AbstractThe account of hydrostatic levelling operations in the Netherlands published by A. Waalewijn [1] in 1965 aroused considerable interest in the Ordnance Survey which at that time was considering the use of hydrostatic levelling in connection with the possible construction of a barrage across the Thames estuary. This project entailed a detailed investigation of tidal movements in the estuary from tide gauges to be installed on offshore forts. The datums of the gauges were to be connected to the geodetic levelling network. The water gaps involved were too great for optical levelling whether by conventional methods or by the trigonometric method of transfer described by J. W. Williams [2], and it appeared that hydrostatic levelling might be the answer to the problem.