A Visual Notation for the Integrated Representation of OWL Ontologies

Stefan Negru

and Steffen Lohmann

Faculty of Computer Science, Alexandru Ioan Cuza University, General Berthelot 16, 700483 Iasi, Romania

Institute for Visualization and Interactive Systems (VIS), Universit¨atsstraße 38, 70569 Stuttgart, Germany

Keywords:

Ontology Visualization, Visual Notation, OWL, Semantic Web, Knowledge Visualization, Ontologies.

Abstract:

The paper presents a visual notation for the Web Ontology Language (OWL) providing an integrated view

on the classes and individuals of ontologies. The classes are displayed as circles, with the size of each circle

representing its connectivity in the ontology. The individuals are represented as sections in the circles so that

it is immediately clear from the visualization how many individuals the classes contain. The notation can

be used to visualize the property relations of either the classes (conceptual layer) or of selected individuals

(integrated layer), while certain elements are always shown for a better understanding. It requires only a small

number of graphical elements and the resulting visualizations are comparatively compact. Yet, the notation is

comprehensive, as it deﬁnes graphical representations for all OWL elements that can be reasonably visualized.

The applicability of the notation is illustrated by the example of the Friend of a Friend (FOAF) ontology.

1 INTRODUCTION

The Web of Data has attracted large interest for both

data publishers and consumers in the last years. Sev-

eral organizations have begun publishing their data in

RDF (Resource Description Framework)

and link-

ing it with other data, leading to a signiﬁcant growth

in RDF triples available on the Web of Data (Bizer

et al., 2009). As (Dadzie and Rowe, 2011) point out

“making sense of such data presents a huge challenge

to the research community”. User-centered visualiza-

tions are considered essential to help people under-

standing the data.

Ontologies play a key role in this context. An on-

tology is deﬁned as “a set of knowledge terms, includ-

ing the vocabulary, the semantic interconnections and

some simple rules of inference and logic, for some

particular topic” (Hendler, 2001). The primary pur-

pose of an ontology is to classify things in terms of

semantics for a speciﬁc domain. Ontologies provide

the backbone for publishing RDF data in a semanti-

cally meaningful way. Especially the Web Ontology

Language (OWL)

is widely used to deﬁne the con-

ceptual schema, the classes, individuals, and the re-

lationships between them. However, OWL is mainly

designed for use by applications in order to support

interoperable data exchange, machine processing, and

http://www.w3.org/RDF/

http://www.w3.org/2004/OWL/

reasoning. Even though it provides great means to de-

scribe and integrate information on the Web, its text-

based representation is difﬁcult to understand for av-

erage users.

While the World Wide Web Consortium (W3C)

provides a notation for the visualization of RDF

graphs

, it does not provide a notation for OWL.

Since any OWL ontology can be represented as RDF

graph, it can also be visualized using the RDF no-

tation. However, as every single RDF statement is

separately visualized, this can result in very large vi-

sualizations with plenty of nodes and edges. Such

RDF visualizations of OWL ontologies are not only

hard to read but they also fail to adequately reﬂect the

OWL semantics, as the OWL constructs are split up

into multiple RDF triples.

Therefore, research has addressed the issue of

providing visualizations for OWL ontologies. How-

ever, most of the existing approaches representclasses

and individuals in a mutually exclusive manner, with-

out providing an integrated view. Moreover, differ-

ent graphical representations are used to depict the

classes, individuals, properties, and their relation-

ships. An integrated view on ontologies that uses a

uniform graphical representation can therefore be re-

garded as crucial for a better understanding of ontolo-

gies and linked data.

http://www.w3.org

http://www.w3.org/RDF/Validator/

308

Negru S. and Lohmann S..

A Visual Notation for the Integrated Representation of OWL Ontologies.

DOI: 10.5220/0004373003080315

In Proceedings of the 9th International Conference on Web Information Systems and Technologies (WEBIST-2013), pages 308-315

ISBN: 978-989-8565-54-9

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

In order to ﬁll this gap, this paper presents a vi-

sual notation for OWL ontologies that represents both

classes and individuals in a comprehensive yet com-

pact manner. A central goal is to provide both expert

and non-expert users with an easy-to-understand no-

tation optimized for OWL ontologies. The key idea

of the approach is to offer an integrated view on on-

tologies, where classes are represented as differently

sized circles that are shown along with the individuals

they contain.

