3d Gis Spatial Data Model And Visualization Techniques

With the rapid development of technologies such as computer software and hardware, graphics, spatial measurement and spatial data storage, it is inevitable for geographic information system (GIS) turning from 2D to 3D. The study of 3D spatial data model is of great importance in 3D GIS application and is the basis of spatial information visualization. To organize and manage 3D data efficiently and to construct interactive 3D spatial model are the keys to successful application of 3D GIS. Currently, applications of 3D GIS mainly focus onsome special areas, including geology, mine, digital city and so on, with limited function for its undevelopment theory and technology. Especially in the field of geology, there still exist a series of problems unresolved, some of which are constructing 3D surface model of ore body based on drill hole data, representing 3D spatial objects integrally by means of model integration, realizing visualization under 3D environment and real-time walkthrough for large-scale terrain. The settlement of these problems will undoubtedly have a greater impetus to the development of 3D GIS. Therefore, aiming at the above problems, related studies are carried out as follows:(1) In view of characteristics of 3D geological data, a layered representation method for 3D model and its modeling technology based on discrete algorithm are put forward. The modeling process is as follows: firstly, on the basis of original geological drill hole data, the layered data can be obtained by forecasting the attributes value of information unknown areas in geological section planes using self-adaptive neural network; secondly, based on dynamic discrete network model, the topological relations for control points of adjacent layer can be established automatically, and then the surface model of 3D ore body can be obtained; finally, the OpenGL is adopted to render the established 3D surface model, and the model dynamic interactive algorithm is realized as well.(2) On the basis of redefining the geological block model, a hybrid data structure model called BOT model (block octree tetrahedron model) is presented, which combines octree and tetrahedral network (TEN) structures. Using BOT generation algorithm, the block model is redivided with octree as general shape description and TEN as partial precise description, in which different gradations of grey are used to stand for different attributes of unit blocks. To reduce memory space a linear BOT coding technology is proposed as well to implement data compression based on Morton code. (3) In view of characteristics of triangular irregular network (TIN) and constructive solid geometry (CSG), a new integrated model structure is put forward, which combines data structures of constrained TIN and CSG. In this model, TIN is used to represent terrain and CSG to represent buildings. The two simulation models are integrated by extracting ground contour lines of buildings as constrained conditions when carrying out triangulating terrains. And the visualization for all parts of the integrated model can be processed synchronously.(4) During the flyover of large-scale terrain, in order to eliminate the popping effect of switching among levels of detail and to increase the frame rates with high image quality, a new bottom-up modeling strategy is put forward, which constructs simplified terrain triangle mesh globally and updates mesh nodes dynamically. Hybrid culling technique based on blocks and triangle faces and simplified computing method for screen-space errors are employed to select appropriate terrain nodes rapidly. Then the Delaunay terrain mesh is updated by adding nodes, deleting nodes and modifying locally: At the same time self-adaptive control for screen-space error tolerance is achieved during the terrain flyover.To validate the effectiveness of the above work, related prototype systems are developed. Results of simulation experiments demonstrate that: ore body layered representation model reduces the influence of human factor with receivable precision and simplifies modeling process effectively; BOT model holds merits of both octree and TEN and has the ability of precisely representing 3D objects and complicated spatial topological relations; constrained TIN and CSG integrated model can perform 3D visualization for different objects (terrain and buildings) simultaneously, which reduces 3D scene render time effectively; view-dependent real-time rendering algorithm for large-scale terrain eliminates popping effect effectively, and has a higher frame rate compared with conventional algorithms, particularly suitable for close-distance flyover simulation of large-scale terrain.