ADJUSTMENT OF THE PRINCIPAL GRAVITY OBSERVATIONS IN GREAT BRITAIN

Summary The gravimeter surveys made by the Anglo‐American Oil Company (now the Esso Petroleum Company), the Anglo‐Iranian Oil Company, the Geological Survey, and the Department of Geodesy and Geophysics of Cambridge University depend on about 600 base stations, of which those in any one survey are connected by a strong network of first order measurements of gravity differences. There are sufficient connections between base stations of different surveys for all the surveys to be compared and combined into one uniform system. The largest discrepancies between surveys arise from errors in the calibration factors of the gravmeters and the surveys have therefore been adjusted to six precise pendulum observations at stations between Aberdeen and Southampton. The adjustment has been carried out in three stages, The first is the preliminary adjustment of sets of observations made with the same instrument. The second is the calculation of gravity values at 120 principal stations selected from all 600 base stations, many of the 120 being common to two or more surveys; pendulum observations are included and the gravimeter calibration factors are determined. The gravity values are expressed as differences from the value at the Pendulum House, Cambridge Observatory. In the third stage, which it is hoped to report elsewhere, the remaining base station values are revised to agree with the 120 principal stations and the calculated calibration factors. It is shown that the discrepancies between this procedure and the simultaneous adjustment of all 600 base station values are not serious. The standard deviations of single observations as determined from the internal errors of surveys range from 0.05 mgal to 0.27 mgal. Comparisons between different surveys indicate larger errors and it is possible that stations are not re‐occupied exactly by different observers. Observations are weighted according to the variability between surveys, and the residuals of measured differences of gravity are generally consistent with this. Of the 300 observations between the 120 principal stations, 3 have residuals greater than three times the standard deviation. The values calculated for the 120 stations have standard deviations mostly between 0.1 and 0.2 mgal. Secular changes of heights of stations may in time lead to greater uncertainty. The difference between the value of gravity at the Pendulum House, Cambridge Observatory, and that at the site of the absolute determination at the National Physical Laboratory, Teddington, is 72.17 mgal, s. d. 0 008 mgal, and the value of gravity at Cambridge on the Potsdam system is therefore 981 268‐5 mgal, assuming the value at Teddington to be 981 196.3 mgal (Cook, 1952 a). It is, however, recommended that gravity values within the British Isles should continue to be based on the value of 981 265 mgal at Cambridge (Bullard and Jolly, 1936).