Rule-based isobath generalisation using the Triangle Region Graph: uniting soundings, isobaths and constraints through a navigational surface

Abstract

Navigational charts play a vital role in a ship's safety while navigating the seas, rivers or lakes. With most of the features and obstructions being out of sight -below sea-level - these charts are more critical than e.g. topographic maps. For routing but also positioning, depth information is a key aspect on these charts. This depth information is available in either depth contours, coloured depth areas or individual spot soundings. However with the data originating from accurate but usually erratic survey data, a visualisation of raw data is not sufficient for use in a navigational chart directly. It would not clearly convey the information to a human operator in one sight, and thus this visualisation is in need of generalisation: a simplified and hopefully more useful representation of the same data. In navigational isobath generalisation four main generalisation constraints can be distinguished: legibility, morphology, functional and topological. Isobaths should always be topological valid and safe (hard constraints). Generalised isobaths may never indicate an area being deeper than it was actually measured in the first place, and thus can only be moved to their deeper side. The objective of yielding a legible and useful chart concretely means having smooth and distinguishable lines. Irrelevant details (isolated pits) should be omitted and dangerous obstacles (isolated shoals) can be emphasised. Yielding a legible isobath representation is in constant compromise with the constraint of representing morphology as good as possible. Every operation increasing legibility of the chart lead to a reduced representation of the actual morphology (and with it a reduction of navigable space). This ever going compromise is what makes navigational isobath generalisation such a complex task. Choices are different for different chart scales, chart purposes and even different areas within each chart (e.g. fairways, anchorages, nature reserves require different amounts and types of generalisation).