GraphDB-SE Geo-spatial Extensions

Version 1 by barry.bishop
on Mar 13, 2012 18:39.

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Version 2 by Dimitar Manov
on Jul 17, 2014 17:26.

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OWLIM-SE GraphDB-SE has special support for 2-dimensional geo-spatial data that uses the [WGS84 Geo Positioning RDF vocabulary (World Geodetic System 1984)|]. Special indices can be used for this data that permit the efficient evaluation of special query forms and extension functions that allow:
* locations to be found that are within a certain distance of a point, i.e. within the specified circle on the surface of the sphere (Earth), using the nearby(...) construction;
* locations that are within rectangles and polygons, where the vertices are defined using spherical polar coordinates, using the within(...) construction.

The following jar files (included with the OWLIM GraphDB distribution) must be on the classpath in order for the geo-spatial extensions to function correctly: jsi-1*.jar, log4j-1*.jar, sil-0*.jar, trove4j-2*.jar

h1. Geo-spatial query syntax

The special syntax used to query geo-spatial data makes use of 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 OWLIM-SE GraphDB-SE are treated differently by the query engine. The following special syntax is supported when evaluating SPARQL queries (the descriptions all use the namespace:

{{omgeo: <[]>}}
h1. Implementation details

Knowledge of the implementation's algorithms and assumptions will allow users to make the best use of the OWLIM-SE GraphDB-SE geo-spatial extensions. The following points are significant and can affect the expected behaviour during query answering:
* Spherical Earth -- the current implementation treats the Earth as a perfect sphere with a radius of 6371.009km;
* Only 2-Dimensional points are supported, i.e. there is no special handling of geo:alt (metres above the reference surface of the Earth);