Research On The Techniques Of Building A Virtual Geographic Environment For Forest Fire Spreading

Forest fire is a kind of tragic nature disaster. It not only can cause tremendous losses of national possessions and people lives, but also can break the balance of ecosystem and reduce the fertilities of soil. Then it can also lead to other nature disaster, such as soil and water losses, debris flow and so on. Therefore based on the techniques of virtual geographic environments, the model of forest-fire spreading and researching on the forest-fire behaviors to forecast the direction and state of fire spreading by multi-dimension visualization techniques, how to find a novel and modern way to improve the efficiency of fire fighting and find a optimization plan to put out the fire is the main aim of this thesis. Our work is as follow: First, we review the development of the model of forest-fire spreading and its visualization. The contents include the way to build and realize the spreading model, the techniques of multi-dimension visualization, the management information system of forest-fire and fire shape simulating. On the base of comparing these methods, discussing the development of forest-fire fighting and combining the theory of virtual geographic environments, we raise the idea to build a virtual geographic environment for forest-fire spreading. After expounding the occur mechanism of the forest-fire and the theory on designing the spreading model and appraising the factors affecting the speed of fire spread, we adopt the Rothermel's surface fire model(1972) and gain the value of all fuel parameters through the local-investigating data, the relative article and forest mapping. Then we simulate the process of forest-fire spreading on 3D terrain, using the Huygens'theory. We deeply discuss the techniques on geometric modeling. Under the condition of simulating virtual forest landscape, the primary landscapes are terrain and the tree. According to terrain rendering and modeling, we discuss the advantage and disadvantage between GRID and TIN, and in our program, we can read the data by geo-tiff format, which consists the elevation information and other geographic information, render the terrain with GRID data using OpenGL graphic library and realize some functions of special analysis on 3D terrain. In the tree geometric modeling, we adopt two methods (Billboard technique and geometric modeling) to build the virtual forest. The result indicates that Billboard is the best way to simulate large-scale scene, so in our prototype system adapt this method. Aim at the question that how to render large-scale scene, we discuss some methods to real-time rendering it under condition of the hardware level at present. These methods involve tile based rendering, LOD, hierarchical bounding volume. According to the feature of forest scene data, based on Octree hierarchical bounding volume, we adopt linear edge function test to improve the efficiency of pre-clipping to improve the rendering frame. Based on research on all kinds of techniques of fire shape modeling and the principle of particles system deep, we use the face particle and Billboard to simulate large-scale forest-fire. Synthesizing all techniques above expounded, after the process of System Analysis, Modules Design and system integration and so on, we complete a prototype system that realize the forest-fire spreading in a virtual forest environment.