=Paper=
{{Paper
|id=Vol-155/paper-7
|storemode=property
|title=Representing the Hierarchy of Industrial Taxonomies in OWL: The gen/tax approach
|pdfUrl=https://ceur-ws.org/Vol-155/paper7.pdf
|volume=Vol-155
}}
==Representing the Hierarchy of Industrial Taxonomies in OWL: The gen/tax approach==
https://ceur-ws.org/Vol-155/paper7.pdf
Representing the Hierarchy of Industrial Taxonomies
in OWL: The gen/tax Approach
Martin Hepp
Digital Enterprise Research Institute (DERI), University of Innsbruck
Florida Gulf Coast University, Fort Myers, FL, USA
martin.hepp@deri.org
Abstract. Existing taxonomies are valuable input for creating ontologies,
because they reflect some degree of community consensus and contain, readily
available, a wealth of concept definitions plus a hierarchy. However, the
transformation of such taxonomies into useful ontologies is not as
straightforward as it appears, because simply taking the hierarchy of concepts,
which was originally developed for some external purpose other than ontology
engineering, as the subsumption hierarchy using rdfs:subClassOf can yield
useless ontologies. In this paper, we (1) illustrate the problem by analyzing
OWL and RDF-S ontologies derived from UNSPSC (a products and services
taxonomy), (2) detail how the interpretation and representation of the original
taxonomic relationship is an important modeling decision when deriving
ontologies from existing taxonomies, (3) propose a novel “gen/tax” approach to
capture the original semantics of taxonomies in OWL, based on the split of each
category in the taxonomy into two concepts, a generic concept and a taxonomy
concept, and (4) show the usefulness of this approach by transforming eCl@ss
into a fully-fledged products and services ontology.
Keywords. Ontology engineering, OWL, reuse, taxonomies, UNSPSC,
eCl@ss, e-business
1. Introduction
Standard taxonomies exist in different problem domains and contain many concept
definitions plus a hierarchy. UNSPSC, a standard taxonomy for products and
services and often referred to as a business ontology, contains 20,789 categories
(in version 7,0901), and eCl@ss, a similar but more expressive standard, contains
25,658 categories plus 5,525 precisely defined object and datatype properties (in
version 5.1de). For a quantitative analysis of the content and domain coverage of
products and services taxonomies, see [1] and [2]. However, the transformation of
such taxonomies into useful ontologies is not as straightforward as it appears,
because simply taking the hierarchy of concepts, which was originally developed
for some external purpose other than ontology engineering, as the subsumption
hierarchy using rdfs:subClassOf can yield useless ontologies. Often, the original
meaning of the taxonomic relationship is “A is in some context a more specific
category of B” rather than a strict subClassOf relationship with the typical
semantics “For all A being a subordinate node of B, every instance of A shall
2 Martin Hepp
also be an instance of B”. UNSPSC, for example, treats “ice” as a subordinate
node of “beverages”, and eCl@ss has “docking stations” as a subordinate node of
“computers”.
Our paper is related to the following previous works: First, the analysis of the
meaning of taxonomic relationships, especially the fundamental work of [3].
Second, methodologies for and experiences with the reuse of consensus in existing
standards for the creation of ontologies. This is the most relevant field of work for
this paper. [4] discusses the transformation of tangled hierarchies, as e.g. such
derived from ambiguous “broader than / narrower than” taxonomies in library
science, into formal ontologies. [5] presents the experiences gained while
transforming the constructs of an existing semantic net in the medical domain into
an OWL ontology. [6] is a detailed description of creating products and services
ontologies based on UNSPSC and eCl@ss. [7] shows the reuse and semantic
enrichment of an existing taxonomy, and demonstrates this for the Art and
Architecture Thesaurus (AAT). [8] and [9] are consequent works of this stream of
research.
The structure of the paper is as follows: In section 2, we describe the problem of
representing the hierarchy of existing taxonomies in derived ontologies. In section
3, we propose a novel “gen/tax” approach, based on the separation of generic and
taxonomy concepts. In section 4, we discuss this approach and show how it can be
successfully applied to the representation of eCl@ss in OWL. Section 5 concludes
the paper.
2. Representing the Hierarchy of Taxonomies in OWL and RDF-S
In this section, we show that in informal taxonomies, the meaning of the
taxonomic relationship and the intension of the concepts are tangled, and that
the interpretation and representation of the hierarchy of a given taxonomy in
an ontology language is an important modeling decision that affects the
usefulness of the resulting ontology.