In the following, we ﬁrst summarize related work

in Section 2. In Section 3, we present the developed

visual notation for OWL ontologies. In Section 4, we

illustrate the applicability of the notation by the exam-

ple of the Friend of a Friend (FOAF) ontology. The

paper ends with a conclusion and an outlook on future

work in Section 5.

2 RELATED WORK

Several approaches for the visual representation of

ontologies and linked data have been presented in the

last years (Guo and Chan, 2011; Dadzie and Rowe,

2011; Katifori et al., 2007; Geroimenko and Chen,

2006). Most of them are based on a two-dimensional

plane, though some propose to use 3D representa-

tions. Little surprising, most users “seem to dislike

chaotic and too cluttered overviews” but “tend to pre-

fer visualizations that offer the possibility of an or-

derly and clear browsing of the presented informa-

tion” (Katifori et al., 2007). Finding a good balance

between comprehensiveness and ease-of-use is one of

the key challenges. Therefore, many works show ei-

ther only a part of the ontology or end up with very

complex representations.

A number of visualization approaches have been

implemented as plugins for ontology editors like

Prot´eg´e

. Some of them (e.g. OWLViz

) simply visu-

alize the class hierarchies of ontologies, while others

(e.g. OntoGraf

), allow for more comprehensivevisu-

alizations that include a representation of individuals

and different property relations. Yet, no clear distinc-

tion between property types or between classes and

individuals is made in the graphical representation of

OntoGraf. The notation rather relies on colors and

abstract symbols to indicate different node and link

types.

A more complete OWL visualization is provided

by GrOWL (Krivovet al., 2007), a graphical editor for

http://protege.stanford.edu

http://protegewiki.stanford.edu/wiki/OWLViz

http://protegewiki.stanford.edu/wiki/OntoGraf

OWL ontologies. It has been particularly designed for

the sublanguage OWL DL and the semantics of De-

scription Logic (DL), for instance, by offering sepa-

rate visualizations for the TBox, ABox, and RBox of

an ontology (Rudolph, 2011). However, the graphical

representation quickly becomes complex and large in

size, as every individual is deﬁned in a separate node.

Work has also been done in the direction of

representing ontologies as UML class diagrams

mostly by deﬁning mappings between elements from

OWL and UML (Craneﬁeld, 2001). The Object

Management Group (OMG) formally speciﬁed such

mappings in the Ontology Deﬁnition Metamodel

(ODM)

. One implementation in this context is OWL-

GrEd (Barzdins et al., 2010), a graphical OWL ed-

itor that uses UML elements for the representation

of ontologies. A disadvantage of these approaches is

again their limited visual scalability, especially when

it comes to the representation of a large number of

individuals.

Other approaches focus on more speciﬁc visual-

ization tasks. For instance, the RelFinder (Heim et al.,

2010) visualizes relationships between individuals in

ontologies and makes these relationships interactively

explorable. It nicely depicts a limited set of instance

data and helps to discover commonalities, but it does

not provide a global picture of the ontology.

OntoTrix (Bach et al., 2011) presents a hybrid net-

work visualization technique that uses both node-link

diagrams and adjacency matrices to represent parts

of the ontology. Though an interesting combination

of visualization techniques, it relies on the users’ ca-

pabilities to correctly interpret the matrix representa-

tions and the used color-coding, which can be prob-

lematic at least for average users.

Finally, there are also visualization approaches

that represent the classes and individuals in ontolo-

gies as differently sized circles. One such work is

CropCircles (Wang and Parsia, 2006), which visu-

alizes the OWL class hierarchy using nested circles.

The size of the circles is proportional to the position

of the classes in the hierarchy. OOBIAN Insight

also makes use of nested circles to visualize ontology-

based data. Like in GrOWL, the visualization quickly

becomes complex and large in size, making it hard

to read. Another example of visualizing ontologies

with nested circles is the DOPE Browser presented in

(Stuckenschmidt et al., 2004), where individuals are

grouped into circle-shaped class representations and

linked together to form a cluster map.

The review of related work showed that most ex-

http://www.uml.org

http://www.omg.org/spec/ODM/

http://dbpedia.oobian.com

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309

isting approaches focus on the terminological part of

ontologies (i.e. the classes and their relationships),

while little emphasis is placed on the individuals be-

longing to the classes (i.e. the actual data). Further-

more, many of the proposed visualizations do not pro-

vide an explicit description of the visual notation, i.e.

a speciﬁcation that clearly deﬁnes the semantics of the

graphical elements.

In order to overcome these limitations, we devel-

oped a visual representation that allows for an inte-

grated view on the TBox and ABox of ontologies.

