We introduce a new approach for the generation and the generalisation of visually smooth depth-contours for hydrographic charts. Unlike most current approaches, it strictly respects the safety constraint that dictates that the resulting chart may not indicate a depth shallower than originally measured. The main idea is to construct a smooth surface using a Voronoi-based interpolation method. This surface is represented using a triangulation, modified using a series of generalisation operators, and ultimately depth-contours are extracted directly from this surface. We report on experiments made with real-world datasets, and we compare our results with existing approaches