SEMANTIC-BASED SPATIAL INFORMATION

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Oct 21, 2013 (3 years and 9 months ago)

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1

Emmanuel Stefanakis

Dr. Emmanuel Stefanakis is an Assistant Professor at Harokopio
University of Athens in the Department of Geography. His main
research interests include geographic information systems, spatial
decision support systems, knowledge and dat
abase systems, spatial data
infrastructures, cartography, geovisualization, and web technology.



Poulicos Prastacos

Dr. Poulicos Prastacos is Director of Research in the Regional Analysis
Division of the Institute of Applied and Computational Mathematics
,
FORTH. His main research interests include GIS applications for
transportation, environment, regional planning; statistical, geographic
and environmental databases; web GIS and applications on the internet;
development of land use, transportation, and re
gional economic models;
implementation of decision support systems.


SEMANTIC
-
BASED SPATIAL INFORMATION
INFRASTRUCTURES: INTEGRATING DATA AND
SERVICES INTO A SINGLE COLLECTION

Emmanuel Stefanakis
1

and Poulicos Prastacos
2

1
Department of Geography, Harokopi
o University, 70, El. Venizelou Ave., 17671 Athens,
Greece,
estef@hua.gr


2
Institute of Applied and Computational Mathematics, Foundation for Research and
Technology,
Vassilika Vouton, P.O. Box 1527, 71110 Heraklion, Gre
ece,
poulicos@iacm.forth.gr



A
bstract.

Nowadays, special attention is given towards the development of effective
spatial data infrastructures

(SDI’s) at regional, national or international level. The main
sco
pe of such an infrastructure is to promote the accessibility and usability of geospatial
data. In parallel, there is an urgent need to share geospatial services. This leads to the
development of
spatial information infrastructures

or
service
-
driven infrast
ructures
. The
paper provides an overview of the role of semantics in a spatial data infrastructure or
service
-
driven infrastructure (we adopt a common acronym, i.e.,
SDI
), and the alternative
architectures of the middleware between the clients (users and a
pplications) and the servers
(data and services).



2

Keywords.

Semantics, Interoperability, Spatial Data Infrastructures (SDI), Spatial
Information Infrastructures.


1.

INTRODUCTION

Nowadays special attention is given towards the development of effective
spat
ial data
infrastructures

(SDIs) at regional, national or international level (Williamson et al., 2004).
SDIs are frameworks of policies, institutional arrangements, data, services, technologies,
and people with a common scope, i.e., to promote the accessib
ility and usability of
geospatial content (data and services).


The construction of an SDI presumes that the participating organizations have agreed on
the adoption of common vocabularies, practices, standards, technical specifications and
operational com
ponents (Nebert, 2004). Hence, an SDI is not a simple data set or
repository. Instead, an SDI hosts: (a) geographic content (data and services); (b) sufficient
description of this content (metadata); (c) effective methods to discover and evaluate this
cont
ent (data catalogues); (d) tools to visualize the data (e.g., web mapping); and (e)
services and software tools to support specific application domains.


An SDI architecture is usually conceptualized as a
three
-
tier architecture

(Evans, 2003).
At the top
layer (the client) reside the users and applications; at the bottom layer (the
server) reside the geospatial data; and at the middle layer (the middleware) reside all
services that assist the accessibility to the data repositories (Figure 1). Obviously, th
e
middle layer is the one that facilitates the worldwide access to and the use of online
geospatial data.



Figure 1.
The three
-
tier architecture of current SDI.


3


Currently, SDI paradigm focuses on the exchange of geospatial data; i.e., the middle layer
s
upports the discovery and retrieval of data available at the bottom layer; and this is
consistent to the term spatial
data

infrastructure.


On the other hand, it is widely recognized (e.g., Bernard et al., 2005), that future SDI
research must focus more o
n the users’ needs and the design of appropriate geoinformation
services and service architectures to satisfy these needs. In other words, the development
of service
-
driven infrastructures; or spatial
information

infrastructures will enhance
infrastructure
s capabilities.


This may be accomplished by enriching both the bottom and middle layers (Figure 2). The
bottom layer will also accommodate geospatial services (i.e., analytical software tools)
along with data. As regards to the middle layer, existing disc
overy and retrieval services
will be extended to apply to both the data and services available at the bottom layer.
Additionally, the middle layer will incorporate software tools and services that may be
used by the top layer (users and applications) and h
ence enhance the whole SDI
functionality.


