CARTOGRAPHIC EXTENSIONS TO OGC WEB MAP SERVICES -
SPECIFICATIONS FOR A MAP AND DIAGRAM SERVICE
I. Iosifescu Enescu, M. Hugentobler, L. Hurni
iosifescu@karto.baug.ethz.ch
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).