Spatial searches with SPARQL

This module was first released with Jena 2.11.0. It was last released in Jena 3.12.0.

Jena provides a GeoSPARQL implementation.

This is an extension to Apache Jena ARQ, which combines SPARQL and simple spatial query. It gives applications the ability to perform simple spatial searches within SPARQL queries. Spatial indexes are additional information for accessing the RDF graph.

The spatial index can be either Apache Lucene for a same-machine spatial index, or Apache Solr for a large scale enterprise search application.

Some example code is available here.

Illustration

This query makes a spatial query for the places within 10 kilometres of Bristol UK (which as latitude/longitude of 51.46, 2.6).

PREFIX spatial: <http://jena.apache.org/spatial#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>

SELECT ?placeName
{
    ?place spatial:nearby (51.46 2.6 10 'km') .
    ?place rdfs:label ?placeName
}

How to Use it by Code

Create Spatial Dataset

import org.apache.jena.query.spatial.EntityDefinition
...
// In Lucene, "entityField" stores the uri of the subject (e.g. a place), 
// while "geoField" holds the indexed geo data (e.g. latitude/longitude).
// Using fields "uri" and "geo":
EntityDefinition entDef = new EntityDefinition("uri", "geo"); 

// index in File system (or use an in-memory one)
Directory dir = FSDirectory.open(indexDir); 

// The baseDataset can be an in-memory or TDB/SDB file based one which contains the geo data. Join together into a dataset.
Dataset spatialDataset = SpatialDatasetFactory.createLucene(baseDataset, dir, entDef); 
...

Supported Geo Data for Indexing and Querying

Builtin Geo Predicates

There are mainly 2 types of RDF representation of geo data, which are both supported by jena-spatial:

1) Latitude/Longitude Format (in gonames, DBPedia and Linked Geo Data)

PREFIX geo: <http://www.w3.org/2003/01/geo/wgs84_pos#>
:EGBB rdf:type :LargeAirport ;
    geo:lat "52.4539"^^xsd:float ;
    geo:long "-1.74803"^^xsd:float .
:EGBB_String rdf:type :LargeAirport ;
    geo:lat "52.4539" ;
    geo:long "-1.74803" .

2) Well Known Text (WKT) Literal (in DBPedia and Linked Geo Data)

PREFIX ogc: <http://www.opengis.net/ont/geosparql#>
:node1000032677 a :Geometry ;
    ogc:asWKT "POINT(7.338818000000001 51.4433324)"^^ogc:wktLiteral .
airports:EGBB_Fake_In_Box rdf:type airports_sc:LargeAirport ;
    ogc:asWKT "Polygon ((-2.0 51.2, 1.0 51.2, 1.0 51.8, -2.0 51.8, -2.0 51.2))"^^wkt:wktLiteral.

For 2) WKT, DBPedia uses geo:geometry, while Linked Geo Data adopts ogc:asWKT and geo:geometry.

The builtin predicates that can be automatically processed by jena-spatial include: 1) geo:lat, geo:long; 2) geo:geometry, ogc:asWKT.

Important note In order to read geo data in 2) WKT literal format, jena-spatial uses JTS Topology Suite, which is under LGPL licence. jena-spatial does not make a hard dependency on JTS. In other words, if an end user just uses the feature of 1), there’s no need to depend on JTS (i.e. nothing needs to be done). If they want 2), they can make it by setting the SpatialContextFactory of EntityDefinition to JtsSpatialContextFactory, which is an optional choice. In this way, the JTS libs should be in the classpath. Here’s the sample code:

import org.apache.jena.query.spatial.EntityDefinition
...
EntityDefinition entDef = new EntityDefinition("uri", "geo");

// use JtsSpatialContextFactory to support 2) WKT literals (optional)
entDef.setSpatialContextFactory("com.spatial4j.core.context.jts.JtsSpatialContextFactory");
...

Custom Geo Predicates

However, there may be more predicates for other data sources for both 1) and 2). jena-spatial provides an interface for consuming all kinds of custom geo predicates. You can simply add predicates to let jena-spatial recognize them using EntityDefinition:

import org.apache.jena.query.spatial.EntityDefinition
...

EntityDefinition entDef = new EntityDefinition("uri"", "geo");

// custom geo predicates for 1) Latitude/Longitude Format
Resource lat_1 = ResourceFactory.createResource("http://localhost/jena_example/#latitude_1") ;
Resource long_1 ResourceFactory.createResource("http://localhost/jena_example/#longitude_1") ;
entDef.addSpatialPredicatePair(lat_1, long_1) ;

// custom geo predicates for Well Known Text (WKT) Literal
Resource wkt_1 = ResourceFactory.createResource("http://localhost/jena_example/#wkt_1");
entDef.addWKTPredicate( wkt_1 );

See more supported geo data examples

Load Geo Data into Spatial Dataset

spatialDataset.begin(ReadWrite.WRITE);
try {
    Model m = spatialDataset.getDefaultModel();
    RDFDataMgr.read(m, file);
    spatialDataset.commit();
} finally {
    spatialDataset.end();
}

Now the spatial dataset is ready for spatial query.

Property Function Library

The prefix spatial is <http://jena.apache.org/spatial#>.

 Property name   Description 
?place spatial:nearby (latitude, longitude, radius [, units, limit])
?place spatial:withinCircle (latitude, longitude, radius [, units, limit])		 Query for the ?place within the radius distance of the location of (latitude, longitude). The distance units can be: “kilometres”/“km”, “miles”/“mi”, “metres”/“m”, “centimetres”/“cm”, “millimetres”/“mm” or “degrees”/“de”, which are delivered as the optional strings (the default value is “kilometres”). limit is an optional integer parameter for the limit of the query results (if limit<0, return all query results).
?place spatial:withinBox (latitude_min, longitude_min, latitude_max, longitude_max [, limit]) Query for the ?place within the box area of (latitude_min, longitude_min, latitude_max, longitude_max).
?place spatial:intersectBox (latitude_min, longitude_min, latitude_max, longitude_max [, limit]) Query for the ?place intersecting the box area of (latitude_min, longitude_min, latitude_max, longitude_max).
?place spatial:north (latitude, longitude [, limit]) Query for the ?place northing the location of (latitude, longitude).
?place spatial:south (latitude, longitude [, limit]) Query for the ?place southing the location of (latitude, longitude).
?place spatial:west (latitude, longitude [, limit]) Query for the ?place westing the location of (latitude, longitude).
?place spatial:east (latitude, longitude [, limit]) Query for the ?place easting the location of (latitude, longitude).

See ESRIs docs on spatial relations

Spatial Dataset Assembler

The usual way to describe an index is with a Jena assembler description. Configurations can also be built with code. The assembler describes a “spatial dataset” which has an underlying RDF dataset and a spatial index. The spatial index describes the spatial index technology (Lucene or Solr) and the details needed for each.

A spatial index has an EntityDefinition which defines the properties to index, the name of the lucene/solr field used for storing the URI itself (e.g. “entityField”) and its geo information (e.g. latitude/longitude as “geoField”), and the custom geo predicates.

For common RDF spatial query, only “entityField” and “geoField” are required with the builtin geo predicates working well. More complex setups, with multiple custom geo predicates besides the two fields are possible. You also optionally use JtsSpatialContextFactory to support indexing WKT literals.

Once setup this way, any data added to the spatial dataset is automatically indexed as well.

The following is an example of a TDB dataset with a spatial index.

## Example of a TDB dataset and spatial index

PREFIX :        <http://localhost/jena_example/#>
PREFIX rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs:    <http://www.w3.org/2000/01/rdf-schema#>
PREFIX tdb:     <http://jena.hpl.hp.com/2008/tdb#>
PREFIX ja:      <http://jena.hpl.hp.com/2005/11/Assembler#>
PREFIX spatial: <http://jena.apache.org/spatial#>

## ---------------------------------------------------------------
## This URI must be fixed - it's used to assemble the spatial dataset.

:spatial_dataset rdf:type     spatial:SpatialDataset ;
    spatial:dataset   <#dataset> ;
    ##spatial:index   <#indexSolr> ;
    spatial:index     <#indexLucene> ;
    .

<#dataset> rdf:type      tdb:DatasetTDB ;
    tdb:location "--mem--" ;
    tdb:unionDefaultGraph true ;
    .

<#indexLucene> a spatial:SpatialIndexLucene ;
    #spatial:directory <file:Lucene> ;
    spatial:directory "mem" ;
    spatial:definition <#definition> ;
    .

<#definition> a spatial:EntityDefinition ;
    spatial:entityField      "uri" ;
    spatial:geoField     "geo" ;
    # custom geo predicates for 1) Latitude/Longitude Format
    spatial:hasSpatialPredicatePairs (
         [ spatial:latitude :latitude_1 ; spatial:longitude :longitude_1 ]
         [ spatial:latitude :latitude_2 ; spatial:longitude :longitude_2 ]
         ) ;
    # custom geo predicates for 2) Well Known Text (WKT) Literal
    spatial:hasWKTPredicates (:wkt_1 :wkt_2) ;
    # custom SpatialContextFactory for 2) Well Known Text (WKT) Literal
    spatial:spatialContextFactory
         "com.spatial4j.core.context.jts.JtsSpatialContextFactory"
    .

then use code such as:

Dataset spatialDataset = DatasetFactory.assemble( "spatial-config.ttl", 
    "http://localhost/jena_example/#spatial_dataset") ;

Key here is that the assembler contains two dataset definitions, one for the spatial dataset, one for the base data. Therefore, the application needs to identify the text dataset by its URI ‘http://localhost/jena_example/#spatial_dataset’.

Working with Solr

Besides Lucene, jena-spatial can work with Solr for spatial query, powered by Lucene / Solr 4 Spatial and Solrj.

It’s required to add the field definitions for “entityField” and “geoField” respectively in schema.xml of Solr. The names of the fields in EntityDefinition should be in accordance with those in schema.xml. Here is an example defining the names of “entityField” as “uri” and “geoField” as “geo”:

<field name="uri" type="string" indexed="true" stored="true" required="true" multiValued="false" /> 
<field name="geo"  type="location_rpt"  indexed="true" stored="true"  multiValued="true" />

The fieldType of “entityField” is string, while that of “geoField” is location_rpt:

<fieldType name="string" class="solr.StrField" sortMissingLast="true" />
<fieldType name="location_rpt" class="solr.SpatialRecursivePrefixTreeFieldType" geo="true" distErrPct="0.025" maxDistErr="0.000009" units="degrees" />

Additionally, in solrconfig.xml, there should be 2 requestHandlers defined for querying and updating the spatial data and the index.

<requestHandler name="/select" class="solr.SearchHandler"></requestHandler>
<requestHandler name="/update" class="solr.UpdateRequestHandler"></requestHandler>

The above is the least required configuration to run jena-spatial in Solr. For more information about the configuration, please check the Lucene / Solr 4 Spatial documentation.

There are also some demonstrations of the usage of Solr in the unit tests of jena-spatial. They use a EmbeddedSolrServerwith a SOLR_HOME sample here.

Working with Fuseki

The Fuseki configuration simply points to the spatial dataset as the fuseki:dataset of the service.

<#service_spatial_tdb> rdf:type fuseki:Service ;
    rdfs:label                      "TDB/spatial service" ;
    fuseki:name                     "ds" ;
    fuseki:serviceQuery             "query" ;
    fuseki:serviceQuery             "sparql" ;
    fuseki:serviceUpdate            "update" ;
    fuseki:serviceReadGraphStore    "get" ;
    fuseki:serviceReadWriteGraphStore    "data" ;
    fuseki:dataset                  :spatial_dataset ;

Building a Spatial Index

When working at scale, or when preparing a published, read-only, SPARQL service, creating the index by loading the spatial dataset is impractical. The index and the dataset can be built using command line tools in two steps: first load the RDF data, second create an index from the existing RDF dataset.

Build the TDB dataset:

java -cp $FUSEKI_HOME/fuseki-server.jar tdb.tdbloader --tdb=assembler_file data_file

using the copy of TDB included with Fuseki. Alternatively, use one of the TDB utilities tdbloader or tdbloader2:

$JENA_HOME/bin/tdbloader --loc=directory  data_file

then build the spatial index with the jena.spatialindexer:

java -cp jena-spatial.jar jena.spatialindexer --desc=assembler_file