32 The Trigonometrical Survey of India. [No. 1 



The Trigonometrical Survey of India, (Communicated by Major 

 J. T. Walker.) 



The following is the first of a Series of papers on matters of general 

 interest connected with the Trigonometrical Survey of India, which 

 it is proposed to extract from the manuscript volumes of the Survey, 

 for publication in the Journal of the Asiatic Society. It is taken 

 from the Introduction to the General Report of the North-East 

 Longitudinal Series of triangles (G. T. Survey, Vol. XV.) drawn up 

 under the Superintendence of Col. Sir Andrew Waugh, when Sur- 

 veyor General of India, by J. B. N. Hennessey, Esq., 1st Assistant 

 G. T. Survey. 



The North-East Longitudinal Series derives its name from the 

 circumstance of its following the course of the corresponding boun- 

 dary of British India. It extends from the valley of the Dehra 

 Dhoon to Purneah, connecting the northern extremities of the 

 Calcutta Meridional Series and the celebrated Great Arc, measured 

 by Cols. Lambton and Everest, on the meridian of Cape Comorin. 

 Its object was to form the most direct connexion practicable between 

 two base lines of verification, one measured in Dehra Dhoon, the 

 other in Purneah. Thus it serves to close and verify the Meridional 

 Series, 10 in number, which lie between the Great Arc and Calcutta 

 Meridional Series and emanate from the longitudinal triangulation, 

 connecting the Calcutta base with the Seronj base on the Great Arc 

 in Central India. 



This is the general system followed in the triangulation of India, 

 which thus resembles in outline the form of a gridiron. At each 

 angle of the gridiron, a base line is measured. The outer series 

 form the frame-work on which the inner ones depend, and are 

 especially valuable for the data they contribute towards the determi- 

 nation of the great problem of geodesy, the accurate measurement 

 of the figure of the earth. By restricting the meridional, or inner 

 series, to distances of 60 to 100 miles apart, all the necessary data 

 for topographical operations are obtained, at a moiety of the cost 

 that would be incurred in throwing a net work of triangles over 

 the whole of India after the manner of European surveys, which 

 require greater detail than is necessary in this country. 



