Design and Realization of Spatial Metadata System

Geographical metadata technologies are key to the integrated management and application of distributed heterogeneous geo-spatial data in Internet/Intranet. This paper discusses conception, classification, standards and applications of geographical metadata, then puts forward some solutions in developing a geographical metadata technical system(GMTS), including designing of geographical metadata base and realization of tool software. As a case, a GMTS of Jilin province agriculture information network(JPAIN) is introduced. Geographical metadata is descriptive information about geographical referenced data. It usually describes content, quality, conditions and other characteristics of spatial data set. In Internet/Intranet based GIS applications,geographical metadata is not only an approach to describe and index data, but also a methodology to support building the operational model of data flow in network system, including data mining, data transforming, data management and data using. So it is necessary to develop powerful tool software in order to capture, check, store, process and use geographical data more effectively. JPAIN is a spatial Intranet, based on which agriculture data is captured, shared and applied in Jilin, a province in northeast China. As a main component of clearinghouse, GMTS provides a mechanism of uniformed resources locating and searching. It works by a serial of request-response processes between WWW Browser, WWW Server, metadata Server, GIS Server and database management system Server. Here is a classical scenario in which a consumer orders some maps by Internet. Step one,he or she (by WWW Browser)accesses metadata base and sends a request of viewing attributes and delineation of the maps. Step two, metadata Server queries the metadata base and sends the result to the consumer. Step three, metadata Server requests GIS Server to create delineation from spatial database and transfer them to Browser with image formats directly supported by HTTP.Step four, the consumer decides whether to buy these maps according to attribute information or not,then fills order forms and puts forward download request. Step five, GIS Server deals with data as the consumer requests,at the same time metadata Server records this process into log file,and all the results are transferred to the consumer. Last step,after an information service process is finished,metadata version is updated by metadata Server. GMTS consists of such components as input subsystem, checking subsystem, searching subsystem, maintaining subsystem and comprehensive application subsystem. Here we used Microsoft Internet Information Server 2.0 as WWW Server, SQL Server 6.5 as metadata base management system, and Internet database connector(IDC), which is also a product of Microsoft corporation, JavaScript, and transact SQL as program-ming languages and development tools. Geographical metadata can be classified into different levels according to their data contents:feature/record level, attribute data level, data file level, database level or data set level. Accordingly, two management strategises of metadata, centralization strategy and decentralization strategy, and two physical storage models, structured model and non-structured model are put forward and analyzed. At the same time,geographical metadata standards are discussed and selected in JPAIN.In metadata capturing subsystem, four kinds of graphical user interfaces can be chosen in or-der to reduce data input wok:①non-spatial short input type;②non-spatial long input type;③spatial short input type; ④spatial long input type. Checking subsystem includes such modules as data integrality and consistency checking, data security checking, and metadata version control.Data seaxching subsystem provides three querying methods. Firstly, searching metadata in specified area in index map and then getting spatial data set:Secondly, searching metadata by logical query in metadata base; thirdly, whole-length text searching. In sum, geographical metadata technical system provides a comprehensive resolu