Chuanchu Wang, Liya Ding, Jiankang Wu
Institute of Systems Science
National University of Singapore
Singapore 119597
email:ccwang,liya,jiankang@iss.nus.sg

ABSTRACT

Geodata sharing and interoperability are the main goals of our virtual geospatial information server, VGIS. VGIS is originally designed to provide one step service for public users who want to access geographical data sources provided by some government departments. Through VGIS, the user can access, view, browse, print and order the data they are interested in. VGIS has an open system architecture. It provides access to common gis systems such as Intergraph, Arc/Info. It can be smoothly extended to include other spatial data sources.

VGIS consists of three sub-systems. One is the client. It consists of a graphic user interface (GUI), used by Internet-user to input their query commands, to view and browse geodata. It is implemented using Java. So it is platform independent. User can run it by a Java-enabled Internet browser. VGIS also provide Java applet to display multimedia information such as text, image, audio and video. This is useful in many applications where rich information type is necessary.

The other two sub-systems are server type. The first one is the catalog server which manages metadata about the location, function, identification, catalog, spatial domain, schema and other information of all geospatial data sources which can be accessed through VGIS in the network environment. Data providers who want their data sources to be open to public need to register with the catalog server by providing all necessary information the catalog server requires. All the metadata information is managed by an database. The primary function of the catalog server is to help to locate the address (URL) of the geospatial data sources. It also helps the user to construct SQL query conditions with client GUI. The information in this server can also be requested when data format conversion is needed.

The second server is called Query/Geoprocessing server. It executes query commands and performs certain operations, such as data format conversion, spatial relationship verification. The Interface of this server is an extended SQL, referred to as Spatial SQL (SSQL). SSQL commands will be parsed and decomposed to sub-query SSQL commands according to the metadata information from the catalog server. The sub-query commands will be sent to correspondent database drivers to retrieve geodata. The returned result from these database drivers will be further processed according to SSQL conditions and integrated as the final result to be sent to the corresponding client.

One feature of VGIS is the transparency of the locations of geospatial data to the end users. User can issue a query command without knowing the destination of the data he wants. The query server can locate the Internet address of the data store by the information available from the catalog server, and the client can receive data from the query server. However, the user can also obtain the information from the catalog server directly about all the services available on the Internet, information on a particular data source. If the user is only interested in the geodata stored in one data store, and he also knows the address of the correspondent GIS system (maybe by the information requested from the directory server), he can communicate to the database driver directly, and the result will be sent directly from DB driver to the end-user, just as in the systems such as GRASSLAND.

To provide very friendly interface, we have developed an icon based visual spatial SQL. The visual SSQL consists of spatial object icons and spatial relationship icons. It is self-explainable. User can use spatial object icons and spatial relationship icons to construct query commands. This exempts the user from understanding the complex syntax of SSQL, which may otherwise easily cause syntax errors.