becomes possible for the topographers to insert the 

 details. It was General Roy who commenced the work in England 

 towards the close of last century by the measurement of a base on 

 Hounslow Heath in 1774, and in 1802 Colonel Mudge was engaged 

 in the measurement of his arc of the meridian from Dunnose to 

 Clifton. The same year saw the commencement of the actual work 

 of the Great Trigonometrical Survey of India in the measurement 

 of Major Lambton's base line near Madras. A graphic account of 

 Lambton's work and a history of the subsecpaent triangulation under 

 Everest. Waugh, and Walker are contained in four chapters of the 

 " Memoir on the Indian Surveys." 



In 1876, at the time that our review of these operations begins, 

 the Trigonometrical Branch was under the control of Colonel 

 J. T. Walker. C.B., R.E., F.R.S., Superintendent, Major-General 

 H. L. Thuillier, CSX, F.R.S.. being Surveyor- General of India 

 and Superintendent of the Topographical Survey. 



Extensive chains of triangulatory measurements had by that time 

 been spread over the Indian Peninsula from the Himalayas to 

 Ceylon, and the principal triangulation was now fast approaching 

 completion. 



The first rough topographical survey of all India was also 

 nearly accomplished, and this naturally suggested a reduction of 

 the establishments of the Survey. In September 1S75 the first 

 step was taken by the Government of India in deciding that the 



