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Design and Implementation of a COM-based Electronic Map System
Relation between TeleMapBase and GeometryObject2D As in Figure 2, TMFields interface represents the collection of geometry and attribute of layer feature in TeleMapBase. TMFields is made up of many TMField. Of all, one TMField interface is related with the interface of geometry element in GeometryObject2D, and the interface of geometry element is stored in the attribute value of TMField, which makes two components relate each other tightly. Developing electronic map system Based on TeleMap The components look like a pile of various building blocks, which implement the different functions respectively. According to the different needs, the components are integrated together and communicate with each other by the standard interfaces to form one system. Using the components to construct the system is not limited in developing platform, which can be implemented in visual development environment such as VB?VC?Delphi. Architecture of electronic map system As in Figure 4, an electronic map system is set up on the basis of spatial database and multimedia database. The system provides the functions such as visualization of vector and raster map, information query, spatial analysis, multimedia representation of objects and others. Most functions can be implemented by calling the methods of TeleMap. The other functions, like playing the multimedia information, can be implemented by the controls of visualization platform.  Figure 4: Architecture of electronic map system The authors have integrated the TeleMap control in VB to accomplish an electronic map system. The system has functions such as: map visualization, zooming in, zooming out, panning, information query, bus line query, spatial analysis, transfer of other data formats and so on. Apply TeleMap to Develop Electronic Map System. The TeleMap control is put into ToolBox, then pulled into the form in VB. Thus the methods and attributes in TeleMap can be called to implement the functions above. Visualization and edit of map As we know, a map consists of many layers that overlap together. In TeleMap, the layers are divided into different types such as point layer, curve layer, polygon layer, stop point layer, arc layer, route layer, raster background layer, vector background layer, user-customized layer and so on. By calling the method NewTMGeoLayerLayer() in TeleMap, system can add a new layer. Because of the data structure of workspace-map-layer, the new layer will be added into the current active map in the workspace automatically. After generating the new layer, gain the interface of layer, set its edit attribute, then select the current operations such as: adding point, adding line, adding polygon, editing node, moving object and so on. The users can call AddFeature() to add the object into the layer, or call the method OperateSelectedFeature() to make the editing operation on the selected objects. Information query Information query falls into two ways. One is to query its attribute according to the geometry element; the other is to query its geometry element according to the attribute of feature. The former is composed of some operations: firstly click the geometry object on the screen, then call the method FindFeaturesInGeometry() to get the selected objects, finally visualize the selected objects with highlight and play their multimedia information and attributes. The latter is to get the geometry object by the means of querying its ID in database, then visualize the object with highlight. Bus line query Use the method ImportTransportData() of TeleMap to import bus line data, and select the start and end stop, then call the method QueryRoute() to query the optimal route, which implements the algorithm of minimum transfer times and minimum distance between two stops. 3/3 GUI of Electronic Map System Figure 5 shows the GUI of the electronic map system: take the map of certain region as the background, use the electronic map system based on TeleMap to visualize the map, then call the functions of bus line query to gain the optimal route.  Figure 5: GUI of electronic map system Conclusion COM technology is the experiential sum-up of software development over the long time, which resolves some problems of baffling the software development and improves the software developing efficiency. Developing the electronic map software by using COM technology is helpful to constructing the open software system, which unites other software more extensively and promotes the popularity and development of electronic map software. References
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