4. Building Distributed Relationships
The real world around us is one of relationships; buildings front onto streets, streets intersect one another, and animal habitat zones depend on the occurrence of specific plant species. In the past, some GIS systems have provided support for feature relationships but these have been restricted in their expressive capability and they have not been suited to relationships that are distributed over the Internet. Some were restricted to simply topological relationships. GML 2.0 changes all of this.GML 2.0 makes use of the XLink and XPointer Specifications to express relationships between geo-spatial entities. This means that such relationships can be expressed between features in the same database or between features across the Internet. Furthermore, GML 2.0 allows relationships to be constructed between GML feature elements in different databases without requiring any modification of the participating databases. No more than read access is required to establish a relationship.The Internet itself was built on the ability of HTML to express linkages between widely distributed web pages. GML 2.0 takes this simple concept further by providing linkages between widely distributed geo-spatial features.
Figure 6.0 GML 2.0 supports distributed feature relationshipsFigure 6.0 shows three GML data stores. One of these is a database of GML road features, while another is a database of GML bridge features. These two databases are assumed to be developed and maintained by separate organizations and to be physically distinct. The third database, that of bridge crossings, is in effect a database of links defining associations between the bridges and the roads that they carry.Relationships in GML 2.0 can themselves be treated as GML features and hence can have their own properties in addition to expressing the associations between distinct features. This might be the case, for example, for a bus route, a traffic intersection or a highway interchange. While GML 2.0 can readily express simple binary relationships using in-line encodings, it can also express complex relationships involving multiple distributed resources.As HTML was critical to the development of the Internet as a linked collection of web pages, GML 2.0 will enable the development of a geo-spatial “Internet” as a linked collection of geo-spatial features.