This paper addresses the problem of predicting the region of safe passage for a ship given widely spaced and random depth soundings. The problem is recognized as one of interpolation and surface generation. Shepard's method of inverse distance weighting (IDW) is presented as an appropriate modelling technique. The history and workings of this technique are described. The parameters of IDW are discussed, and their influence on the outcome is explained. This is followed by a description of the computer programs developed to implement this technique and apply it to the data provided. The resultant safe region is presented in graphical form. A possible testing scheme is presented, followed by a discussion of possible predictors of error. An evaluation system designed to compare one interpolation technique to another is also presented. Finally the advantages and disadvantages of IDW are discussed.