An Ontology Framework for Semantic Web illustrating Ontology Merging Sanjay Kumar Malik1 , SAM Rizvi2

1 University School of Information Technology, GGS Indraprastha University, New Delhi 2 Deptt. of Computer Science,Jamia Millia Islamia, New Delhi

sdmalik@hotmail.com1 , samsam_rizvi@yahoo.com2

Abstract--Ontology’s significance cannot be denied for realizing the vision of semantic web to add semantics or computer understandable data to the existing human readable web as it enables to structure and conceptualize the shared knowledge of a particular domain on web. But Ontology is abstract and involves various complex issues. This paper presents a framework for such multifarious possible issues of ontology which have been briefly presented. One of the significant issues, ontology merging, has been illustrated with an example using protégé 3.4.1 editor.

Keywords: Semantic Web, Ontology, Ontology issues, Ontology framework , merging, protégé, prompt

I. INTRODUCTION

Semantic web refers to the future intelligent web where we will be able to search information meaningfully and more efficiently. It is one of the hot concern of research today in the era of web. It’s architecture as given by Sir Tim Berner’s LEE, founder of web, focuses on Ontology as one of the key layers. Semantic Web extends the current one by giving the web content a well defined meaning, better enabling computers and people to work in cooperation [1 The user community, including standard organizations like Internet Engineering Task Force(IETF) and World Wide Web Consortium(W3C), has directed major efforts in developing methods and procedures for meaningful & intelligent information retrieval on web[2]. Ontology is the platform for sharing the knowledge of various domains on the web but it is abstract and involves various complex issues which needs to be listed together. There is a need to frame all these significant multifarious issues in the form of a framework which have been briefed in this paper. According to Gruber’s definition,“an ontology is an explicit specification of a conceptualization”[3] where explicit means that it cannot be implicitly assumed and should be processable by machines. Mitra et al. [4] defines an ontology O as a directed labelled graph G = (N, E) where N is a finite set of labelled nodes and E is a finite set of

labelled edges. An edge e is written as a triplet (n1, , n2) where n1 and n2 are members of N and is the label of the edge. A class is represented in the form of vertex, relationship as edge, and instance as data records assigned to concepts or relation [5] which is illustrated in the equation: O (ontology) = (N, E), where n1, n2, n3… N (nodes) and e1, e2, e3… E (edges).

Considering all the issues of Ontology is out of scope of this paper. One of the significant issues ,ontology merging, has been focused here and illustrated with an example using protégé ontology editor. Ontology merging refers to the process of development of an extended global ontology from two or more local source Ontologies where the new ontology will unify or in general replace the original source Ontologies [8]. First, a pair of concepts are deemed to be suitable for merging, then proceed for physical merge at the attribute level where all local attributes are considered to be merged and then the process begins with grouping of all the conceptual based information[9] by taking care that the merged attributes does not contain redundant attributes. Merging involves various types, tools, techniques, approaches and challenges which are not in the scope of this paper. Generally, Ontologies are developed without keeping other ontologies in mind which results in most common type of inconsistencies to occur at the time of merging because of reasons like: “same term with different meanings”, “different name for the same concept”, “different definitions for the same concept”[6]. Protégé is a free, open-source ontology editor which supports two ways of modeling ontologies, namely Protégé-Frames and Protégé-OWL[7] where Prompt plug-in/tab is used for merging. “Merging” refers to merging different ontologies about the same subject into a single one that unifies all of them and “Integrating” refers to building a new ontology reusing other available ontologies. First, this paper, lists the various possible issues of ontology and presents a framework for them. Second, it illustrates ontology merging issue by merging similar ontologies of two University Schools by using prompt plugin of protégé 3.4.1 of IP (Indraprastha) University,Delhi.

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II. ONTOLOGY FRAMEWORK

The framework proposed for various issues of ontology is as follows in figure 2.1:

Fig 2.1: Ontology Framework

A few significant issues of Ontology are summarized as follows in table 2:

Table 2 : Summary of a few significant issues of Ontology

Ontology Issue Refers to

Creation Designing and developing an ontology from scratch or appending to an existing ontology

Merging merging different ontologies of same

type about the same subject into a single one that unifies all of them

Integration building a new ontology reusing other available ontologies

Deployment & Implementation

using for real life usage and applications in different domains like medical etc

Maintenance Management of existing ontologies efficiently

Tools/Methodologies Using different

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tools like protégé,swoop etc and methodologies

Selection Choosing an appropriate ontology

Validation/Evaluation Various ways of validating and evaluating an ontology ie in terms of appropriateness, usability and efficiency.

Import/Export Using one ontology into/ from another

Mapping/Matching/Alignment Mapping entities of different ontologies and matching them, align them if required accordingly.

Ontology comparison & ranking

Different parameters for comparing ontologies and rank them on some basis

Query Execution Executing SPARQL Queries on OWL Code for output

Others Evolution, Versioning, Translation, Documentation, web usage mining, Ontology Engineering, Storage etc.

Each one of the above issues is abstract and concern of detailed discussion and research but are not in the scope of this paper.

III. ONTOLOGY MERGING While merging two Ontologies, in the first step, two ontologies are created using protégé 3.4.1, viz, “University School of Chemical Technology(USCT)Ontology” and another as

“University School of Bio Technology(USBT)Ontology”. Then, we discuss how these two ontologies are merged using PROMPT tab in protégé showing various screenshots. Figure 3.1 shows the super class-sub class hierarchy in the USCT Ontology which comprises of super classes like “Persons”, Staff etc and sub classes like “Clerk”.

Figure 3.1 and 3.2 shows the super class-sub class hierarchy of USCT and USBT Ontology which comprises of super classes like “Persons” and “Programmes” etc and sub classes like “Staff” etc and so on.

Figure 3.1: Super class-Sub class Hierarchy in University School of Chemical Technology (USCT) Ontology

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Figure 3.2: Super class-Sub class Hierarchy in University School of Bio Technology(USBT) Ontology

Now, let us merge these two ontologies using “Merge” option of Protégé. In figure 3.2a, we proceed for merging the above two ontologies by selecting the “merge” option in “prompt” tab. “Prompt” may be used to Merge two ontologies into one or compare existing version of ontology with another or similar such other issues.

Then, choose the first and second source ontologies with the algorithm to be used in “initial comparison” like lexical matching etc and then submitted for “source comparison” and “name matching” and finally click to begin merging. Suggestions screen is obtained as shown in fig 3.3.

Figure 3.2a: Selecting Merge Option

Figure 3.3: Suggestion shown by PROMPT tab for merging two ontologies

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Now, click on “New operations” option to select our own suggestions like selecting the classes or subclasses or instances of source ontologies to be merged. Then, proceed for “Do it” option for merging the two ontologies. The merged ontology is obtained as shown in fig 3.4 which depicts merged ontology along with the two source ontologies. Conflicts, if any, may be known from the “conflicts” option. In our case, there are no conflicts due to similar ontologies.

Figure 3.4: Source and Result classes of two ontologies being merged

Finally the “merged ontology” may be viewed from option “classes” as shown in figure 3.5 .

Figure 3.5: Super class-Sub class hierarchy after merging two ontologies: Merged Ontology

A few key issues while merging are : • Structure, modules and assumptions of

Ontology. • Including Entities in Ontology and

their representation in classes etc. • Lexical matching of ontologies. • Finding inconsistencies and mismatches

in merging. • Extracting possible sub ontologies to be

merged, relationships and comparison • Sharing, reusing, mapping , unifying

ontologies and finally analysing the large ontology obtained.

IV. CONCLUSIONS AND FUTURE EXTENSION WORK

This paper briefly lists down various significant issues of an Ontology collectively in the form of a framework. Presenting these issues in the form of a framework may be useful for researchers who are beginner’s in Ontology research to know the various possibilities. Each of these issues is

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abstract and a topic of in-depth research but is out of scope of this paper. Considering an issue of ontology merging with an illustration gives an idea of merging two ontologies using an editor.

In future, as an extension of this paper, these issues of ontology may be collectively or individually may be taken as topic of in-depth research.

V. REFERENCES

[1]Tim Berner’s Lee, J.Hendler, O.Lossila, “The Semantic web.”, Scientific American, May 2001. [2]Tim Berners-Lee , Nigel Shadbolt and Wendy Hall,“The Semantic Web Revisited,” IEEE May/June 2006. [3] T.Gruber, Towards principles for the design of ontologies used for knowledge sharing. Int. J. of Human and Computer Studies, 43(5/6):907–928, 1995 [4] P. Mitra, G. Wiederhold, M. Kersten. A graph oriented model for articulation of ontology interde-pendencies. Proc. Extending DataBase Technologies, Springer, Berlin Heidelberg, 2000, LNCS 1777, 86–100. [5] J. Davies, R. Studer, P. Warren. Semantic Web Technologies Trends and Research in Ontology-based Systems. John Wiley & Sons Ltd, 2006. [6] Huang, Z., van Harmelen F., Teije, A.,T., Groot, P., and Visser, C., “Reasoning with Inconsistent Ontologies: A General Framework”, EU-IST Integrated Project (IP) IST-2003-506826 SEKT, 2005 [7] http://protege.stanford.edu/ [8] Jos de Bruijn, Marc Ehrig, Cristina Feier, Francisco Martin-Recuerda, Francois Scharffe and Moritz Weiten, “Ontology Madiation, Merging, and Aligning”, John Davies, Rudi Studer, Paul Warren, “Semantic Web Technologies: Trends and Research in Ontology-based Systems”, Wiley 2006, pp 102-104 [9] David A. Ostrowski, George M Schleis, “ Enterprise Ontology Merging for the semantic web”, Proceedings of 2008 International conference on semantic web and web services, WorldComp’08, July 14-17, Las Vegas, Nevada, USA [10] Natalya Fridman Noy and Mark A. Musen, “Prompt: Algorithm and tool for automated ontology Merging and Alignment” Stanford Medical Informatics, Stanford University, Stanford, CA 94305-5479.

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