Gis Components and Map Visualization Objects*

In component-based WebGIS, map visualization objects are cornerstones in developing applications. From constructing user interfaces, displaying maps and human-computer interactions to displaying query results and making subject maps, almost all developing work is based on map visualization objects. In this paper, a component-based WebGIS system Geo-Union is introduced, which is built by COM/DCOM technology and can run in Internet/WWW environment. After briefly describing the architecture and the functional partition of Geo-Union, the design and implementation of map visualization objects are described and discussed in details. Map visualization objects in Geo-Union include a map visualization control and three programming objects (map object, display layer object and dynamic layer object). Map visualization control is in charge of displaying map, interacting with users and coordinating other components. The three programming objects are used to manage map data and its display properties. At last, a sample in Visual Basic environment is given to illustrate the use of Geo-Union components and its map visualization objects.