One of the main concerns on Web-based geographical information systems (Web GIS) is related to performance issues, as these datasets need to migrate, as fast as possible, from server to client tiers (Peng & Tsou, 2003). This is the problem of generation and transmission of the digital maps, which are suitable for user needs. Another important issue is to enable fast and easy Internet GIS application deployment. In this chapter, we discuss solutions proposed for Web GIS based on the vector format, particularly the iGIS framework (Baptista, Leite Jr., Silva, & Paiva, 2004). The iGIS is a Web GIS that renders maps using multiresolution techniques, and enables user interaction them, using the W3C SVG specification. The chapter also addresses the issues that affect Web GIS performance, presenting an overview of the techniques that increase Web GIS performance and the results obtained by using the iGIS framework.
Web GIS applications evolved from the delivery of static maps conveyed in raster formats (e.g., JPEG, GIF, PNG) to an actual stage in which users may choose the window, the scale, and possibly also the data layers to be displayed, and the map is generated dynamically from a database and transmitted in the vector format. Web mapping deals at least with two basic problems: map generation, and map transmission.
Regarding map generation, there are, basically, two approaches. The first one is based on the use of a database that stores geometry information at the most precise scale, and each visualization at less precise scales are automatically derived from that, mainly through cartographic generalization (Müller, Lagrange, Weibel, & Salgé, 1995; Weibel & Dutton, 1999). This implies the execution of map generalization procedures on the fly (Harrie, Sarjakoski, & Lehto, 2002; Lehto & Kilpelainem, 2001). The second approach utilizes a multiresolution database in which spatial objects may be associated to a variety of geometric representations that are scale dependant (Zhou, Prasher, & Kitsuregawa, 2001). In this case, it is necessary to process an off-line computation of a multiscale database containing several independent levels of detail.
Associated with the map generation problem, there is also the problem of vector map transmission. A possible solution is the use of progressive data transmission that is well known and successfully applied to raster images, in which coarser versions of the data are displayed before the complete image is downloaded.
Key Terms in this Chapter
On-Demand Map Loading: A scheme for map transmission that sends, to the client, just the map portion that is being visualized.
GIS: Geographical information system is a system of hardware and software used for storage, retrieval, mapping, and analysis of geographic data.
Framework: A set of software components that provides a foundation structure for an application. Frameworks maximize developer productivity and produce more reliable code, as there is reuse of already tested code, which enables not writing an application from scratch.
Multiresolution: A strategy to construct different representations of vector maps in different scales based on the construction of the map objects simplification.
Map Simplification: A set of algorithms to find simpler representations for each map object.
SVG: Scalar vector graphics, is an XML specification of the W3C to deal with vectors graphics in the web.
Spatial Operators: Set of SQL operators to deal with the spatial dimension of the data, and making possible the definition of spatial queries.
Progressive Transmission: Map transmission scheme in which the map server divides the map into a low-resolution version and a set of incremental versions that, once incorporated to a certain map version, will generate a more detailed map version.