32 
The Trigonometrical Survey of India. 
[No. 1 
The Trigonometrical Survey of India, (Communicated hy 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, Yol. NY.) 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 Delira 
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 Are 
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 th^ fneridional, 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. 
