Research on Three-dimensional Water Resources Information System Based on ArcGIS4.2 COM in ArcGIS ArcGIS Desktop is made up of ArcObjects components. ArcMap, ArcCatalog, ArcScene and ArcGlobe modules are all built and developed via “high-level language” by calling ArcObjects interfaces to implement the functionality. ArcObjects is a set of platform-independent software components, written in C++, that provide services to support GIS applications on the desktop, in the form of thick and thin clients, and on the server. It is built by using components technology, and provides multiple interfaces based on COM. These interfaces consist of over 1200 objects which can be used to customize, extend and develop GIS applications. They also support map display, management, storage and other operation. The development using ArcObjects is not a single customization with ArcGIS functionality, but within the whole ArcGIS framework. However, it is expensive to develop with ArcObjects components because the desktop software must be installed on the same machine in the meantime. ArcGIS Engine can be viewed as a set of low-cost and lightweight ArcObjects components. It is available for any standard development environment, including .NET, COM, Java, C++ and all the popular operating system, for example, Windows, UNIX and Linux. As for a GIS application developed with ArcGIS Engine, its implementation needs only a RUNTIME, and not need to install ArcGIS Desktop at the same time, thus decrease the cost of system development greatly. What’s more, ArcGIS Engine has more controls than ArcObjects which has just two: MapControl and PageLayoutControl. Fig.2 shows the mechanism and running process of COM objects in ArcGIS.  Fig.2 Mechanism and running process of COM objects in ArcGIS 5 Implementation 5.1 System developing procedure In order to display the landform of Mengwa flood diversion-detention area in Huaihe River Basin and construct a series of flood scenes with functionality of 3D viewing, roaming, fly-through simulations, querying and so on. The working flow diagram shown in Fig.3 illustrates the developing procedure of system.  Fig.3 The flow diagram for procedure of system development. 5.2 Data The system inputs remote sensing images with high degree of resolution as the base map of 3D scene. The reached area is a broad area relative to the height of the terrain, so the apparent of terrain surface is rather flat. In order to enhance the sense of depth in the scene, and to bring out subtle features in the terrain, the height of the terrain is exaggerated by 5 times. In addition to various basic geographic information layers, several flood layers are also be loaded into the 3D scene, so that the changes of flooding area can be reflected in detail. 5.3 Key technology In the process of system implementation, a comprehensive 3D scene is firstly designed and constructed on ArcGlobe platform. Then secondary develop the application system in the object oriented programming language, Visual Basic 6.0, by calling ArcGIS Engine. By taking advantage of COM methodology, the integrating framework is built and the functionality of 3D viewing, flying simulation, querying and output is implemented. 5.4 Functionality A user-friendly GIS interface was developed to allow for 3D display and easy spatial manipulation. The functionality provided by the system is as follows: (1) Three-dimensional display. To render a 3D scene more quickly, the amount of data required for display is reduced: only the portion of the image within the observer’s field-of view is loaded into memory. The ortho-images have been loaded into the scene, and terrain data with various scales in Mengwa flood diversion-detention area in Huaihe River Basin are visualized in three dimensions. Using perspective, objects farther away are rendered at a lower resolution than objects closer to the observer. To increase the 3D effect of the display, high degree of resolution is used for high relief images, while low resolution is used for low relief images. (2) Data operation. The spatial data loaded into the scene are divided into three types: floating layer, draped layer and elevation layer. Each type of layers can represent the spatial position relative to the earth’s surface. They also can be manually changed from one type to another if needed. The terrain data can be unbounded zoomed, roamed, rotated and refreshed, and any layers can be added or deleted. (3) Thematic mapping. Thematic maps can be created according to different themes. The map can convey the spatial distribution of a wide variety of qualitative and quantitative information. Almost any subject that can be expressed as a geographical distribution can be mapped. For example, the analyzed result can be directly shown on E-map, so that users can clearly learn the spatial distribution of flooding. The thematic maps can be stored in client computer as well as printed output. (4) 3D Pan. Take the elevation data as underlying layer for the scene to be roamed. By making use of the direction keys and other functional keys on keyboard, the video or viewpoint can be adjusted forward, backward, upward or downward to observe from different angles. The functionality facilitates real-time flying through and panning around the 3D scene. (5) Information querying. The system integrates quickly spatial indexing technique and efficient graphics arithmetic, which provide powerful tools for spatial query and analysis, so that user can analyze surfaces, query attribute values at a location on a surface, and analyze the visibility of parts of a surface from different locations. 6 Conclusions The 3D water resources information system of Mengwa flood diversion-detention area in Huaihe River Basin has an advantage given by COM methodology. This system development way can be used in other relevant projects by reason of shorter development period, lower cost and no need for additional GIS secondary languages. One of development trend in 3D water resources information system is simulating flood routing in flood diversion-detention area. So it is necessary to do more research on combining special water flow models with visualized development environment to develop 3D flood routing information system, which can perform more perfectly and support government to make decisions better. Acknowledgements The author gratefully acknowledges Remote Sensing Technology Application Center, IWHR for providing fellowship in carrying out this research work. References
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