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This is a report of the W3C Geospatial Incubator Group (GeoXG) as specified in the Deliverables section of its charter.
In this report we provide an overview and description of geospatial foundation ontologies to represent geospatial concepts and properties for use on the Worldwide Web.
Specifically the report:
The intention is that this report form input for a subsequent W3C geospatial activity to further develop geospatial ontologies and define critical use cases for their application in forming and working with Web resources.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of Final Incubator Group Reports is available. See also the W3C technical reports index at http://www.w3.org/TR/.
This document was developed by the W3C Geospatial Incubator Group. It represents the consensus view of the group, in particular those listed in the acknowledgements, on the use cases, requirements and general approach that should be taken in meeting the identified needs. This is the second report of the Group. The first report presents a basic vocabulary and ontology as an update to the 2003 geo vocabulary. This report presents and overview of proposed foundation geospatial ontologies as extensions and elaborations of the updated geo vocabulary. Earlier informal drafts of this report are archived.
Publication of this document by W3C as part of the W3C Incubator Activity indicates no endorsement of its content by W3C, nor that W3C has, is, or will be allocating any resources to the issues addressed by it. Participation in Incubator Groups and publication of Incubator Group Reports at the W3C site are benefits of W3C Membership.
The geospatial incubator group was chartered to begin addressing issues of location and geographical properties of resources for the Web of today and tomorrow, by taking a concrete step to update the W3C GEO vocabulary, laying the groundwork for a more comprehensive geospatial ontology, and formulating a proposal for a W3C Working Group to develop recommendations to further the Web representation of physical location and geography.
The group's work has been greatly influenced by the work of the Open Geospatial Consortium (OGC), ISO/TC 211, and georss.org. While the rigor of the OGC and ISO/TC 211 General Feature Model is essential for clarity of spatial representations, the breadth and depth of geographic information handling developed by those organizations is considered to be beyond the needs of most Web use cases. The group has followed the lead of GeoRSS in seeking to complement those efforts with a simpler baseline implementation of geospatial resource description for the Web. It has been clear in this work, however, that certain extensions and additions to the baseline are important for fully realizing the use cases defined by the group.
A set of use cases demonstrates the aims in more detail. A set of high level requirements was derived from the use cases that were then formalized for the work presented in the group's reports. A model of foundation ontologies has then been developed that encapsulates particular issues discussed and discovered during the XG's work. Throughout the Incubator Activity, decisions have been taken via consensus during regular telephone conferences, online collaboration, and face to face meetings.
The Incubator Group is now considering the approach of re-forming as a W3C geospatial interest group to further the activities of developing geospatial foundation ontologies and representing a geospatial aspect of other W3C activities.
Based on the use cases and the original high level requirements that were derived from them, a set of more detailed requirements was established. The group decided to adopt the GeoRSS feature model as their baseline representation, following the essential nature of the ISO General Feature Model, which separates the discernment of a feature object such as a city from the particular coordinate geometry property such as a point or a polygon by which it may be represented.
While the group completed work on the geometric model described above, along with its instantiation in RDF and OWL, the group was not able to get as far with models and vocabularies for other more complete and/or advanced geospatial concepts, properties, and relationships. The most common spatial relationships in use, for example, are equals, disjoint, intersects, touches, crosses, within, contains, and overlaps. These can be tremendously useful in many of the group's use cases. For example, in use case 4 the researcher wants to find county-level recycling programs. One would need to describe the fact that a recycling program is within a particular county; or that a semantic model of a facility would want to describe that a printer was within a certain room, and that the room touches hallway A-14. While these core spatial relationships are well established within the geographic sciences, this group was not able to validate them as a necessary and sufficient representation in the Web context, nor develop specific semantic encodings in the initial time frame. The group was able to describe in overview form, however, a set of 7 such geospatial foundation ontologies as a basis for future work.
In summary, the Geo XG has successfully developed a basic geography model that can update the W3C GEO vocabulary, as stated in its charter. The group identified a core set of spatial relationships as well as other significant geospatial ontology components or categories which are described in the present report, but implementation of these in W3C is presented as an item for future work.
The companies and organizations that participated in or supported GEO XG are as follows:
As enabling technologies for the Semantic Web have developed, such as RDF, OWL, SPARQL, and SWRL, much work has been done to build the ontologies required for expressing the common elements of disparate knowledge artifacts. This is true in the realm of geospatial and temporal concepts and relationships, yet the work has not yet reached a level of either consensus or actionability which would allow it to be the basis of knowledge interoperability. In other words, it is not yet ready to support the functionality of a Geospatial Semantic Web, where the geographic properties of knowledge resources can be expressed, discovered, acted upon by machines, and understood by diverse communities.
