In the paper which he read on 19 April 1967, Captain Dickson described in some detail the investigations which have been carried out to determine the underkeel clearance which should be allowed in any given circumstance. These investigations have concentrated on the vertical movement of a tanker under different conditions, particularly in determining squat and the effect of wave motion. However, another factor in the underkeel clearance equation is the charted depth and this also is affected by ship movement, in this case the movement of the surveying vessel which obtained the soundings. In the following note the probable effect of this movement is discussed and suggestions made by which the effect might be reduced.