The “marine Gis” – Dynamic Gis in Action
نویسنده
چکیده
The sea moves: the land usually stays still. It is not surprising that the underlying structure of a land-based GIS is rarely appropriate for marine applications. Add the third spatial dimension and it is clear that an attempt to simulate the sea requires a major overhaul of the appropriate algorithms and data structures. It has seemed obvious to us for some time that spatial data structures need to adapt locally to change, and that the Voronoi diagram provides a conceptually simple framework for which dynamic and kinetic algorithms may be developed. The opportunity to work on real marine data for the Hong Kong area provided the incentive to put our ideas into practice. The challenge was to produce a dynamic three dimensional equivalent to the classical “Pilot Book”, which contains the rules for navigation in the proximity of individual harbours. While we have done some work on true dynamic three dimensional data structures, as required for marine profiling, the Pilot Book application could be achieved with a kinetic two dimensional structure, but in several layers. The terrain (above and below the sea surface) was modelled with the dual Delaunay triangulation, and the coastline at any particular tidal time was captured by its intersection with the current local sea level. This, together with the locations of individual ships and other surface features, was used to form a two dimensional dynamic Voronoi diagram at the sea surface for proximity and collision detection. Other layers were used to indicate fairways, marine markers, submarine contours, etc. However, in order to provide a realistic simulation, we needed to take concepts (and models) from 3D games development and provide marine markers such as lighthouses and buoys, and simulate fog and darkness. We also needed to provide a variety of camera views: overhead and on board a selected ship – a deck view, above and behind, below and behind. This required an appropriate scene graph structure to manage the scales, objects, lights and cameras in order to give us the flexibility required for realistic simulation. The result, while still requiring work (especially on ship navigation) may provide a feasible replacement for the Pilot Book, especially for practice simulations.
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