CARTOGRAPHIC EXTENSIONS TO OGC WEB MAP SERVICES - SPECIFICATIONS FOR A MAP AND DIAGRAM SERVICE
I. Iosifescu Enescu, M. Hugentobler, L. Hurni
This paper makes an analysis of some OGC standards from a cartographic point of view and presents abstract and implementation specifications for a Map and Diagram Service designed to support a wide-range of distributed cartographic applications.
The specifications were developed in the frame of the Integrated Project ORCHESTRA (Open Architecture and Spatial Data Infrastructure for Risk Management) which aims to design an open service-oriented architecture for risk management. As in risk management special attention is paid to an integrated service and data approach, including their spatial, temporal and thematic characteristics, thematic maps are of interest as a decision support tool that visually communicate risk-related geographical concepts like data distributions, variations and aggregations (e.g. distribution of risk levels, densities).
The Map and Diagram Service has its foundations on existing de-facto and de-jure standards like Web Map Service (WMS) and Styled Layer Descriptor (SLD) Specifications. The WMS is an OGC (Open Geospatial Consortium) and ISO (International Standards Organization) standard for web map services which is broadly supported by commercial as well as open source software. Another well-known OGC standard for portrayal of geographic information is the Styled Layer Descriptor (SLD). This is an XML based description language for extending OGC Web Services such as Web Map Services (WMS) and Web Coverage Services (WCS) with a user-defined symbolization. Since these OGC Web Services consist of layers, each layer can be symbolized with user-defined styles (e.g. the layer "hydrology" with a blue colour). But this positive intention has also disadvantages considering that in SLD the styles can mainly be defined for specific layers and so it is difficult to reuse them. Furthermore, it is not possible to visualize multiple data values using diagrams. Only lines, polygons, points, texts and raster images are integrated in the SLD language, which is the cause why WMS instances are often used for topographic map services, but not for thematic map services.
The Map and Diagram Service specifications extend these standards with cartographic features like various diagram types (e.g. pie charts, bar charts), definition of complex point symbols, data distributions, transparency levels for individual layers, advanced texture mapping and definitions of label placement rules. Each cartographic extension will be extensively explained and documented. In addition, the paper will shortly present the Map and Diagram Service implementation specifications which are based on the latest W3C recommendation for web services: WSDL and SOAP (Web Services Description Language/Simple Object Access Protocol).