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Asked: 2026-06-15T05:53:43+00:00

I just started using OWL for a project at the university. The project is

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I just started using OWL for a project at the university.
The project is about pizza, which seems to be a common problem when it comes to RDF and OWL. So what we have to do first is model a pizza that consists of two toppings: tomatoes and cheese. I used the property madeof to connect them. This is a minimal code snippet that I tried to validate at: http://www.mygrid.org.uk/OWL/Validator

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
    xmlns:owl="http://www.w3.org/2002/07/owl#"
    xmlns="http://example.org/pizzeria#">

    <owl:Ontology rdf:about=""/>  

    <!-- ========================= -->



    <owl:Class rdf:ID="Topping"/>

    <Topping rdf:ID="Cheese"/>
    <Topping rdf:ID="Tomato"/>

     <owl:ObjectProperty rdf:ID="madeOf">
            <rdfs:range rdf:resource="#Topping"/>
            <rdfs:domain rdf:resource="#Pizza"/>
     </owl:ObjectProperty>

     <!-- Pizza -->
     <owl:Class rdf:ID="Pizza">
         <!-- comes with Cheese and Tomato -->
         <rdfs:subClassOf>
                 <owl:Restriction>
                     <owl:onProperty rdf:resource="#madeOf"/>
                     <owl:hasValue rdf:resource="#Tomato"/>
                 </owl:Restriction>
         </rdfs:subClassOf>
         <rdfs:subClassOf>
             <owl:Restriction>
                 <owl:onProperty rdf:resource="#madeOf"/>
                 <owl:hasValue rdf:resource="#Cheese"/>
             </owl:Restriction>
         </rdfs:subClassOf>
     </owl:Class>
 </rdf:RDF>

However, the validator tells me:

OWL DL
Individual Value: restriction(a:madeOf value (a:Cheese))
Individual Value: restriction(a:madeOf value (a:Tomato))

OWL Full
Untyped Class: http://example.org/pizzeria#Topping

Which means, I guess, that the Toppings class was not defined (I think I did this…) and that something is wrong with the madeof property. I think I am doing something wrong here and am pretty much stuck at the moment.

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1 Answer

  1. Editorial Team

    Your OWL is fine, the only problem you have is with the XML encoding of RDF. Generally speaking, avoid working directly with RDF/XML if you can. Turtle is a much more compact and readable syntax, and has fewer gotchas than RDF/XML.

    So the gotcha that you fell into is that, while you did define the expansion for the empty prefix:

    xmlns="http://example.org/pizzeria#"

    you didn’t define the base URI for the document. Subject resources like:

    <Topping rdf:ID="Cheese"/>

    will be interpreted as relative to the base URI, rather than as though they had a default blank prefix. See paragraph one, above!

    The fix is to define the base URI as well as the default prefix:

    <rdf:RDF
 xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
 xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
 xmlns:owl="http://www.w3.org/2002/07/owl#"
 xmlns="http://example.org/pizzeria#"
 xml:base="http://example.org/pizzeria#"
>
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