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An Original Way of Building a TIN with Complex Urban Structures

Geographical Information System (GIS) software often models a Digital Terrain Model (DTM) by using triangulations (TINs). Computer Aided Design (CAD) is then used to model complex city structures and superimpose them on the TINs. Since the terrain surface is not connected to the built-up structures, no further spatial analysis (for example, neighbourhood analysis, flow modelling) is possible. Bridges and tunnels are parts of complex urban structures which may be difficult or impossible to be modeled in those TINs. An original way of building the TIN model may help to solve this problem. The use of the “boundary-representation” structure from CAD systems, the triangulation structure of TINs and the Quad-Edge structure from Computational Geometry are combined to alleviate the problem. The Quad-Edge structure forms the base of the model and CAD-type Euler Operators are the intermediate level operators for building the TIN model. To extend the TIN model with bridges, holes and buildings, additional Euler Operators are used. This original way of building the terrain model not only permits the extrusion of buildings and the formation of bridges and tunnels, but also the development of topological queries and analysis, for example, runoff modelling. It is a simple and reliable method and is illustrated with examples of interactive modelling of part of the Hong Kong city.