Furthermore, all mappings between the graphical rep-

resentations of our notation and OWL elements are

clearly speciﬁed.

3 VISUAL NOTATION FOR

ONTOLOGIES

For a better understanding of our visual notation, we

need to relate to ontologies both in terms of Descrip-

tion Logic (DL) (Baader et al., 2003; Rudolph, 2011)

and the Web Ontology Language (OWL): In OWL,

there are classes, properties, and individuals; simi-

larly, in DL we have concepts (classes), roles (prop-

erties), and individuals. In order to clearly separate

concepts from individuals, we adopt the classical DL

distinction of TBox and ABox. In contrast to GrOWL

(Krivov et al., 2007), we do not have a separate RBox

view but included it in the TBox. Together, TBox

and ABox form the knowledge base that can be de-

scribed by one or more OWL ontologies. We can thus

distinguish between three layers that may be visual-

ized: The ﬁrst two layers follow the TBox and ABox

distinction, while their combination – the integrated

layer – represents the complete ontology:

1. Conceptual Layer – Represents the classes,

properties, and their relationships;

2. Instance Layer – Represents the individuals and

their relationships;

3. Integrated Layer – Represents the classes popu-

lated with individuals.

3.1 Representation of OWL Elements

We developed graphical representations for a large

part of the elements deﬁned by OWL. For some OWL

elements, we do not provide a graphical representa-

tion, as we either do not think they can be reasonably

visualized or consider a text-based presentation of the

corresponding information more appropriate. Fur-

thermore, we distinguish between primary elements

Class Label

3 instances

+ 2 inferred

instances

Figure 1: Individuals are visualized as sections of a circle.

that should be visualized as they are key to the un-

derstanding of the ontology, and additional elements

that should only be included in the visualization if

needed. In Table 1, we present the primary OWL el-

ements along with their graphical representations and

short descriptions. The graphical representations of

some additional OWL elements that may also be vi-

sualized are listed in Table 2. Note that some of the

elements are from the RDF Schema (RDFS) vocabu-

lary

– they are included in the notation, as they are

reused in OWL.

Elements from the ontology header, such

as owl:imports, owl:BackwardCompatibleWith,

owl:incompatibleWith, owl:priorVersion, and

owl:versionInfo, are best represented as text

in an infobox that complements the visual-

ization (as in the later described example of

Figure 2). Elements such as owl:onProperty,

owl:Ontology, owl:AllDifferent, owl:differentFrom,

owl:hasValue, owl:distinctMembers, owl:Restriction,

owl:AnnotationProperty, owl:OntologyProperty,

owl:allValuesFrom, owl:someValuesFrom, and

owl:DataRange (rdfs:datatype in OWL 2) are also

not visually represented, since they serve as con-

tainers for other elements or deﬁne constraints for

populating the ontology. They are important for

editing the ontology or for checking its validity but

they can be omitted in the visualization.

Individuals are displayed inside the classes they

are instances of. Each individual makes up one sec-

tion of the circle that represents the class (see Fig-

ure 1). Apart from individuals deﬁned by rdf:type, the

class may include individuals that are deﬁned by an

enumeration or alike (e.g. owl:oneOf). Superclasses

contain the individuals of their subclasses, but these

individuals are shown in gray to avoid confusion and

misinterpretation.

The connectivity of the classes with regard to the

domain described by the ontology is indicated by the

size of the circles. Thissize is determined by the num-

ber of incoming and outgoing property relations (the

more relations, the larger the circle). The owl:Thing

class has a ﬁxed size for the reason that it does usu-

ally not carry important domain information, as we

will discuss in the next section.

http://www.w3.org/TR/rdf-schema/

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Table 1: Graphical representations of primary OWL elements.

Element Graphical representation Description

owl:Class

Class Label

Circle with round label (rdfs:label) in the mid-

dle. Classes from imported ontologies are dis-

played entirely in gray (incl. the label back-

ground).

owl:Thing

Thing

Simple circle with label Thing.

owl:ObjectProperty,

owl:DatatypeProperty

Domain

Range

Object Property

Range

Domain

Datatype Property

Line or arc annotated with the label of the prop-

erty. The label’s background color indicates the

type of the property (object or datatype prop-

erty).

rdfs:subClassOf,

rdfs:subPropertyOf

Property [Subproperty]

Dotted line/arc with an unﬁlled arrow head at

the end pointing to the superclass or superprop-

erty. Subproperties having the same domain and

range as their superproperties can alternatively

be listed in square brackets within the label of

the superproperty.

