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3D Terrain visualization for Web GIS
2. Terrain Modeling
Terrain models can be classified into two types: Digital Elevation Model (DEM) and Triangulated Irregular Network (TIN) [Abdelguerfi et al. 1998]. VRML has build-in visualization mechanisms for both types: ElevationGrid node for DEM and IndexedFaceSet node for TIN. Generally speaking, a DEM is easier to be constructed than a TIN, but for a multi-resolution model, DEM structure may have the problem of terrain tearing between tiles with different resolutions. Although a TIN works well in the multi-resolution case, but its complex algorithms of the transformation between different resolution models impede its use in real-time applications. A DEM is adopted in our work not only because of its easy construction but also its compatibility with a TIN model. A DEM can be regarded as a very special case in TINs. The contour file we have is an ASCII text file containing more than 4,0000 pairs of longitude/latitude coordinate values with elevation values. Pre-processing work is performed to connect broken contour lines and get rid of iterative points. A small 2D visualization application is implemented to give a clear view of contour layers (Figure 1). The lines with warmer colours, i.e. yellow, red and brown, indicates higher elevation contours.  Figure 1: 2D visualization of the contour file To construct the DEM from this contour file, we categorize the points into five cases according to their positions and treat them diversely. The five cases are mountain slopes, mountain tops, mountain ridges/valleys, sea and on-contour. Linear interpolation and cubic spline interpolation methods are employed to calculate elevation values. Since this only involves the given grid point and the contours in its neighboring area, this location-dependent method is much less time-consuming than the existing 3 one-point-to-all-contours methods. It has a good tradeoff between efficiency and accuracy. With this method, theoretically we can generate terrain models with any level of detail. Figure 2 illustrates the terrain model we constructed for later 3D display on the Web with a resolution of 512x256 grids.  Figure 2: 512x 256 terrains shaded and wireframe 3. Terrain information Visualization on Web VRML gives poor performances on visualization of large-scale areas because that it is an interpreted language, and not a compiled one. Therefore, assistant technology is necessary for better performance. One of the most common methods is to use Level of Detail (LOD) [Coors and Flick 1998, Kofler et al. 1998, Pajarola 1998]. VRML has its embedded LOD node; however, its fast speed of transformation between different levels is achieved at the price of long initializing time. When a VRML file is launched, terrain data of all levels need to be downloaded together, which normally will take longer time than what users can tolerate. The better way would be to change LOD while interaction or navigation. Some work has been carried out on progressive rendering of VRML. The Web3D Consortium [Web 2] set up a specific working group for the streaming VRML issue in 1998. However, due to the limitation of VRML specification itself, implementations in this area, especially its efficiency for large data sets is still under development with no breakthrough. Other researchers [Koh & Chen 1999] proposed an alternative way by implementing a new VRML browser that downloads 3D models in a progressive format, based on the MPEG-4 verification model source code. However, so far no further work for its application has been published. In terms of graphics, the efforts on progressive VRML are exciting. But for a Web GIS, the purpose of visualization is not only a vivid representation of reality but also a proper interpretation of information. The progressive rendering of terrain is accompanied with the problem of the presentation of other terrain-related information, for example, roads. When the surface of terrain is changed to another LOD, roads on it are supposed to follow the change too; otherwise, tearing occurs between the terrain surface and the roads. In one word, unless the progressive rendering of other information can also be achieved, the solo progressive rendering of terrain does not make much sense to a Web GIS. |