In both architectures the middle layer plays the role of the
middleware

between the clients
(users and applications) and the servers (data and services). Its role is supported by a set of
mechanisms, as follows (N
ebert, 2004): (a) sufficient descriptions of the geospatial content
(both data and services), i.e., the
metadata
; (b) discovery of distributed collections of
geospatial content through their metadata descriptions, i.e., the
catalogue gateway
; and (c)
metap
hors to discover and access the geospatial content, i.e., the
interfaces
.




4

Figure 2.
The three
-
tier architesture of future SDI.



All mechanisms above must be constructed in such a way that enable the effective sharing
and use of geospatial content. T
he incorporation of rich
semantics

at all these components
affects definitely the efficiency of the middleware (Nebert, 2004). Semantics enable the
interoperability between different data collections and services. The scope of this paper is
to highlight th
e presence of semantics into these components, and discuss the alternative
architectures of the middleware.


In the sequel, we use the single acronym
SDI
to represent both spatial data and information
infrastructures. Although, several other acronyms can
be found in the literature (such as SII
standing for spatial information infrastructures) we maintain the widely used acronym SDI,
which can also stand for service
-
driven infrastructures.


The discussion is organized as follows. Sections 2 to 4 comment the

role of semantics and
related methods in the components of an SDI and how they assist the role of the
middleware. Specifically, Section 2 focuses on metadata; Section 3 on the catalogue
gateway; and Section 4 on the interfaces. Section 5 presents briefly
the efforts and products
of the organizations developing standards for SDIs. Then Section 6 shows the alternative
architectures for SDIs. Finally, Section 7 concludes the discussion.


2.

METADATA

As regards to the
metadata
, they convey


by nature


the seman
tics of the SDI content
(Nogueras
-
Iso et al. 2005). Metadata helps people who use geospatial content find the data
and services they need and determine how best to use it. Ideally, the metadata structures
and definitions should be referenced to a standard.

Metadata standards (Nogueras
-
Iso et al.,
2005) for geospatial content


although in use


are currently under continuous revision
and testing.


Professional communities have developed metadata standards for geospatial content. These
standards describe bot
h the structure of data and services and the intended semantic
content for specific data themes. Currently, the most prominent metadata standards are
(Nebert, 2004): the Content Standard for Digital Geospatial Metadata


CSDGM (FGDC,
1998); and the ISO 191
15 (ISO
-
TC211, 2003).


5


These standards define the schema for describing geographic information and services and
provide information about the identification, the extent, the quality, the spatial and
temporal schema, the spatial reference, and the distribu
tion of digital geographic data.
Additionally, both standards contain guidelines to develop
metadata profiles
, which allow
their customization to specific community needs.


CSDGM has been adopted and implemented in the U.S., Canada, U.K. and other countri
es
around the world. ISO 19115 has been expressed through its companion specification, ISO
19139, into eXtensible Markup Language (XML) and is currently used for the creation of a
North American Profile of Metadata for the U.S., Canada, and Mexico. The sam
e standard
will also be used by the INfrastructure for SPatial Information in Europe (INSPIRE, 2004).


3.

CATALOGUE GATEWAY

As regards to the
catalogue gateway
, semantics play again a key role, provided that they
assist and guide the discovery and evaluatio
n of useful content from an SDI. The catalogue
exploits the metadata of the multiple resources hosted in an SDI.


Obviously, catalogues can be achieved through the use of common descriptive
vocabularies (metadata), a common discovery and retrieval protoco
l and a registration
system for metadata collections (Nebert, 2004). At this point the issue of
interoperability

between metadata schemas emerges. According to the ISO, interoperability is defined as
the capability to communicate, execute programs, or tran
sfer data among various
functional units in a manner that requires the user to have limited or no knowledge of the
characteristics of those units.


The catalogue gateway handles multiple metadata collections, which are distributed and
heterogeneous
. The h
eterogeneity of the metadata collections may be considered as
syntactic
,
schematic

or
semantic

heterogeneity (Bishr, 1998). Syntactic heterogeneity
refers to the differences in the logical data models (e.g., relational versus object
-
oriented)
or in spatial

representations (e.g., raster versus vector). Schematic heterogeneity refers to
the differences in the conceptual data models (the data are organized in different schemas
and structures). Semantic heterogeneity refers to the differences in meaning, interp
retation
or usage of the same or related data. Semantic heterogeneity is further divided into
naming


6

(i.e., synonyms and homonyms) and
cognitive

(i.e., different conceptualizations)
heterogeneity.


