What are geo-spatial extensions¶

GraphDB provides support for 2-dimensional geo-spatial data that uses the WGS84 Geo Positioning RDF vocabulary (World Geodetic System 1984). Specialised indexes can be used for this type of data, which allow efficient evaluation of query forms and extension functions for finding locations:

• within a certain distance of a point, i.e. within a specified circle on the surface of a sphere (Earth), using the nearby(...) construction;
• within rectangles and polygons, where the vertices are defined by spherical polar coordinates, using the within(...) construction.

The WGS84 ontology contains several classes and predicates:

How to create a geo-spatial index¶

Execute the following INSERT query:

PREFIX ontogeo: <http://www.ontotext.com/owlim/geo#>
INSERT DATA { _:b1 ontogeo:createIndex _:b2. }

If all geo-spatial data is indexed successfully, the above update query will succeed. If there is an error, you will get a notification about a failed transaction and an error will be registered in the GraphDB log files.

Geo-spatial query syntax¶

The Geo-spatial query syntax is the SPARQL RDF Collections syntax. It uses round brackets as a shorthand for the statements, which connect a list of values using rdf:first and rdf:rest predicates with terminating rdf:nil. Statement patterns that use custom geo-spatial predicates, supported by GraphDB are treated differently by the query engine.

The following special syntax is supported when evaluating SPARQL queries. All descriptions use the namespace: omgeo: <http://www.ontotext.com/owlim/geo#>

PREFIX geo-pos: <http://www.w3.org/2003/01/geo/wgs84_pos#>

PREFIX geo-ont: <http://www.geonames.org/ontology#>

PREFIX omgeo: <http://www.ontotext.com/owlim/geo#>

SELECT distinct ?airport

WHERE {

 ?base geo-ont:name "Seoul" .

 ?base geo-pos:lat ?latBase .

 ?base geo-pos:long ?longBase .

 ?link omgeo:nearby(?latBase ?longBase "50mi") .

 ?link geo-ont:name ?airport  .

 ?link geo-ont:featureCode geo-ont:S.AIRP .

}

PREFIX geo-pos: <http://www.w3.org/2003/01/geo/wgs84_pos#>

PREFIX geo-ont: <http://www.geonames.org/ontology#>

PREFIX omgeo:   <http://www.ontotext.com/owlim/geo#>

SELECT ?feature ?lat ?long

WHERE {

 ?link omgeo:within(45.85 9.15 48.61 13.18) .

 ?link geo-ont:featureCode geo-ont:R.TNL .

 ?link geo-ont:name ?feature .

 ?link geo-pos:lat ?lat .

 ?link geo-pos:long ?long .

}

PREFIX geo-pos: <http://www.w3.org/2003/01/geo/wgs84_pos#>
PREFIX geo-ont: <http://www.geonames.org/ontology#>
PREFIX omgeo:   <http://www.ontotext.com/owlim/geo#>
SELECT ?feature ?lat ?long
WHERE {
?link omgeo:within( "51.45" "-2.59"
  "54.99" "-3.06"
  "55.81" "-2.03"
  "52.74"  "1.68"
  "51.17"  "1.41" ) .
  ?link geo-ont:featureCode geo-ont:S.CAVE .
  ?link geo-ont:name ?feature .
  ?link geo-pos:lat ?lat .
  ?link geo-pos:long ?long .
}

Extension query functions¶

At present, there is just one SPARQL extension function:

PREFIX geo-pos: <http://www.w3.org/2003/01/geo/wgs84_pos#>
PREFIX geo-ont: <http://www.geonames.org/ontology#>
PREFIX omgeo:   <http://www.ontotext.com/owlim/geo#>

SELECT distinct ?airport_name
WHERE {
  ?a1 geo-ont:name "Bournemouth" .
  ?a1 geo-pos:lat ?lat1 .
  ?a1 geo-pos:long ?long1 .
  ?airport omgeo:nearby(?lat1 ?long1 "80mi" ) .
  ?airport geo-ont:name ?airport_name .
  ?airport geo-ont:featureCode geo-ont:S.AIRP .
  ?airport geo-pos:lat ?lat2 .
  ?airport geo-pos:long ?long2 .
  ?a2 geo-ont:name "Brize Norton" .
  ?a2 geo-pos:lat ?lat3 .
  ?a2 geo-pos:long ?long3 .
  FILTER( omgeo:distance(?lat2, ?long2, ?lat3, ?long3) < 80)
}
ORDER BY ASC( omgeo:distance(?lat2, ?long2, ?lat3, ?long3) )

Implementation details¶

Knowing the implementation’s algorithms and assumptions allow you to make the best use of the GraphDB geo-spatial extensions.

The following aspects 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 6371.009km radius;
• Only 2-dimensional points are supported, i.e., there is no special handling of geo:alt (metres above the reference surface of the Earth);
• All latitude and longitude values must be specified using decimal degrees, where East and North are positive and -90 <= latitude <= +90 and -180 <= longitude <= +180;
• Distances must be in units of kilometers (suffix ‘km’) or statute miles (suffix ‘mi’). If the suffix is omitted, kilometers are assumed;
• omgeo:within( rectangle ) construct uses a ‘rectangle’ whose edges are lines of latitude and longitude, so the north-south distance is constant, and the rectangle described forms a band around the Earth, which starts and stops at the given longitudes;
• omgeo:within( polygon ) joins vertices with straight lines on a cylindrical projection of the Earth tangential to the equator. A straight line starting at the point under test and continuing East out of the polygon is examined to see how many polygon edges it intersects. If the number of intersections is even, then the point is outside the polygon. If the number of intersections is odd, the point is inside the polygon. With the current algorithm, the order of vertices is not relevant (clockwise or anticlockwise);
• omgeo:within() may not work correctly when the region (polygon or rectangle) spans the +/-180 meridian;
• omgeo:nearby() uses the great circle distance between points.