Establishment of precise gravity network of airport stations in India

A precise gravity network of thirty-five stations based on the first order gravity station at Palam airport, New Delhi (979.13433 gals—University of Wisconsin 1969 value) was established during April–June 1971, covering the entire country, in order to use them as reference bases for any future gravity surveys in India with a repeatability of ±0.05 mgal or less. The instrument, a LaCoste-Romberg geodetic gravimeter No. G-84, was transported by air over the network of airport stations embracing Trivandrum in the south, Srinagar in the north, Bombay in the west and Mohanbari in the east. The four airport stations in New Delhi, Calcutta, Madras and Bombay which were more precisely established by a large number of repeat observations were utilised as base stations for facilitating easy occupation of the remaining thirty-one stations within their respective zones. The observations were reduced by procedure which permits automatic removal of instrumental drift from the observed readings. According to the depicted drift curve, the instrumental drift though comparatively small, is found not exactly linear due to the possible tare effect observed at the initial stage and also the resulting creep drift that might have been developed during transportation of the gravimeter by air. The final results along with their probable errors of the order of ±0.01 mgal for base stations and ±0.03 mgal for other stations relative to the adopted value at Palam airport, are given in Table 1. Fourteen of the sites occupied are reoccupations of stations already established by the University of Wisconsin in 1963, and the results of the old and the new measurements as given in Table 2, are in remarkable agreement, which ensures the correctness of the calibration factors of the present instrument relative to that of the Wollard's LaCoste-Romberg gravimeter No. G-1-A actually employed in the 1963 measurements.