rdfs:domain,

rdfs:range

Domain

Range

Property

Arrow starting at the domain and pointing to the

range. Datatype properties do not need an ar-

row, as they are always directed to the literal or

data value.

owl:inverseOf

Property

Inverse Property

Domain

Range

Line/arc with arrow heads at both ends and an-

notated with the label of the property and its in-

verse counterpart. Filled arrow heads at the end

of the labels indicate the direction of the prop-

erties (i.e. their range).

owl:disjointWith

Dashed line/arc with \\ symbol as label.

owl:FunctionalProperty,

owl:InverseFunctionalProperty,

owl:TransitiveProperty

Domain

Range

Functional Property

Domain

Range

Inverse Functional Property

InvF

Domain

Range

Transitive Property

Object property representation with an F for

functional properties, InvF for inverse func-

tional properties, and T for transitive properties

on top of the property label.

owl:SymmetricProperty

Class Label

Symmetric Property

Object property representation with identical

domain and range, i.e. start and end point of

the arrow is the same class.

owl:cardinality,

owl:minCardinality,

owl:maxCardinality

Domain

Range

Cardinality Property

min:1

max:1

Exact, minimal, and/or maximal cardinality

constraints are written above the property label.

owl:equivalentClass

Class Label,

Equivalent Class

Label

Colored ring around a class. The labels of

all equivalent classes are shown as comma-

separated list in the label, each colored in the

same font as the corresponding ring.

owl:unionOf Dashed line/arc with ⊔ symbol as label.

owl:intersectionOf Dashed line/arc with ⊓ symbol as label.

owl:ComplementOf Dashed line/arc with ¬ symbol as label.

Literals, data types and

data values

Literal

Rectangle containing the literal or data value.

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311

Table 2: Graphical representations of additional OWL elements.

Element Graphical representation Description

owl:DeprecatedClass

Class Label

Class representation colored in dark gray (with white

label background).

owl:DeprecatedProperty

Domain

Range

Property

Object/datatype property representation colored in

dark gray.

owl:equivalentProperty

{Property1, Property2}

Labels of equivalent properties are listed in curly

brackets.

rdfs:comment,

rdfs:seeAlso,

rdfs:isDeﬁnedBy

rdfs:label of individuals

Label

Inverted literal representation (i.e. black rectangle

with white font).

owl:sameAs

individual1 sameAs individual2

Inverted literal representation listing the labels of the

individuals connected with the keyword ‘sameAs’.

3.2 Speciﬁc Issues

While we developed the visual notation, we came

across some issues that we would like to address

in the following. One of these issues concerns the

owl:Thing and owl:Nothing elements. According to

the OWL speciﬁcation

, all classes are subclasses

of owl:Thing, while owl:Nothing is the subclass of

all classes. Many visualization approaches do not

represent these two elements (Katifori et al., 2007;

Guo and Chan, 2011). This is understandable if we

consider that owl:Nothing is an empty class and that

owl:Thing is usually not relevant for a particular do-

main but serves mainly as root class in the ontology.

However, at some point we might want to include the

owl:Thing class in the visualization to facilitate un-

derstanding of the ontology (e.g. to indicate that a

property does not have a speciﬁc domain or range).

In these cases, we recommend to use multiple repre-

sentations of owl:Thing in order to not give it an im-

portance in the visualization that it does not deserve.

For the basic set operations owl:intersectionOf,

owl:unionOf, and owl:ComplementOf, we reuse the

corresponding DL symbols (⊓, ⊔, and ¬), since they

are well-known to expert users. Because all three

OWL elements deﬁne new(anonymous) classes to de-

scribe the union, intersection, or complement of other

classes, we represented them as circles. If the domain

or range of a property links to one of these elements,

the line or arc of that property is directly connected to

the corresponding circle representation.

Another issue is the question of what happens if

certain classes, properties, or individuals do not have

an rdfs:label. In these cases, we recommend to dis-

http://www.w3.org/TR/owl-ref/

play a part of the URI as label (e.g. the characters

that follow the last slash or number sign). Related

to that, the URIs of classes, properties, or individuals

can generally be represented with the graphical repre-

sentation used for rdfs:comment (see Table 2).

We also took into account that color-blindness

does not have a strong effect on the readability of the

visual representation. In particular, key information is

never purely encoded by color. For instance, though

different colors are used for object and datatype prop-

erties, this information is also clear without color-

coding, as object properties always link pone individ-

ual with another, while datatype properties link indi-

viduals with data values.

