Print This Post Application Schemas Help Build the Geo-Web

Issue Date: February – 2005, Posted On: 2/1/2005


RON LAKE, is president, Galdos Systems Inc.; e-mail:rlakeatgaldosincdotcom  (rlakeatgaldosincdotcom)  

The "Geo-Web" is an open, global or regional community of geographic information services that provides access to diverse data and geoprocessing functions. It's embedded in a broader "semantic web" that will eventually provide a seamless information backdrop for much of our interaction with the world.Geography Markup Language (GML) is emerging as a key information carrier in the Geo-Web through its ability to enable user- and community-defined vocabu laries incorporating the constructs of time, location and spatial extent. Such vocabularies are formally defined as GML Application Schemas, and an understanding of such schemas is essential to understand the Geo-Web.

Defining Objects of Interest

GML provides a set of geographic primitives (e.g., feature, geometry, etc.), a thin semantics-oriented structure layered on Extensible Markup Language (XML) and the concept of a user-defined application schema.Application schemas define objects of interest to a particular community such as forestry, homeland security, oceanography, etc. Because the communities overlap, the object definitions also overlap.GML application schemas reflect this by incorporating definitions from other communities and application schemas. I see a broad network of such schemas emerging, reflecting the relationships and overlap among different information communities.

Application Schemas

An application schema is written in XML Schema, imports one or more GML core schemas and is structured according to GML rules for writing application schemas. Each application schema defines geographic objects such as roads, buoys, mines or railway stations.

Each component or object definition in the application schema consists of an element declaration (e.g., ukusa:Buoy) and a complex type (XML Schema) definition that assigns the properties. The properties have values that can be geometries, topologies, times, etc., or simple types such as integers, floats or strings. Properties also can represent relationships among objects in GML.

GML application schemas describe the terms or objects of interest to a particular community and are meant to be shared. They can be hosted on the Internet as flat files or offered through a Web service such as the OGC Web Feature Service (WFS) or Web Registry Service (WRS). The OGC WFS provides a specific interface (DescribeFeatureType) that enables the client to determine the GML application schema(s) supported by the WFS in question.

A WRS can provide an entire catalog of schemas as well as manage schema components as separate logical objects–rather than as components of schema files–and support their classification in terms of feature taxonomies. Moreover, a WRS can provide facilities to associate the schemas or schema components with particular Web services, datasets and organizations or information communities, which can enhance the semantics of GML application schemas.

Web service descriptions also can build on GML application schemas. Web Services Description Language describes the interfaces to a Web service in terms of the messages exchanged between the service and its clients. Part of this description is in terms of the message elements and their types. Where this involves geographic information, the elements and types can be expressed in a GML application schema.

Contributions and Roles

GML application schemas play an important role in defining an information community's vocabulary and Web services. But how does this contribute to an understanding of the semantics of these services and data definitions? What's the role of the application schemas from an ontological perspective?

GML is, at best, a weak ontology language. It can't describe object behavior or semantic equivalence, and it doesn't have any mechanism to
make assertions about the objects it defines. Nonetheless, it plays an important role in the "semantic" Geo-Web by expressing the types of interest to a particular community–providing a typology for the community rather than a full ontology. This can easily be supplemented (e.g., incorporating OWL markup inside the schema) to add additional semantic content.

Because GML objects defined in application schemas can be classified in taxonomies through a WRS, GML application objects can be connected with topic maps or other classification schemes. In addition, snippets of ontological languages can be inserted into application schema components.

Such information contributes to the understanding of what an object is, without having to transport the information in every instance. Such integration of richer semantics into GML application schemas will happen during the next few years, increasing the utility of GML application schemas.

Read the article on their website: