=Paper=
{{Paper
|id=None
|storemode=property
|title=Graphical Representation of OWL 2 Ontologies through Graphol
|pdfUrl=https://ceur-ws.org/Vol-1272/paper_59.pdf
|volume=Vol-1272
|dblpUrl=https://dblp.org/rec/conf/semweb/ConsoleLSS14
}}
==Graphical Representation of OWL 2 Ontologies through Graphol==
https://ceur-ws.org/Vol-1272/paper_59.pdf
Graphical Representation of
OWL 2 Ontologies through Graphol
Marco Console, Domenico Lembo, Valerio Santarelli, and Domenico Fabio Savo
Dipartimento di Ingegneria Informatica, Automatica e Gestionale “Antonio Ruberti”
S APIENZA Università di Roma
{console,lembo,santarelli,savo}@dis.uniroma1.it
Abstract. We present Graphol, a novel language for the diagrammatic represen-
tation of ontologies. Graphol is designed to offer a completely visual representa-
tion to the users, thus helping the understanding of people not skilled in logic. At
the same time, it provides designers with simple mechanisms for ontology edit-
ing, which free them from having to write down complex textual syntax. Through
Graphol we can specify SROIQ(D) ontologies, thus our language essentially
captures the OWL 2 standard. In this respect, we developed a basic software tool
to translate Graphol ontologies realized with the yEd graph editor into OWL 2
functional syntax specifications.
1 Introduction
Ontologies have become popular in recent years in several contexts, such as
biomedicine, life sciences, e-commerce, enterprise applications [9]. Obviously, it is
very likely that people operating in such contexts are not experts in logic and generally
do not possess the necessary skills to interpret formulas through which ontologies are
typically expressed. This turns out to be a serious problem also in the development of
an ontology. Indeed, ontologists usually work together with domain experts, the former
providing their knowledge about ontology modelling and languages, the latter provid-
ing their expertise on the domain of interest. During this phase, communication between
these actors is fundamental to produce a correct specification.
The use of a graphical representation for ontologies is widely recognized as a means
to mitigate this communication problem. At the same time, the possibility of specifying
ontologies in a graphical way might bring software analysts and experts in conceptual
modelling to approach ontology modelling, since they would be provided with mecha-
nisms that are close in spirit to those they usually adopt for software design.
Various proposals in this direction exist in the literature, but to date graphical lan-
guages for ontology have not become very popular, especially for the editing task.
Among various reasons, we single out the following points: (i) many languages for
graphical representation of ontologies do not capture the current standard OWL 2, and
their extension to it is not straightforward (see, e.g., [3,8,7,6,1]); (ii) other proposals
require the use of formulas mixed with the graphical representation (see, e.g., [4,1]);
(iii) popular ontology management tools, such us Protégé1 or TopBraid Composer2 ,
offer visualization functionalities, but do not support a completely graphical editing.
1
http://protege.stanford.edu
2
http://www.topquadrant.com/tools
To meet the main disadvantages mentioned above, in this paper we present our
proposal for graphical specification and visualization of ontologies, and introduce the
novel Graphol language, whose main characteristics can be summarized as follows:
– Graphol is completely graphical (no formulae need to be used in our diagrams) and
adopts a limited number of symbols. In Graphol, an ontology is a graph, whose
nodes represent either predicates from the ontology alphabet or constructors used
to build complex expressions from named predicates. Then, two kinds of edges
are adopted: input edges, used to specify arguments of constructors, and inclusion
edges, used to denote inclusion axioms between (complex) expressions.
– Graphol has a precise syntax and semantics, which is given through a natural en-
coding in Description Logics.
– Such enconding shows that Graphol subsumes SROIQ(D), the logical underpin-
ning of OWL 2.
– Graphol is rooted in a standard language for conceptual modeling: the basic com-
ponents of Graphol are taken from the Entity-Relationship (ER) model. Notably,
simple ontologies that correspond to classical ER diagrams (e.g., some OWL 2 QL
ontologies) have in Graphol a representation that is isomorphic to the ER one.
– Graphol comes with some basic tools that support both the graphical editing and
the automatic translation of the diagrams into a corresponding OWL 2 specifica-
tion, to foster the interoperation with standard OWL reasoners and development
environments.
We have adopted Graphol in various industrial projects, where we have produced
large ontologies with hundreds of predicates and axioms. In such projects we could
verify the effectiveness of the language for communicating with domain experts. At the
same time, we exploited Graphol in the editing phase: all ontologies realized in these
projects have been indeed completely specified in our graphical language, whereas an
OWL functional syntax encoding thereof has been obtained automatically through the
use of our translator tool. One of these experiences is described in [2], where the impact
of the use of Graphol on the quality of the realized ontology is widely discussed.
We also conducted some user evaluation tests, where both designers skilled in con-
ceptual modelling (but with no or limited experience in ontology modelling) and users
without specific logic background were involved. From these tests, we obtained promis-
ing results about the effectiveness of our language for both visualizing and editing on-
tologies. A complete description of our evaluation study is given in [5].
For a complete description of both the syntax and the semantics of Graphol we refer
the reader to [5] and to the Graphol web site3 , where it is also possible to download
currently available software tools for our language. In the rest of the paper we instead
discuss how the Graphol demonstration will be carried out.
2 The Graphol demonstration
In this demo we will show the process we devised to obtain an OWL 2 ontology starting
from the specification of a Graphol diagram. Such a process relies on both existing
3
http://www.dis.uniroma1.it/˜graphol
Fig. 1: A simple Graphol ontology
open source tools and original software components. More in detail, to draw a Graphol
ontology we make use of the yEd editor for graphs4 , which we equip with a palette
containing all and only the symbols needed for Graphol. yEd allows us to save the
ontology in GraphML5 , a popular XML-based file format for encoding graphs.
An example of a Graphol ontology obtained through yEd is given in Figure 1. In
the figure, the reader can see that in Graphol classes (i.e., Person, Car maniac, Car),
object properties (i.e., is owner of car), and data properties (i.e., age) are modeled
by labeled rectangles, diamonds, and circles, respectively, similarly to ER diagrams.
The white (resp. black) square labeled with exists is a graphical constructor that takes
as input a property, through a dashed arrow whose end node is a small diamond, and
returns the domain (resp. the range) of the property. Such squares can have also different
labels, to denote different constructs. In the example, the label (4,-) on the white
square taking as input the is owner of car property specifies a cardinality restriction
on the domain of such property, i.e., it denotes all individuals participating at least 4
times to is owner of car. The solid arrow always indicates a subsumption relation.
This means that Car maniac is a subclass of Person, and also of the complex class
obtained through the cardinality restriction, which implies that a car maniac owns at
least four cars.
Furthermore, the ontology
in the example says that the
domain of is owner of car is
Person, its range is Car, and
also that each Person has an
age, and that the domain of
age is Person. Also, the ad-
ditional dash orthogonal to the
edge connecting age to its do-
main specifies that this property
is functional.
The above example uses
only a limited sets of construc-
tors available in Graphol. Par-
ticipants to the demo will be
provided with the yEd editor Fig. 2: The Graphol2OWL tool
and the Graphol palette to draw
their own ontologies, experiencing the entire expressive power of the language.
4
http://www.yworks.com/en/products_yed_about.html
5
http://graphml.graphdrawing.org/
To both check the correctness of the specification and translate it into OWL 2, we
developed a dedicated tool. The tool provides a syntactic validation of a given diagram:
while parsing the GraphML file, if a portion of the graph is found that does not respect
the Graphol syntax, the tool reports an error to the user in a pop-up window and visual-
izes this portion by means of an external yEd viewer. A screenshot of this tool showing
an error identified in a Graphol diagram is given in Figure 2. In this example, the error
consists in linking a class to a property with a solid arrow, which actually corresponds
to a wrong subsumption between a concept and role.
The translator to obtain OWL 2 encodings from Graphol will be used during the
demo. We will also show the compatibility of the produced OWL 2 functional syntax
file with popular tools for ontology editing and management, like Protégé 6 .
3 Future Work
Our main future work on Graphol is the development of editing tools (stand-alone sys-
tems or plugins of existing ontology development environments) tailored to the specifi-
cation of ontologies in our graphical language and integrated with state-of the art rea-
soners. At the same time, we are working to improve ontology visualization in Graphol,
by investigating mechanisms to automatically extract ontology views at different levels
of detail on the basis of specific user requests.
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6
A preview of the demo is available at http://www.dis.uniroma1.it/˜graphol/
research.html.