A Virtual Solution for Integrating Coastal Web Atlases
Yassine Lassoued
, Dawn Wright
, Luis Bermudez
and Omar Boucelma
Coastal and Marine Resources Centre, University College Cork, Naval Base - Haulbowline, Cobh - Co. Cork, Ireland
Department of Geosciences, Oregon State University, Corvallis, Oregon, U.S.A.
Southeastern Universities Research Association (SURA), Washington DC, U.S.A.
Laboratoire des Sciences de l’Information et des Systmes
Avenue Escadrille Normandie Niemen, 13397 Marseille, France
Data Semantics, Semantic Web Technologies, Information retrieval, Geographic Information Systems (GIS),
Ontologies, Catalogue Service for the Web (CSW), Mediation.
In recent years significant momentum has occurred in the development of Internet resources for decision
makers and scientists interested in the coast. Chief among these has been the development of coastal web
atlases (CWAs). While multiple benefits are derived from these tailor-made atlases (e.g., speedy access to
multiple sources of coastal data and information), the potential exists to derive added value from the integration
of disparate CWAs, to optimize decision making at a variety of levels and across themes. This paper describes
the development of a semantic mediator prototype to provide a common access point to coastal data, maps and
information from distributed CWAs. The prototype showcases how ontologies and ontology mappings can be
used to integrate different heterogeneous and autonomous atlases, using the Open Geospatial Consortium’s
Catalogue Services for the Web.
The vast and heterogeneous amount of geospatial data
on the World Wide Web causes users to be informa-
tion overloaded (Kashyap and Sheth, 2000). Search
engines return millions of results, including non rel-
evant information, that are rarely taken into account.
A user (e.g. scientist or a coastal response manager)
would like to have as much relevant information as
possible integrated for a particular event and region.
This requires 1) providing a mechanism to allow uni-
fied discovery and access to distributed and heteroge-
neous data and 2) categorizing the results in a conve-
nient vocabulary to the end user.
Practically, data discovery relies on documenta-
tion provided as part of metadata (notably discovery
metadata) such as the dataset title, abstract, extent,
keywords, etc. In the context of distributed resources,
this information is present in different heterogeneous
formats, and systems, according to several existing
metadata models and standards.
The International Standardization Organization
(ISO) has recently defined metadata standards for
geospatial data, notably the ISO-19139 (ISO, 2006)
standard. The aim is to harmonize metadata represen-
tation and implementation by conforming to a unified
model, a unified structure and a unified format. The
Open Geospatial Consortium (OGC), in their turn, fo-
cus on developing standards for querying and trans-
porting data and metadata over the Internet. Specifi-
cally, the OGC Catalogue Service for the Web (CSW)
(Nebert and Whiteside, 2005) specification defines a
standard for advanced querying and transporting of
metadata records over the Web.
Despite these harmonization efforts, problems still
arise when dealing with metadata semantics. Ter-
minology used to describe similar data can vary be-
tween specialities or regions, which can further com-
plicate data searches and integration. For instance,
usage of of the word ”seabed” in Europe versus use
of the word ”seafloor” to describe the same feature
in North America is a good example of this scenario,
as is the interchangeable use of ”coastline” versus
”shoreline” in both regions. From both human and
computational standpoints, users need assurance that
the concepts, terminology, and even the abbreviations
that are shared between two or more individuals, sys-
tems, or organizations are understood by all to mean
Lassoued Y., Wright D., Ber mudez L. and Boucelma O. (2008).
ONTOLOGY-BASED MEDIATION OF OGC CATALOGUE SERVICE FOR THE WEB - A Virtual Solution for Integrating Coastal Web Atlases.
In Proceedings of the Third International Conference on Software and Data Technologies - ISDM/ABF, pages 192-197
DOI: 10.5220/0001891001920197
the same thing. In this way the quality of data retrieval
and subsequent data integration are greatly increased.
In this paper, we describe an ontology-based
mediation approach for performing geospatial data
search across different organizations. We use ontolo-
gies as the means to define semantics for metadata
values (terms such as keywords, places, etc.) within
organizations, but also to link terms from different or-
ganizations. An organization or a group of organi-
zations populate their metadata using a local CSW.
Their metadata records use a given ontology of terms
called local ontology. Human or machine users for-
mulate CSW requests using a common ontology of
metadata terms, called global ontology. A CSW me-
diator rewrites the user’s request into CSW requests
over local CSWs using their own (local) ontologies,
collects the results and sends them back to the user.
The paper is organized as follows. In section 2
we sketch the problem through a concrete integration
example, while in section 3, we present state of the
art interoperability techniques and technologies. In
Section 4 we detail our CSW mediation approach, and
in section 5 we present the implemented prototype.
Finally, we conclude in section 6.
The example described in this article is drawn from
the coastal web atlases (CWAs) integration problem
addressed by a new International Coastal Atlas Net-
work (ICAN) initiative (Wright et al., 2007).
A CWA is a Web geographic information system
composed primarily of coastal GIS data (vector data,
coverages, raster grids, and images), their associated
metadata, and thematic information about data (such
as textual descriptions, references, images, etc.).
Integrating several CWAs requires three different
levels of integration: integrating the GIS data, inte-
grating their metadata, and integrating the thematic
information that accompany data. The problem we
are focusing on in this article is metadata interoper-
ability, in other terms performing data discovery and
search across different CWAs in a transparent way.
We report here on the development of a prototype
as a proof-of-conceptto inter-relate metadata between
two initial CWAs: the Marine Irish Digital Atlas or
MIDA, [http://mida.ucc.ie], and the Oregon Coastal
Atlas or OCA, [http://www.coastalatlas.net]. It may
not be immediately obvious how Oregon and Ireland
may need to be interoperable, but these two mature
atlas efforts can be used as a testbed for interoperabil-
Both MIDA and OCA atlases implement the OGC
CSW for querying and delivering metadata records.
Metadata records in each of the CSWs use a given
local ontology (e.g. keywords, places, titles, etc.). In
order to facilitate search across both atlases, a central-
ized system needs to be implemented, which will pro-
vide unified and transparent access to the local CSWs.
Ideally, metadata records from both MIDA and OCA
will not be copied at the integrated level as both at-
lases are autonomous and are subject to evolution.
Rather the integration system will act as a mediator
(Wiederhold, 1992) that uses a common terminology
(metadata ontology) and will translate user queries,
on the fly, into queries over the atlases’ CSWs using
their own ontologies.
This section presents the state of the art of existing
techniques and technologies that are related to the
problem of semantic interoperability of GIS metadata
management systems. The following subsections will
explore the use of CSW to facilitate syntactic interop-
erability (c.f. subsection 3.1) and the use of media-
tion as integration approach (c.f. subsection 3.2).
3.1 OGC Catalogue Service for the Web
CSW (Nebert and Whiteside, 2005) is an OGC ab-
stract specification for supporting the ability to pub-
lish and search collections of descriptive information
(metadata) for data, services, and related information
objects. CSWs allow a unified access to metadata
records within a community or an organization, thus
harmonizing GIS resources discovery and search. For
this reason, CSWs are required for coastal atlases in
order to facilitate syntactic and schematic interoper-
CSWs support several operations. We focus here
on the
tt GetRecords operation for searching metadata
records, possibly using filters, such as keyword, lo-
cation and time search.
There exist several implementations of CSW. For
instance, both MIDA and OCA use GeoNetwork
[http://geonetwork-opensource.org] as a CSW imple-
3.2 Mediation
The database (DB) community has extensively stud-
ied and developed data integration approaches and
systems leading to, among others, a virtual approach
Integrating Coastal Web Atlases
to data integration called mediation (Wiederhold,
A mediation system provides the user with a uni-
form interface of the different data sources via a com-
mon model. In a typical mediation architecture (c.f.
Figure 1), several distributed data sources use their
own data schemas, called local schemas or source
schemas. Users pose queries over a common refer-
ence schema, called global schema. The mediator
uses mapping rules between the global schema and
the local schemas in order to rewrite the user’s query
into queries over the local data sources. It extracts and
reformulates the responses conforming to the global
schema and combines them in order to construct a re-
sponse which is as complete as possible. Mediators
often use a wrapper for each data source, for translat-
ing queries from the mediator’s query language into
the data source’s query language, and for converting
the source’s data into the mediator’s data model. All
is transparent to the user. That is, the user ignores
where and how data are stored and how the mediator
manages to retrieve data from their sources.
Query flow
Data flow
Source nSource 2Source 1
Mapping Rules
Global Schema
Wrapper Wrapper Wrapper
Figure 1: Typical Mediation Architecture.
Several mediation systems and prototypes have
been developed: examples of such systems are On-
tomet (Bermudez, 2004), TSIMMIS (Garcia-Molina
et al., 1997), PICSEL (Goasdoue et al., 2000), In-
formation Manifold (Kirk et al., 1995), AGORA
(Manolescu et al., 2001). Most of these existing
mediation approaches focus on data schemas hetero-
geneity and data complementarity and try to build re-
sponses which are as complete as possible.
The targeted integrated Coastal Web Atlas, called
atlas or super atlas, is a virtual atlas that of-
Please note that the term ”global” does not refer to the globe in
this context. Rather, it is the term used by the database community
fers transparent access to a variety of distributed and
heterogeneous local coastal atlases. The notion of
”virtual”, in this context, means that local atlas re-
sources are not integrated or copied at the integrated
level. Rather, they remain at their locations and are
remotely accessed, harmonized and integrated on the
fly depending on users’ requests. This allows a high
degree of independence and autonomy for the local
atlases and facilitates extendibility in an architecture
where atlases can be added and removed at any time
without affecting the global atlas, provided that they
implement core services including OGC CSW for
the delivery of metadata, Web Map Services (WMS)
(de la Beaujardiere, 2004) for the delivery of maps,
and WFS (Vretanos, 2005) for the delivery of vector
In this article, we focus on the data discovery and
search aspects. We propose an ontology-based me-
diation approach for OGC CSWs. The solution dif-
fers from the classical mediation approaches cited in
subsection 3.2 in the way that it deals with metadata
which are already in the same format, XML, and have
the same ISO-19139 schema. It does not try to com-
bine information from different sources (i.e. atlases or
CSWs); rather it focuses on the semantic values con-
tained within the metadata records and tries to solve
semantic conflicts between different applications, do-
mains, organizations, or simply CSWs. Our ap-
proach is ontology-based, i.e. it uses ontologies for
representing the semantics of data values as well as
for matching concepts of local ontologies with con-
cepts of the global ontology.
4.1 Architecture
The global atlas introduced above offers a virtual
CSW, called global CSW, which acts as a CSW me-
diator and which offers unified and transparent access
to the atlases’ CSWs. As illustrated in Figure 2, users
of the global CSW are provided with a global ontol-
ogy of terms
. The user refers to the global ontology
and formulates a CSW
request (c.f. sub-
section 3.1) using an area of interest and keywords de-
fined in the global ontology. The global CSW rewrites
the user’s request into CSW requests over the local
atlases’ CSWs using their local ontology terms, exe-
cutes the so-obtained requests, and collects metadata
records (responses) from local CWAs.
This architecture facilitates extensibility as new
catalogue services can be added and removed at any
to refer to the integrated data schema in a mediated approach as
opposed to local schemas.
The current use case topic that the ICAN group is fo-
cussing on is coastal erosion
ICSOFT 2008 - International Conference on Software and Data Technologies
OCA OntologyMIDA Ontology
Mapping Ontologies
CSW Query
CSW Response
Import Link
Global Ontology
CSW Mediator
X Ontology
Figure 2: Ontology-Based CSW Mediation Architecture.
time without affecting the global CSW, provided that
they come with the ontologies for the terms used by
their metadata records and that mappings between
these terms and the global ontology’s terms are pro-
vided. Another advantage of this architecture is that
the global CSW acts itself as a catalogue service,
which in its turn can be queried by another external
application or even integrated in a similar CSW me-
diation architecture, as a local CSW.
4.2 Global and Local Ontologies
A (global or local) CSW uses an ontology which de-
fines the terms used as values in its metadata records
(for example thematic keywords, places, etc.). In the
initial CSW mediator prototype, we only focus on val-
ues for keywords provided as part of metadata. Con-
forming to the ISO-19115 standard, five types of key-
words are defined: discipline, theme, place, temporal,
and stratum. For each atlas, an ontology of terms re-
lated to these five keyword types is defined. Relation-
ships between the terms contained within one ontol-
ogy are provided as part of the same ontology.
is a
is a
is a
Figure 3: Place terms from the MIDA ontology.
Figure 3 illustrates an extract of place keywords
from the MIDA ontology. Examples of places are
respectively Europe, Ireland and Cork. Relationships
between places such as ”Cork is within Ireland”, and
”Ireland is within Europe”, can be expressed. This
helps improve keyword search using an inference en-
gine. For example, if the place keywords for a dataset
only contain the term ”Cork”, and a user queries the
metadata catalogue using the place term ”Europe”
or ”Ireland” they still will get this dataset in the re-
sponse, because Cork is in Ireland and Ireland is in
Europe, which also infers that Cork is in Europe.
Ontologies are expressed in the OWL-DL lan-
guage (Herman, 2007) in the ICAN CSW mediator
4.3 Ontology Mappings
Ontology mappings link the global ontology to the lo-
cal ontologies. This link is crucial as it is the only
means to allow the CSW mediator rewrite user re-
quests expressed with terms from the global ontol-
ogy into requests over the local CSWs using their own
terms. Therefore, they act, as semantic translators.
For each local ontology, an OWL ontology called
mapping ontology defines the mappings between the
local ontology and the global one. A mapping ontol-
ogy imports both a local and the global ontology and
defines relationships between their concepts. An ex-
ample showing extracts of the MIDA and the OCA
mapping ontologies is illustrated in Figure 4.
In Figure 4, terms preceded by prefix global
are from the global ontology. Those preceded by
prefixes mida and oca are respectively from the
MIDA and OCA ontologies. Relationships repre-
sented with thin lines are defined as part of the lo-
cal ontologies. Those represented with thick lines are
defined as part of the mapping ontologies. For ex-
ample, Coastal Protection and Shore Stabilization are
defined in the MIDA and OCA mapping ontologies
as narrower terms than Human Responses to Coastal
4.4 Query Rewriting and Execution
Query rewriting is the most important task of the
mediator as it refers to the process of rewriting a
user query posed over a global schema into queries
over local data sources (in this context CSWs). We
mainly focus on the
CSW requests in
this section. Consider a human or software user
that poses a
request, searching for
metadata records available through the global CSW.
For instance, the following is a
formulated by a user interested in metadata records
about data covering any region all over the world and
related to Human Responses to Coastal Change.
Integrating Coastal Web Atlases
oca:Jettiesoca:Seawalls oca:RipRap
Figure 4: Extracts of the MIDA and the OCA Mapping Ontologies.
<?xml version="1.0" encoding="UTF-8"?>
<Filter xmlns="http://www.opengis.net/ogc"
<PropertyIsLike wildCard="%"
singleChar="_" escape="\">
<gml:lowerCorner>-180 -90</gml:lowerCorner>
<gml:upperCorner>180 90</gml:upperCorner>
As we only deal with keywords semantics in this
paper,the query rewriting process is quite simple. The
process consists in translating the global keywords
contained in the user’s queries into local keywords
and rewriting the initial request using the so-obtained
terms. In order to do so, the mediator starts by pars-
ing the user’s request. It identifies the clauses related
to keywords, and extracts the corresponding keyword
literals. For instance, in the example above, the only
clause related to keywords is the one delimited by the
tag, and the corresponding
keyword is Human Responses to Coastal Change.
For each local atlas, the CSW mediator uses its in-
ference engine to obtain all the local atlas’ terms that
are equivalent to, or narrower than, the keyword lit-
eral considered. Next, the initial clause is replaced
by a disjunction of clauses, each containing a key-
word literal corresponding to one of the so-obtained
local keywords. For instance, in the example above,
the CSW mediator will translate keyword Human Re-
sponses to Coastal Change into the MIDA keywords
Coastal Protection and Coastal Defence Structure.
Thus the corresponding clause will be rewritten ac-
cording to MIDA as follows:
<PropertyIsLike wildCard="%" singleChar="_"
<PropertyIsLike wildCard="%" singleChar="_"
This process is repeated for each clause in the re-
quest’s filter, of course by avoiding repetition of key-
words in a disjunction of clauses. Each so-obtained
request is sent to the corresponding
local CSW. Records obtained as results from the local
CSWs are then collected and sent back to the user.
A first version of the global coastal atlas prototype
has been implemented in Java, using the Jena 2
framework for inference purposes and is available at
The prototype allows the user to:
Select keywords from the global ontology;
Select an area of interest;
Submit a query, which will generate a CSW
requests to the global CSW.
ICSOFT 2008 - International Conference on Software and Data Technologies
The CSW mediator will consult a registry of atlases
and identify the atlases that may have data within the
bounding box selected, as a bounding box is associ-
ated with each atlas representing its geographic ex-
tent in order to optimize query execution by avoid-
ing rewriting queries over CSWs with no data cover-
ing the area of interest. Next, the atlas mediator will
translate the user’s request according to the process
described in subsection 4.4, collect the responses and
send them back to the user through the graphical in-
The atlas mediator prototype described in this pa-
per is a first step towards atlas integration as part of
a new International Coastal Atlas Network (ICAN).
The prototype showcases how ontologies and ontol-
ogy mappings can be used to integrate different het-
erogeneous and autonomous atlases (or information
systems), particularly OGC CSWs.
The next step of the ICAN initiative is to integrate
WFS and CSW mediation techniques in order to de-
fine a more complete approach for integrating both
data and metadata. Also, thematic information will be
considered and interfaces will be specified for sharing
this type of information, which is highly important in
atlases. The aim is to define a complete solution for
integrating CWAs.
An initial evaluation revealed that the number of
inferred keywords in rewritten queries can be dra-
matic depending on how general the user’s selected
keywords are. In some cases, one global keyword can
correspond to more than sixty local keywords of a lo-
cal atlas. This results in huge queries whose execu-
tion can be time consuming, especially when a large
number of users are connected to the global atlas at
the same time. In addition, no effort has been made to
rank metadata records according to relevance, date, or
spatial proximity. Future work will take these prob-
lems into consideration in order to optimize query
rewriting and execution as well as results presenta-
Bermudez, L. E. (2004). Ontomet: Ontology Metadata
Framework. PhD thesis, Drexel University, Philadel-
phia, USA.
de la Beaujardiere, J. (2004). OGC Web Map Service In-
terface (Version 1.3.0). Open Geospatial Consortium
Garcia-Molina, H., Papakonstantinou, Y., Quass, D., Ra-
jaman, A., Sagir, Y., Ullman, J., Vassalos, V., and
Widom, J. (1997). The TSIMMIS approach to me-
diation: Data models and Languages. Journal of In-
telligent Information Systems.
Goasdoue, F., Lattes, V., and Rousset, M.-C. (2000). The
Use of CARIN Language and Algorithms for Infor-
mation Integration: The PICSEL System. Interna-
tional Journal of Cooperative Information Systems,
Herman, I. (2007). Semantic Web Web Ontology Lan-
guage (OWL). W3C.
ISO (2006). ISO/PRF TS 19139 – Geographic information
Metadata XML schema implementation. Interna-
tional Standardization Organization.
Kashyap, V. and Sheth, A. (2000). Information Broker-
ing Across Heterogeneous Digital Data – A Metadata-
based Approach. Kluwer Academic Publishers, Nor-
well, Massachusetts, USA.
Kirk, T., Levy, A. Y., Sagiv, Y., and Srivastava, D. (1995).
The Information Manifold. In Knoblock, C. and Levy,
A., editors, Information Gathering from Heteroge-
neous, Distributed Environments, Stanford University,
Stanford, California.
Manolescu, I., Florescu, D., and Kossmann, D. (2001).
Answering XML Queries over Heterogeneous Data
Sources. In Proceedings of VLDB, pages 241–250.
Nebert, D. and Whiteside, A. (2005). OGC Catalogue Ser-
vices Specification. Open Geospatial Consortium Inc.
Vretanos, P. A. (2005). Web Feature Service Implementa-
tion Specification. Open Geospatial Consortium Inc.
Wiederhold, G. (1992). Mediators in the Architecture of
Future Information Systems. IEEE Computer, pages
Wright, D., Watson, S., Bermudez, L., Cummins, V.,
Dwyer, N., O’Dea, L., Nyerges, T., Benoit, G.,
Berman, M., Helly, J., and Uhel, R. (2007). Report
on Coastal Mapping and Informatics Trans-Atlantic
Workshop 2: Coastal Atlas Interoperability. Internal,
unpublished workshop proceedings.
Integrating Coastal Web Atlases