| With the development of electronic map technology, map database technology and geographic information technology, the research in the area of GIS technology has become more lucubrated. The expression form of geographic information has been gradually extended to three dimensions. Electronic sand table which is also called 3D geographic information system is a combination of RS, GIS and 3D simulation technology. It makes up for the defects of traditional sand table by using real 3D geographic data. It also achieves dynamic query, real-time flight, and decision analysis and so on. Currently, the application of electronic sand table has been pervasive in every aspect of people's life.This thesis is based on 3D graphics, virtual reality theory, virtual terrain generation technology, terrain simplification technology and texture synthesis technology. The main works are as follows.Firstly, this thesis does research on terrain data models and compares the different models in data sampling method, feature, accuracy, cost and so on. This thesis extracts elevation values on the basis of contour and employs interpolation to generate practical DEM data.Secondly, this thesis studies the OpenGL-based development principles of 3D graphs,3D texture mapping technology and shadow rendering technology. By utilizing texture mapping technology provided by OpenGL, this thesis overlays texture graph on elevation data to generate realistic 3D terrain scenes.Thirdly, this thesis analyzes human-computer interaction in real-time roaming, and uses Double Buffering and Display List technology of OpenGL to solve the problem of real-time terrain display. Also, it accomplishes two forms of roaming. The first one is manual roaming which is controlled by mouse and keyboard. The second one is automatic roaming which is achieved by timer and interpolation. By combining these two forms, this thesis provides a solution to human-computer interaction in roaming.Last but not the least, this thesis employs OpenGL development Kit and Visual C++ to complete 3D sand table system on Windows platform. This system achieves the rendering of terrain, sky and texture in scene and many GIS functions such as zoom, pan, and spatial analysis and so on. Also it provides controllable flight mode and satisfies the requirement for real-time interaction in roaming. |