Finally, it should be noted that the notation does

not make a distinction between the three OWL sub-

languages OWL Lite, OWL DL, and OWL Full, as

this is not relevant for the visual representation. It is

also compliant with OWL 2

and all its proﬁles.

3.3 Advantages

An advantage that is immediately visible from Table 1

is the small number of graphical elements needed to

represent a relatively large number of OWL elements.

Basically, we used only four types of symbols in dif-

ferent variations for the notation:

• classes = circles (size indicates the connectivity of

a class);

• individuals = sections in the circles;

• properties = lines/arcs (with different arrow

heads, line types, etc.);

http://www.w3.org/TR/owl2-overview/

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312

• literals, data types and values = rectangles.

Maybe the biggest advantage of the notation is the

integrated view that allowsto represent individuals in-

side the classes they are instances of.

4 USE CASE

We applied our visual notation to the well-known

Friend of a Friend (FOAF)

ontology to give an il-

lustration on how it can be used. Although FOAF

consists of a relatively small number of classes and

properties, it nicely showcases most of the graphical

representations of our notation.

4.1 Conceptual Layer

Figure 2 depicts the visual representation of the con-

ceptual layer of the FOAF ontology, i.e. it shows the

classes and subclasses along with their property rela-

tions. The circles of the classes Person, Agent, and

Document have the largest size, since these classes

have most property relations. As mentioned in the

previous section, we do not consider owl:Thing to be

of large interest in ontology visualizations. However,

we represented it using the multiplication technique

mentioned above, since some FOAF properties either

explicitly deﬁne it as domain or range or do only de-

ﬁne one of these two.

As the classes Spatial Thing

, Person

(from

the PIM ontology) and Concept

are imported from

other ontologies, they are displayed in gray (see de-

scription of the class notation in Table 1). The colored

ring around the circle representing the class Agent re-

sults from the fact that it is equivalent to the class

Agent

from the Dublin Core vocabulary. Its name

is added to the label of the circle and displayed in

the same color as the ring (in this case green). The

subproperty relations directing to page are explicitly

shown because the domain and range axioms of the

subproperties weblog, tipjar, openid, and is primary

topic of differ from those of page. If this was not

the case they could have been represented in the short

form that we also used for the subproperty relation of

homepage in Figure 2 and that is mentioned in Ta-

ble 1.

The class Label Property and the properties

phone, focus, funded by, logo, theme, DNA checksum,

http://xmlns.com/foaf/spec/ (version 0.98)

http://www.w3.org/2003/01/geo/wgs84 pos

http://www.w3.org/2000/10/swap/pim/contact

http://www.w3.org/2009/08/skos-reference/skos

http://purl.org/dc/terms/Agent

Given name, nickname, title, and name have no re-

lationships with the rest of the classes or properties.

Hence, they are listed separately in the top right of

Figure 2. Information from the header of the FOAF

ontology is shown in the infobox at the bottom right.

4.2 Integrated Layer

We also created a visual representation of the inte-

grated layer of the FOAF ontology using our nota-

tion and some sample data. This sample data con-

sists of 28 individuals that are distributed on the

FOAF classes as follows: 5 × Person, 7 × Docu-

ment, 5 × Image, 2 × Group, and 9 × Organiza-

tion. Though all these individuals are represented

as sections in the circles, the properties of only one

selected individual are shown (namely, “Jane Doe”).

The FOAF proﬁle of that individual in RDF/XML

syntax is as follows:

<rdf:RDF

xmlns:rdf="http://www.w3.org/1999/02/

22-rdf-syntax-ns#"

xmlns:foaf="http://xmlns.com/foaf/0.1/">

<foaf:Person rdf:ID="janedoe">

<foaf:name>

jane doe

</foaf:name>

<foaf:title>

Mrs.

</foaf:title>

<foaf:givenname>

jane

</foaf:givenname>

<foaf:family_name>

doe

</foaf:family_name>

<foaf:nick>

jane

</foaf:nick>

<foaf:mbox rdf:resource=

"mailto:janedoe@example.com"/>

<foaf:homepage rdf:resource=

"www.example.com/janedoe"/>

<foaf:depiction rdf:resource

="janedoe.jpg"/>

<foaf:workplaceHomepage rdf:resource

="www.work.com/janedoe"/>

<foaf:knows>

<foaf:Person>

<foaf:name>

john doe

</foaf:name>

<foaf:mbox rdf:resource

="mailto:johndoe@xample.com"/>

</foaf:Person>

</foaf:knows>

</foaf:Person>

</rdf:RDF>

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Agent, Agent

Document

Label

Property

Online

Account

Project

Spatial

Thing

Person

Group

Organiza-

tion

Imagine

Personal

Prole

Document

Online Chat

Account

Online

E-commerce

Account

Online

Gaming

Account

Person

depiction

image

focus

funded by

logo

phone

publications

schoolHomepage

theme

thumbnail

work info

homepage

Thing

Literal

family_name

Literal

Title: Friend of a Friend (FOAF) vocabulary

About: http://xmlns.com/foaf/0.1/

Description: The Friend of a Friend (FOAF) RDF

vocabulary, described using W3C RDF Schema and

the Web Ontology Language.

workplage

homepage

account

service homepage

account

interest

member

past

project

current

project

personal

mailbox

topic_interest

is primary

topic of

primary

topic

page [hompage]

topic

depicts

made

maker

knows

based near

myerBriggs

rstName

plan

surname

age

birthday

gender

status

Concept

Thing

focus

openid

Given Name

nickname

title

DNA checksum

geekcode

Literal

lastName

sha1sum (hex)

account

name

InvF

weblog

tipjar

InvF

Literal

sha1sum of a

personal

mailbox URI

name

Literal

AIM chat ID, ICQ chat ID, MSN chat ID,

Yahoo chat ID, jabber ID, skypeID

Figure 2: Visual representation of the conceptual layer of the FOAF Ontology (version 0.98).

Online

Account

Person

Personal

Prole

Document

Online Chat

Account

Online

E-commerce

Account

Online

Gaming

Account

depiction

image

publications

schoolHomepage

work info

homepage

Thing

Literal

workplage

homepage

account

service homepage

account

interest

past

project

current

project

personal

mailbox

topic_interest

is primary

primary

topic

page

[hompage]

topic

depicts

made

maker

myerBriggs

plan

surname

age

birthday

gender

status

openid

geekcode

account

name

InvF

weblog

tipjar

InvF

Yahoo chat ID, jabber ID, skypeID

Jane Doe

Jane

Doe

mailto:janedoe@example.com

Thing

1 instance

Mrs.

title

workplage

homepage

Spatial

Thing

Person

5 instances

knows

janedoe

johndoe

Group

2 instances

Organization

9 instances

Agent

16 inferred

instances

Image

5 instances

janedoe.jpg

Document

7 instances

+ 5 inferred

instances

www.example.com/janedoe

www.work.com/janedoe

homepage

InvF

topic

depicts

depiction

personal

mailbox

InvF

name

givenname

family_name

5 inferred

instances

Figure 3: Visual representation of the integrated layer of the FOAF Ontology (with sample data).

Figure 3 shows the integrated view for that indi-

vidual. We can observe how ‘Jane Doe’ is linked in

the ontology by following her relations with other in-

dividuals and with associated data. The general topol-

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ogy of the graph visualization and the positions of the

classes are roughly the same as in the conceptuallayer

presented in Figure 2. Also, the subclass and dis-

joint relations between the classes are still displayed.

However, classes without individuals have been faded

out, as the integrated layer focuses on the classes that

contain individuals. The number of individuals each

class contains is also shown in the integrated layer.

The notation distinguishes between individuals that

are inferred from subclasses and individuals that are

instances of the class itself. Inferred individuals are

presented in a light gray while the other individuals

have random colors.

5 CONCLUSIONS

In this paper, we presented a visual notation for OWL

ontologies. The notation provides a comprehensive

yet compact representation of the ontology that re-

quires only a relatively small number of graphical el-

ements. In contrast to most existing approaches, it

explicitly deﬁnes a graphical representation for each

OWL element that can be reasonable visualized. Fur-

thermore, it offers an integrated view on ontologies

that shows classes (circles) along with their individ-

uals (sections in the circles) and property relations

(connecting lines and arrows). We illustrated the ap-

plicability of the visual notation on the example of the

FOAF ontology.

The speciﬁcation of the notation is published un-

der a persistent URL on the web

so that it can easily

be found and used by others to visualize OWL on-

tologies. Furthermore, we plan to develop a tool that

implements the notation and adds interactive features

to it, allowing for a more detailed exploration of the

classes, individuals, and relationships in ontologies.

ACKNOWLEDGEMENTS

This work was partially supported by the European

Social Fund in Romania, under the responsibility of

the Managing Authority for the Sectorial Operational

Program for Human Resources Development 2007-

2013 [grant POSDRU/107/1.5/S/78342].

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