GraphDB-SE has special support for 2-dimensional geo-spatial data, which uses the WGS84 Geo Positioning RDF vocabulary (World Geodetic System 1984). Special indices can be used for this data, which permit the efficient evaluation of special query forms and extension functions that allow finding locations, which are:
The WGS84 ontology can be found at: http://www.w3.org/2003/01/geo/wgs84_pos and contains several classes and predicates:
Before the geo-spatial extensions can be used, the geo-spatial index must be built. This is achieved by inserting and committing a dummy statement (that is not actually stored), which uses a special predicate:
If the indexing is successful, the above update succeeds, otherwise an exception is thrown. Information about the indexing process and any errors can be found in the log. Note that this update will fail if there is no geo-spatial data in the repository, i.e. no statements describing resources with latitude and longitude properties.
The Geo-spatial query syntax is the SPARQL's RDF Collections syntax. This syntax uses round brackets as a shorthand for the statements, connecting a list of values using rdf:first and rdf:rest predicates with terminating rdf:nil. Statement patterns that use one of the special geo-spatial predicates, supported by GraphDB-SE, are treated differently by the query engine. The following special syntax is supported when evaluating SPARQL queries. All descriptions use the namespace:
At present there is just one SPARQL extension function.
Knowledge of the implementation's algorithms and assumptions allow users to make the best use of the GraphDB-SE geo-spatial extensions. The following points are significant and can affect the expected behaviour during query answering:
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