XXI. India's contribution to geodesy

The science of Geodesy has long been indebted to India for Valuable data contributing to a knowledge of the Figure of the Earth. Early in the present century the Great Trigonometrical Survey was commenced, as a basis for the general topography of the Peninsula, by Major Lambton, who found that in order to obtain exact determina­tions of the latitudes and longitudes of the trigonometrical points from his geodetic calculations, it was necessary to obtain more exact values of the numerical elements of the Earth’s figure. He therefore commenced measuring with great care and exacti­tude a meridional chain of triangles, and determined the latitudes of certain stations in this chain, thereby producing a meridional arc which he carried up from Cape Comorin, in latitude 8°, to the parallel of 18°. After his death this arc was extended 11° further to the north by Colonel Everest, who brought it up to the nearest point to the Himalayan ranges at which it seemed probable that the astronomical latitudes would not be sensibly affected by the attraction of the mountain masses. This work was completed by the year 1842, furnishing a meridional arc 21° in length, which is generally known as the Great Arc of India, and has invariably been employed in all subsequent investigations of the figure of the Earth. From that time onwards chains of principal triangulation have been carried all over India, in gridiron fashion, that is to say, by a system of meridional series at varying distances of 1° to 3° apart, on either side of the primary Great Arc, tied together by longitudinal series on the parallels of Madras, Bombay, and Calcutta, and along the northern frontier. Thus a large number of additional meridional chains of triangles have been furnished, several of which have been converted into meridional arcs by the addition of latitude observations at a large number of the stations. In addition to this, a large number of longitudinal arcs have been acquired by the electro-telegraphic determinations of the astronomical differences of longitude between certain of the trigonometrical stations. This work, so far as available up to the year 1878, was employed by Colonel Clarke, C. B., of the Ordnance Survey of Great Britain and Ireland, in his latest determination of the figure of the Earth. But very much more geodetic material has since become available.