When taking the categories found in a taxonomy as the basis for the creation of an
ontology, we face a fundamental problem: Unless there is a formal definition of
the semantics of the taxonomic relationship, the intensions of the category
concepts (e.g. the product classes) are not determined independently from the
interpretation of the taxonomic relationship. In other words: If we lack a formal
definition of either the hierarchical relationships or the category concepts, then
how we understand the taxonomic relationship determines the shape of the
category concepts and vice versa. Our choice of the interpretation of the
taxonomic relationship affects the intension of the category concepts, and a chosen
definition of the intension of the category concepts is compatible with only a
specific interpretation of the taxonomic relationship. As a consequence, we have
Representing the Hierarchy of Industrial Taxonomies in OWL: The gen/tax Approach 3
some degree of choice over the intension of the ontology classes derived from the
categories in the source taxonomy by selecting the interpretation of the taxonomic
relationship.
Two examples might illustrate this fundamental problem: The hierarchies of both
UNSPSC and eCl@ss were created on the basis of practical aspects of
procurement, treating those commodities that “somehow” belong to a specific
category, as descendents of this closest category. This makes “ice” a subclass of
“non-alcoholic beverages” in UNSPSC and “docking stations” a subcategory of
“computers” in eCl@ss. Now, we still can read the taxonomic relationship as a
strict “rdfs:subClassOf” relationship (i.e. each instance of “ice” is also an instance
of “non-alcoholic beverages” and each instance of “docking station” is also an
instance of “computers”). Then, however, the intension of the class “computers” is
no longer any computer, but the concept “computer” solely from the perspective of
cost accounting or spend analysis, where an incoming invoice for a docking station
can be treated as an incoming invoice for a computer. Similarly will “non-
alcoholic beverages” no longer represent all non-alcoholic beverages, but the
union of non-alcoholic beverages and related commodities. The problem arises
only because we have to narrow down the semantics of the original informal
standard when turning it into an ontology. Basically, each source taxonomy
contains two concepts for each category node: First the generic concept of the
respective category (e.g. “computer”) and second the intersection of this concept
with a concept “element in this taxonomy”, the latter reflecting all the implicit
assumptions of the creators of the taxonomy and the constraints resulting from the
interpretation of the taxonomic relationship.
The most straightforward approach to deal with this is to define a transitive
relationship “taxonomySubClassOf” that can be used to represent the original
hierarchy in the taxonomy. Then, the original hierarchy would not be falsely used
as the subsumption hierarchy, turning instances of “ice” into instances of
“beverages”, but would still be available for queries. However, this is not possible
in neither OWL Lite, OWL DL, nor RDF-S. In OWL Lite and OWL DL, it is not
possible to define a transitive relation that links classes; such can only be
annotation properties that cannot be assigned a formal semantics. In RDF-S, it is
not possible to define transitivity of properties.
There are at least the following three approaches of transforming a given
taxonomy into an OWL Lite or DL ontology:
1. Create one class for each taxonomy category and assume that the meaning
of the taxonomic relationship is equivalent to rdfs:subClassOf.
2. Create one class for each taxonomy category and represent the taxonomic
relationship using an annotation property taxonomySubClassOf in OWL.
3. Treat the category concepts as instances instead of classes and connect
them using a transitive object property taxonomySubClassOf.
4 Martin Hepp
In RDF-S, only the first two alternatives are possible, with taxonomySubClassOf
being a regular RDF property for solution 2. Approach 1 is chosen by both
available transformations of UNSPSC into products and services ontologies [10]
[11]. Solution 2 seems to be the most straightforward alternative, since the specific
meaning of the taxonomic relationship is captured using a specific property, and the
classes can represent generic product concepts. The problem with this approach is
that, in OWL Lite and OWL DL, a property that connects classes with classes can
only be an annotation property. Thus, it cannot be made a transitive property, and
an OWL Lite or OWL DL reasoner will only see explicit statements. In other
words, if class A is a taxonomy-SubClassOf of class B and class B is a
taxonomySubClassOf of C, then the reasoner will not infer that class A is also a
taxonomy-SubClassOf of class C. This limitation can be avoided by making the
products and services concepts instances instead of classes, as described in solution
3. Then, the property “taxonomySubClassOf” can be an owl:ObjectProperty and
can be made transitive. The downside of this approach is that one absolutely needs
OWL Lite or OWL DL reasoning support in order to process the transitive nature
of the property. We also think that it is not very intuitive to model categories, which
are meant as abstract classes, in the form of instances.
3. The gen/tax Approach
In this section, we describe a fourth approach of transforming existing taxonomies
into domain ontologies. Our approach is based on the idea of deriving two
concepts for each taxonomy category, one reflecting the generic concept and
another reflecting the taxonomy concept. The advantage of this solution is that it
works with the “intersection” of RDF-S and OWL Lite, i.e. it does not require
reasoning capabilities beyond rdfs:subClassOf. This limited requirement on the
ontology language has also advantageous effects from an implementation
perspective.
The basic idea of this “gen/tax” approach is as following:
1. We create two separate concepts for (1) the generic category and (2) the
respective taxonomy category.
2. We arrange the taxonomy concepts in a rdfs:subClassOf hierarchy that is
identical to the original order in the taxonomy, but don’t do this for the
generic concepts. The generic concepts are just named classes without any
support for subsumption, but can be manually augmented at a later point
in time. This split allows for capturing the hierarchy of taxonomy
concepts without linking the generic concepts to incorrect superordinate
classes.
Representing the Hierarchy of Industrial Taxonomies in OWL: The gen/tax Approach 5
3. In order to ease annotation, we create one “annotation” class for each
taxonomy node, which becomes an rdfs:subClassOf of both the respective
generic and the respective taxonomy concept. With this construct, a single
rdf:type statement is sufficient to make a resource an instance of both the
generic and the taxonomy concept.
In the following, we illustrate the gen/tax approach using an example. The example
is based on the sample products and services taxonomy shown in Figure 1.
Radio and TV
TV Set Radio
TV Portable Radio
b/w TV Color TV
Maintenance Radio Antenna
Fig. 1. Example of a products and services taxonomy
Fig. 2 illustrates how the two grey-shaded categories in Fig. 1 can be represented
using the gen/tax approach. The generic concept “TV Set” represents all TV Sets.
The taxonomy concept “TV Set” represents “TV Set”-related instances in the
context of the original taxonomy (“everything that shall be treated as a TV Set in
the context of the ordering purpose of the original taxonomy”). The annotation
concept, being a subclass of both, is just for convenience reasons and allows
making an instance, e.g. the concrete TV set model “sony:TV-123” an instance of
both the generic and the taxonomy concept with one single rdf:type statement.
TV Set 1 TV Set
2
(Generic) (Taxonomy)
rdfs:subClassOf rdfs:subClassOf
TV Sets
(Annotation
rdfs:subClassOf
rdf:type
TV Maintenance
1 TV Maintenance
(Generic) sony:TV-123 2
(Taxonomy)
rdfs:subClassOf
rdfs:subClassOf
TV Maintenance
(Annotation)
rdf:type
ex:myTVService
Fig. 2. The gen/tax approach: Separating the generic concept from the
taxonomic concept
This approach allows preserving the original order of the taxonomy without
narrowing down the intensions of the concepts to one application domain, and puts
only minimal requirements on the expressiveness of the ontology language. One
6 Martin Hepp
might argue that the generic classes are of little value, because they are just named
classes. However, there are two counterarguments: First, more semantics cannot
be automatically deduced from the input taxonomy. It might be desirable to infer
more about these classes, but this means manual ontology engineering work. Keep
in mind that the taxonomies under discussion can be rather big with more than
20,000 concepts and are also volatile with multiple releases per year. Second, in
combination with a library of properties, already a flat set of consensual, named
classes can bring a lot for the application domain, e.g. in e-business.
The application of this approach for describing products is shown in Fig. 3: The
TV maintenance service ex:tv-set-repair is an instance of the annotation class “TV
Set Maintenance”. This makes it also an instance of the generic product class “TV
Set Maintenance (Generic)” and the taxonomy concept “TV Set Maintenance
(Taxonomy)”. The second is a subclass of “TV Set (Taxonomy)”, but the first is
not a subclass of “TV Set (Generic)”.
TV Set 1 TV Set
2
(Generic) (Taxonomy)
rdfs:subClassOf rdfs:subClassOf
TV Sets
(Annotation
rdfs:subClassOf
TV Maintenance 1 TV Maintenance 2
(Generic) (Taxonomy)
rdfs:subClassOf
rdfs:subClassOf
TV Maintenance rdf:type
rdf:type (Annotation)
rdf:type rdf:type
ex:tv-set-repair
Fig. 3. Usage of the gen/tax ontology for product description
This yields exactly the distinction we want: When searching for a TV maintenance
service, we look for instances of the generic class, and when looking for all items
that belong to the taxonomy category, we use the taxonomy concept. For example,
a store manager might want to find all products in the TV set segment. In this case,
he or she also wants to find TV set cabling and maintenance, so the query will be
based on the taxonomy concept. The taxonomic concepts on the right side are the
narrow interpretations of the taxonomic categories and are arranged in the original
hierarchical order. The generic concepts on the left side are the original, broad
variants of the taxonomy concepts, not constrained by the implications of reading
the taxonomic relationships as rdfs:subClassOf. The convenience classes in the
middle ease the annotation task, since an instance can be made an instance of both
the generic and the taxonomy concept by just one rdf:type statement.
Representing the Hierarchy of Industrial Taxonomies in OWL: The gen/tax Approach 7
4. Validation and Discussion
As a validation of the gen/tax approach, we transformed the latest version of
eCl@ss into a fully-fledged products and services ontology, called eClassOWL.
The project will be available at http://www.heppnetz.de/eclassowl. In the following,
we give a short example of the usage of the resulting ontology for product
description in the Semantic Web. We assume that “Fendt Supermower“ is an
agricultural machine (eCl@ss category AKK255001), its weight is 125.5 kg, and
the manufacturer name is "Fendt". As concept identifiers, we use the primary key
(e.g. “AAA001001”) plus a prefix for classes and properties (“C_”, and “P_“). The
annotation class has the resulting string as its concept identifier. The generic class
has an additional “-gen” and the taxonomic class has an additional “-tax”. In other
words, the eCl@ss category “agricultural machine” (primary key “AKK255001”)
is represented using the following three concepts: (1) C_AKK255001 for the
annotation concept, (2) C_AKK255001-gen for the generic concept, and (3)
C_AKK255001-tax for the taxonomic concept. Assumed that the ID for this
product instance is “machine1”, the respective product description using the
eCl@ss ontology would be as follows:
125.50
Fendt
Fendt Supermower1234
Now, we want to search for all agricultural machines in the ontology that
weigh less than 160 kg. The respective RDQL query would be:
SELECT ?x, ?weight, ?productName, ?vendor WHERE
(?x, , )
(?x, , ?vendor)
(?x, , ?productName)
(?x, , ?weight)
AND ?weight <160
Because we want to get only instances of the generic product category, the class to
be used in the query is C_AKK255001-gen, not C_AKK255001-tax. The later
could be used to determine all products that fall in the respective taxonomy
category. For example, a store manager might want to see all products in this
product segment, including maintenance and spare parts for agricultural machines.
Just changing the class ID in the query to C_AKK255001-tax would return
exactly that.
5. Conclusion
In this paper, we have proposed a novel “gen/tax” approach that allows the
mechanized transformation of taxonomies into OWL or RDF-S ontologies based
8 Martin Hepp
on the representing the original taxonomy node in two concepts, one for the generic
concept and one for the taxonomic concept, plus a third convenience class for
easing annotation. We have applied our approach to the task of creating an industry-
strength products and services ontology based on eCl@ss. The approach has the
disadvantage that it increases the number of classes in the resulting ontology. We
do not think that this is a significant problem, especially since performance-wise,
this will likely be outweighed by possibility to use a very simple and more scalable
reasoner. It is noteworthy that ontology languages without the limitations of OWL
or RDF-S allow a more straightforward modeling of the original semantics by just
creating a transitive relationship “taxonomySubClassOf”. This is especially true for
all variants of WSML and WRL. Still it is important to observe the fact that the
interpretation of the taxonomic relation in informal taxonomies is a crucial
modeling decision when deriving ontologies from such existing taxonomies. In
WSML and WRL, our findings can be represented without the OWL/RDF-S-
specific workaround of separate classes per each category in the taxonomy.
Acknowledgements: The work presented in this paper is partly funded by the
European Commission under the project DIP (FP6-507483), the TransIT
Entwicklungs- und Transfercenter at the University of Innsbruck, and Florida Gulf
Coast University.
References
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RNTD Content, Coverage, and Maintenance", accepted for IEEE ICEBE 2005, Beijing, China, 2005.
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