5. Enabling Geo-spatial interoperability
Within the OpenGIS Consortium, work is underway on a number of specifications that are critical to the future development of distributed spatial systems. These include interfaces for:
- Requesting geo-spatial features.
- Describing map styles.
- Requesting maps and map generation.
- Invoking feature coordinate transformations.
- Definition of and request for coordinate transformations.
- Geo-coding and Gazetteer requests.
- Image and map annotation
Each of these specifications is itself dependent on GML 2.0. GML 2.0 is thus playing a critical role in enabling geo-spatial interoperability.GML 2.0 supports geo-spatial interoperability in a number of ways. The first is that GML provides a common schema framework for the expression of geo-spatial features. While GML builds on XML Schema it provides a more constrained model for expression of a geo-spatial feature type in terms of the properties that characterize that feature type. This means that one can readily compare features by looking at their corresponding feature schemas.GML further supports interoperability by providing a common set of GML geometry types. While two different schema authors might for example model a road in different ways they can share the same mechanisms for geometry description and it is then very likely that one can interpret the correspondence between the two schemas. This is illustrated in Figure 7.
Figure 7.0 Simple Street or Road UML ModelFigure 7.0 shows two classes, one describing a Road and the other describing a Street. The properties of these two schemas are clearly different, although they have a common geometry description, achieved by each author using the common gml:centerLineOf geometry property. GML assures geometry level interoperability.
6. The Future of GML
With GML 2.0, Geography Markup Language has reached a stage of maturity that enables the construction of real spatial datasets, the interchange of spatial information and the construction of distributed spatial relationships. We anticipate that GML 2.0 will have a significant impact on the geo-spatial industry and most importantly in the domain of location-based services.GML 3.0 slated for this fall will offer many enhancements while retaining backwards compatibility with GML 2.0. Some of the features to look for include topology support, new geometry classes, events, histories and feature time stamps, units of measure, metadata, and coverages. GML is moving forward to enable the geospatial web.