The group considered a more comprehensive system of spatial / geospatial foundation ontologies and has described in overview form the following critical examples.
This ontology is intended to provide formal representation of ISO and OGC standards for feature discernment (General Feature Model or GFM). The GFM is the general model of which the geo vocabulary has been developed as a very restricted subset. While the latter can adequately represent a large proportion of Web resource, many important applications such as routing, earth observation, and imagery interpretation require a more complete representation. Sub-ontologies of the Feature Ontology included here cover geometries, coverages (primarily rasters and grids), and observations. Several draft translations of relevant ISO and OGC standards into OWL ontologies are found at http://loki.cae.drexel.edu/~wbs/ontology/list.htm
This ontology might be thought of as the "50 types of features that everyone can agree on".
As an example of this type of ontology, the Ordnance Survey has published an ontology of administrative boundaries in the UK, which can be found at http://www.ordnancesurvey.co.uk/ontology along with ontologies for other feature types.
There are at least 8 commonly accepted 2D topological relationships such as RCC8 or Egenhofer relations, but there are probably a number of other "vernacular" relationships which would be useful, such as "next-door-to". One of many examples of spatial relationship ontologies has been published by Ordnance Survey at http://www.ordnancesurvey.co.uk/ontology/SpatialRelations.owl
Place names are an important means of geolocating resources, at least to some approximation. Some work has been done on globally useful placename ontologies and individual taxonomies exist, but have not really been brought together as a common reference. A useful reference is the ontology work at http://www.geonames.org/ontology/
The numerical counterpart of a place name scheme, in a similar sense that TCP-IP is a counterpart of DNS, are coordinate reference systems (CRS) and spatial reference grids. While WGS84 might be sufficient globally, other CRS's are important for local geography, for measurement, or for different views of the globe (e.g. polar). OGC and ISO provide standards (for example, ISO 19111) for models and XML encodings of coordinate reference systems. A draft ontology for this standard was developed at Drexel University (http://loki.cae.drexel.edu/~wbs/ontology/2004/09/iso-19111.owl)
A useful quantitative way of both geolocating and indexing a resource involves identifying the grid cell of a geospatial grid scheme (pyramid of successively smaller cell sizes) in which it is located. Correspondence between grid scheme and map tile scheme allows background map tiles to be easily discovered and presented together with co-located resources.
Data quality and provenance are critical to deriving real benefit from geography and geolocation on the Web. A general approach to descriptive metadata is modeled in ISO 19115 and rules for XML encoding are specified in ISO 19139. Metadata which is operational specifically for use with Web resources have not yet been either modeled or codified in the form of an ontology. Fundamental issues such as the time relevance and time validity of a geospatial Web resource (for example, a map image) as well as representation of such properties in a URI have yet to be resolved.
Evolving standards such as OWL-S have raised the bar on formal and actionable descriptions of Web services, but elaborations are needed of the manner in which the closely coupled content of most geospatial Web services affects their process model and expected interface behavior. Some work on geospatial semantic service models can be found in the OGC Discussion Paper 06-002r1 at http://portal.opengeospatial.org/files/?artifact_id=15198
The Geospatial Incubator Group began with a simple mission and a likely candidate vocabulary. Consideration of the charter use cases indicated that extensions of this vocabulary were likely to be necessary, but were a more ambitious Web standards challenge. The group has proposed 7 foundation ontologies as the basis for future work to provide these extensions.
The open questions raised throughout the XG process as reported in this document are collated and presented below in no particular order of priority.
The following terms are used throughout this report. Definitions have been collected from W3C glossaries where possible and provided a priori where necessary.
Assertion Any expression which is claimed to be true. [W3C definition source]
Resource Anything that might be identified by a URI. [W3C definition source]
Schema (pl., schemata) A document that describes an XML or RDF vocabulary. Any document which describes, in a formal way, a language or parameters of a language. [ W3C definition source]
Vocabulary A collection of vocabulary terms, usually linked to a document that defines the precise meaning of the descriptors and the domain in which the vocabulary is expected to be used. When associated with a schema, attributes are expressed as URI references. [This definition is an amalgam of those provided in Composite Capability/Preference Profiles (CC/PP): Structure and Vocabularies 1.0 and OWL Web Ontology Language Guide.]
Vocabulary term An attribute that can describe one or more resources using a defined set of values or data type. Attributes may be expressed as a URI reference. See also descriptor and expression.
Well-formed Syntactically legal. [W3C definition source]
The editors acknowledge significant contributions from: