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R2RML: A Step-by-step Tutorial

Christophe Debruyne

ADAPT, Trinity College Dublin
WISE, Vrije Universiteit Brussel

1 Introduction

The goal of this tutorial is to familiarize the reader with some core concepts of R2RML. The R2RML engine we will use is R2RML-F, though we will not avail of any functionality outside of R2RML's specification. I assume that the reader has downloaded or installed an R2RML engine.

While R2RML was intended for relational databases, R2RML-F allows one to access CSV files as relational tables. This tutorial will use this feature so that a relational database will not be required for this tutorial. We use the H2 Database Engine to access CSV files as tables. This means that column names are capitalized (i.e., emp becomes EMP).

The CSV file we will transform into RDF is weatherstations.csv. This CSV file was published by Dublinked.ie with a CC BY 4.0 license. This file contains information on weather stations; their names, locations, the agency responsible for that stations, and a URL to a page with the weather readings of that station. The CSV file contains the following rows (with the first being the header):

Name Weather_Reading Agency LAT LONG
M50 Blanchardstown ... National Roads Authority 53.3704660326011 -6.38085144711153
M50 Dublin Airport ... National Roads Authority 53.4096411069945 -6.22759742761812
Dublin Airport ... Met Éireann 53.4215060785623 -6.29784754004026

Note that the column "Weather_Reading" contains URLs in the file. We have omitted those for brevity. We will transform the contents of this CSV file into RDF using the GeoSPARQL. In GeoSPARQL, there is a distinction between features and geometries. Features are the "things" that one can represent on a map, and geometries are the way those things are represented using points, lines, polygons, etc. For each record in our CSV file, we thus have information on weather stations (the feature) and the longitude and latitude that constitute a point on a map (the geometry). Features and geometries are connected using the geo:hasGeometry predicate.

Before we start, we will first create the configuration file for our R2RML-F engine. The contents of that file, which we call weather-config.properties is as follows:

CSVFiles = weatherstations.csv
mappingFile = ./weather-mapping.ttl
outputFile = ./weather-output.ttl
format = TURTLE

This informs the engine that weatherstations.csv will be transformed into RDF using the mapping provided by ./weather-mapping.ttl. The RDF will be formatted into TURTLE and written to weather-output.ttl.

We now create weather-mapping.ttl and add the following prefixes:

@prefix rr: <http://www.w3.org/ns/r2rml#> .
@prefix fcc: <http://www.example.org/ont#> .
@prefix geo: <http://www.opengis.net/ont/geosparql#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix geo2: <http://www.w3.org/2003/01/geo/wgs84_pos#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

The namespace geo is used for GeoSPARQL. The names spaces rr, rdfs, and xsd refer to the namespaces for R2RML, RDFS, and XSD respectively. We will use geo2 for publishing the longitude and latitude in another vocabulary (next to GeoSPARQL). We furthermore assume the existence of an ontology prefixed with fcc for some concepts and relations.

2 A Triples Map for Weather Stations

2.1 Generating Resources for Weather Stations

We first create the triples map for weather stations (the feature). Every triples map needs a logical table and a subject map. The subject map is responsible for creating the subjects and any type declarations.

# PREFIXES APPEAR HERE

<#WeatherStation>
  a rr:TriplesMap ;

  rr:logicalTable [ rr:tableName "WEATHERSTATIONS" ] ;

  rr:subjectMap [
    rr:template "http://data.example.org/ws/{NAME}" ;
    rr:class geo:Feature ;
    rr:class fcc:WeatherStation ;
  ] ;

  # PREDICATE OBJECT MAPS FOR <#WeatherStation> WILL APPEAR HERE
  .

For every row in the table, we use NAME to create the URI of the subject. We also declare that each subject is an instance of geo:Feature and fcc:WeatherStation.

We now execute the mapping with:

$ java -jar r2rml/r2rml.jar weather-config.properties

And six triples should be generated:

<http://data.example.org/ws/Dublin%20Airport>
        a       <http://www.opengis.net/ont/geosparql#Feature> , <http://www.example.org/ont#WeatherStation> .

<http://data.example.org/ws/M50%20Dublin%20Airport>
        a       <http://www.opengis.net/ont/geosparql#Feature> , <http://www.example.org/ont#WeatherStation> .

<http://data.example.org/ws/M50%20Blanchardstown>
        a       <http://www.opengis.net/ont/geosparql#Feature> , <http://www.example.org/ont#WeatherStation> .

We will not provide the whole output for each step in this tutorial, but we will provide a snippet of the expected output instead.

2.2 Generating Labels for Weather Stations

We will now provide labels for Weather Stations. We know those labels are in English, so we can use that column for both the default label (i.e., with no language tag) and English labels. The predicate we will use is rdfs:label. Since we are going to use the same predicate for both labels, we only need to declare one Predicate Object Map with one predicate (for rdfs:label) and two object maps (one for the default label and one for the English label).

  rr:predicateObjectMap [
    rr:predicate rdfs:label ;
    rr:objectMap [ rr:column "NAME" ] ;
    rr:objectMap [ rr:column "NAME" ; rr:language "en" ] ;
  ] ;

The resulting triples for one of the weather stations are as follows:

<http://data.example.org/ws/Dublin%20Airport>
        <http://www.w3.org/2000/01/rdf-schema#label>
                "Dublin Airport" , "Dublin Airport"@en .

2.3 Generating the Coordinates with geo2

Now we will use the geo2 namespace to publish the longitude and the latitude. They need to be published as xsd:double. If we do not provide any instructions, R2RML prescribes that literals should be generated. We extend the mapping with two predicate object maps.

  rr:predicateObjectMap [
    rr:predicate geo2:lat ;
    rr:objectMap [ rr:column "LAT" ; rr:datatype xsd:double ] ;
  ] ;

  rr:predicateObjectMap [
    rr:predicate geo2:long ;
    rr:objectMap [ rr:column "LONG" ; rr:datatype xsd:double ] ;
  ] ;

These predicate object maps result in:

<http://data.example.org/ws/Dublin%20Airport>
        <http://www.w3.org/2003/01/geo/wgs84_pos#lat>
                "53.4215060785623"^^<http://www.w3.org/2001/XMLSchema#double> ;
        <http://www.w3.org/2003/01/geo/wgs84_pos#long>
                "-6.29784754004026"^^<http://www.w3.org/2001/XMLSchema#double> .

You will notice that the engine will generate literals if you remove the rr:datatype xsd:double statements. I encourage you to try that.

2.4 Providing Weather Readings

We will now provide the weather readings (the URLs) as a resource with both rdfs:seeAlso and fcc:withWeatherReading. We can thus use one predicate object map with two predicates and one object map. The column WEATHER_READING contains a URL, but R2RML states that term maps with a rr:column generate literals. If we want to generate resources, however, we need to declare that in the mapping. The mapping looks as follows:

  rr:predicateObjectMap [
  rr:predicate rdfs:seeAlso, fcc:withWeatherReading ;
    rr:objectMap [ rr:column "WEATHER_READING" ; rr:termType rr:IRI ] ;
  ] ;

This mapping results in:

<http://data.example.org/ws/Dublin%20Airport>
        <http://www.w3.org/2000/01/rdf-schema#seeAlso>
                <http://www.met.ie/latest/reports.asp> ;
        <http://www.example.org/ont#withWeatherReading>
                <http://www.met.ie/latest/reports.asp> .

I encourage you to remove the rr:termType rr:IRI statement and to see what happens.

3 A Triples Map for Geometries

Before we can generate the relationships between features and geometries, we need to generate the geometries. In this section, we will create a triples map for geometries. In the next section, we will relate the two.

The triples map for geometries is pretty straightforward. It uses the same table and has a subject map that generates URIs for each geometry. The URIs are different as features are different from geometries. Each geometry is also declared to be an instance of geo:Geometry. Both longitude and latitude are used for the URI.

We have one predicate object map for the generation of WKT literals. Both longitude and latitude are used to fill in a template. R2RML prescribes that templates are used to generate IRIs. Here, however, we have to generate a literal (typed as geo:wktLiteral). The rr:datatype declaration will inform the engine that literals have to be produced as only literals can be data-typed.

<#Geometries>
  a rr:TriplesMap ;

  rr:logicalTable [ rr:tableName "WEATHERSTATIONS" ] ;

  rr:subjectMap [
    rr:template "{NAME}" ;
    rr:class geo:Geometry
  ] ;

  rr:predicateObjectMap [
    rr:predicate geo:asWKT ;
      rr:objectMap [
        rr:template "POINT({LONG} {LAT})" ;
        rr:datatype geo:wktLiteral
      ] ;
  ] ;
  .

Geometries then look as follows:

<http://data.example.org/geom/-6.22759742761812/53.4096411069945>
        a       <http://www.opengis.net/ont/geosparql#Geometry> ;
        <http://www.opengis.net/ont/geosparql#asWKT>
                "POINT(-6.22759742761812 53.4096411069945)"^^<http://www.opengis.net/ont/geosparql#wktLiteral> .

<http://data.example.org/geom/-6.29784754004026/53.4215060785623>
        a       <http://www.opengis.net/ont/geosparql#Geometry> ;
        <http://www.opengis.net/ont/geosparql#asWKT>
                "POINT(-6.29784754004026 53.4215060785623)"^^<http://www.opengis.net/ont/geosparql#wktLiteral> .

<http://data.example.org/geom/-6.38085144711153/53.3704660326011>
        a       <http://www.opengis.net/ont/geosparql#Geometry> ;
        <http://www.opengis.net/ont/geosparql#asWKT>
                "POINT(-6.38085144711153 53.3704660326011)"^^<http://www.opengis.net/ont/geosparql#wktLiteral> .

4 Tying it all together

Now that we have a triples map for geometries, we can create a predicate object map that relates features and geometries. It is true that in this simple case we can avail of a "simple" predicate object map, but we will now illustrate a predicate object map referring to another triples map with join conditions. The R2RML engine will thus create triples based on an SQL join. We extend the first triples map as follows:

  rr:predicateObjectMap [
    rr:predicate geo:hasGeometry;
    rr:objectMap [
      rr:parentTriplesMap <#Geometries> ;
      rr:joinCondition [
        rr:child "NAME" ;
        rr:parent "NAME" ;
      ] ;
    ] ;
  ]

The two logical tables are joined using the column NAME resulting in:

<http://data.example.org/ws/Dublin%20Airport>
        <http://www.opengis.net/ont/geosparql#hasGeometry>
                <http://data.example.org/geom/-6.29784754004026/53.4215060785623> .

5 Separating Longitude and Latitude in a Different Graph

Note: the "solution" of this section are to be found in the files weather-mapping-graph.ttl and weather-config-graph.properties.

Assuming we want to keep the statements using geo2 in a separate named graph. This means we have to use an RDF serialization that supports graphs. Both predicate object maps and subject maps can be graph statements. Comprehending which triples end up in which graphs can be tricky, but we recommend reading R2RML's algorithm. In our case, separating those triples is straightforward.

First, we need to change our configuration file so that another serialization format is used. We also change the file extension of the output file to ensure that best practices are complied with. We will use TriG in this tutorial, which is an extension of TURTLE.

CSVFiles = weatherstations.csv
mappingFile = ./weather-mapping.ttl
outputFile = ./weather-output.trig
format = TRIG

Then we change the two predicate object maps of our weather triples map by adding a rr:graph statements.

  rr:predicateObjectMap [
    rr:graph <http://data.example.org/graph/geo> ;
    rr:predicate geo2:lat ;
    rr:objectMap [
      rr:column "LAT" ;
      rr:datatype xsd:double ;
    ] ;
  ] ;

  rr:predicateObjectMap [
    rr:graph <http://data.example.org/graph/geo> ;
    rr:predicate geo2:long ;
    rr:objectMap [
      rr:column "LONG" ;
      rr:datatype xsd:double ;
    ] ;   
  ] ;

R2RML states that if the graph maps of both the subject map and the predicate object map are empty, then triples are written to the default graph; otherwise the triples are written to the union of both graph maps. Since the subject map has no graph maps (i.e., {}), the union is {} U {http://data.example.org/graph/geo}. In other words, those triples will appear in the named graph http://data.example.org/graph/geo and not in the default graph:

# TRIPLES IN DEFAULT GRAPH OMITTED

<http://data.example.org/graph/geo> {
    <http://data.example.org/ws/Dublin%20Airport>
            <http://www.w3.org/2003/01/geo/wgs84_pos#lat>
                    "53.4215060785623"^^<http://www.w3.org/2001/XMLSchema#double> ;
            <http://www.w3.org/2003/01/geo/wgs84_pos#long>
                    "-6.29784754004026"^^<http://www.w3.org/2001/XMLSchema#double> .

    <http://data.example.org/ws/M50%20Dublin%20Airport>
            <http://www.w3.org/2003/01/geo/wgs84_pos#lat>
                    "53.4096411069945"^^<http://www.w3.org/2001/XMLSchema#double> ;
            <http://www.w3.org/2003/01/geo/wgs84_pos#long>
                    "-6.22759742761812"^^<http://www.w3.org/2001/XMLSchema#double> .

    <http://data.example.org/ws/M50%20Blanchardstown>
            <http://www.w3.org/2003/01/geo/wgs84_pos#lat>
                    "53.3704660326011"^^<http://www.w3.org/2001/XMLSchema#double> ;
            <http://www.w3.org/2003/01/geo/wgs84_pos#long>
                    "-6.38085144711153"^^<http://www.w3.org/2001/XMLSchema#double> .
}

6 Generating Geometries as Blank Nodes

Note: the "solution" of this section are to be found in the files weather-mapping-blank.ttl and weather-config-blank.properties.

Before we continue, it is important to note that blank nodes with the same identifier in different graphs are not considered representing the same resource. In other words, given a blank node with identifier x in graph1 and a blank node with the same identifier x in a different graph2, then those two blank nodes do not represent the same thing. If you want to relate nodes across graphs, then you have to use IRIs (or avail of ontology axioms or rules to infer sameness).

Let's assume that we do not want to "publish" our geometries as resources with an IRI, but as blank nodes. In R2RML, this is easily achieved by stating that the rr:termType of the subject map is rr:BlankNode. We will need something that generates values to determine when the term map generates the same blank node. The values generated by the subject map are used for internal blank node identifiers. In R2RML-F, these values are kept in a dictionary. While the original template did generate unique URI strings, this is not necessary for blank nodes and we could choose a simpler representation (e.g., {LONG}-{LAT}).

We amend the subject map of the geometries triples map as follows:

  rr:subjectMap [
    rr:template "{LONG}-{LAT}" ;
    rr:class geo:Geometry ;
    rr:termType rr:BlankNode ;
  ] ;

After running the R2RML engine, we see that it generates the following triples:

<http://data.example.org/ws/Dublin%20Airport>
        <http://www.opengis.net/ont/geosparql#hasGeometry>
                [ a       <http://www.opengis.net/ont/geosparql#Geometry> ;
                  <http://www.opengis.net/ont/geosparql#asWKT>
                          "POINT(-6.29784754004026 53.4215060785623)"^^<http://www.opengis.net/ont/geosparql#wktLiteral>
                ] .

I encourage the reader to add a row to the CSV file with different values for Name, Weather_Reading, and Agency, but the values of LAT and LONG of one of the previous rows. Even though it does not make sense in the "real world", the reader will notice that two records will generate one blank node and all statements are merged. In other words, two weather stations will point to the same blank node. If the reader were to add a fifth row by copying the first and choosing different values for LAT and LONG, the reader will notice that this weather station will refer to two different geometries.

7 Some Final Considerations

In this tutorial, we covered a common occurrence in databases; the "grouping" of several concepts in a table. The city of an address, for instance, can appear as a column in an Address table. With R2RML, we can create mappings for projections.

8 Conclusions

With this step-by-step tutorial, I hope the reader has a better understanding of R2RML. I believe we have covered most of the R2RML W3C Recommendation and that the reader has everything they need to create and execute their own mapping. Feel free to contact me with feedback.

License

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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