Semantic heterogeneity between metadata collections caus
es most integration problems.
These problems refer to (Bernard et al., 2003): (a) metadata interpretation, and (b) data
interpretation. As regards to metadata interpretation, different providers may use different
terms. This makes difficult for both the hu
mans and the computers to recognize the
coherence between similar terms. As regards to data interpretation, the properties of the
entities involved in an application domain are expressed based in various standardized
(e.g., CORINE vocabulary for land cover

classification; EAA, 2000) or not vocabularies.
Data and metadata interpretation problems must be overcome when data from different
sources have to be integrated into a single collection. This requires the adoption of
common terms, which are usually provi
ded by a standardized vocabulary with unchanged
terms. All terms coming from other vocabularies will be translated in the adopted
vocabulary using tools for
semantic translation
.


The use of
ontologies

(Fonseca and Egenhofer, 1999; Guarino, 1998; Wache, 2
001) as
semantic translators is a possible approach to overcome the problem of semantic
heterogeneity (Bernard et al. 2003). Ontologies play a critical role in associating meaning
with data such as computers can understand and process data automatically. O
ntologies
may be used (a) to assist the translation of a term (i.e., a concept) from one vocabulary
(i.e., one ontology) into a term from another vocabulary; and (b) to derive super concept
and sub
-
concept relationships.


However, currently ontologies are

at the best able to define data semantics under very
controlled situations. They cannot yet fully support semantic translation. They fail to cope
with the semantics of services (Bernard et al., 2005). Nowadays GI community addresses
the following three re
search issues (Fonseca and Sheth, 2003): (a) the creation and
management of domain specific geo
-
ontologies for both data and services; (b) the
matching geo
-
concepts available in the web to geo
-
ontologies; and (c) the integration of
different ontologies as
regards to both the geographic dimension and the non
-
geographic
domain.



7

4.

INTERFACES

As regards to the interfaces, geospatial applications have special requirements at the user
interface level. This is because the appropriate
visualization of geospatial da
ta

may assist
people to discover or evaluate the geospatial content (Adrienko and Adrienko, 1999; Kraak
and Brown, 2000). Hence, the quality of geovisualization products must be promoted.


Semantics associated to geospatial content may be exploited at this

stage and lead to
effective geo
-
visualizations. Current research on the development of metaphors and GUI’s
as well as on issues related to multiple representation and interoperability of geospatial
data (e.g., ISRPS, 2006) may lead to enhanced visualizati
ons.


Last, but not least, the
services and software tools

hosted by an SDI must be semantic
based in order to be more efficient and assist advanced application domains. The
development of semantic
-
based software for geospatial applications is an emerging

field of
research. Although many semantic
-
based tools have been developed in the past to assist
geospatial applications, these tools have been designed and implemented patchily without
big interference with other tools or an established research agenda. C
urrently this situation
changes dramatically, with several workshops aligned to this new trend, e.g., the
Geovisiualization Conference (EURESCO, 2004), the SeBGIS Workshop (OTM, 2005),
etc.


5.

DEVELOPING STANDARDS

AND SPECIFICATIONS

Open Geospatial Consorti
um (OGC) in parallel with World Wide Web Consortium (W3C)
and ISO develop standards and specifications to support the interoperability between
repositories with geospatial content. Table 1 provides some representative
standards/specifications related to in
formation content.


Table 1.

Standards and Specifications for geospatial content.

Organization

Standard/Specification name

W3C

HyperText Transfer Protocol
(HTTP)

W3C

Simple Object Access Protocol
(SOAP)

W3C

Web Services Description
Language (WSDL)

OGC

Web Map Server (WMS)


8

OGC

Web Coverage Server (WCS)

OGC

Web Feature Service (WFS)

OGC

Geography Markup Language
(GML)

OGC

Catalog Service (CS)

ISO
-
TC211

Rules for Application Schema
(DIS19109)

ISO
-
TC211

Methodology for Feature
Cataloguing (DIS19110)

ISO
-
TC211

Spatial Referencing by
Geographical Identifiers
(CD19112)

ISO
-
TC211

Metadata (IS19115)

ISO
-
TC211

Metadata Implementation
Specification (DTS19139)


The scope of these standards and specifications is to enable an application developer to use
an
y geospatial content (data and services) available on “the net” within a single
environment and a single workflow (McKee and Buehler, 1998). However, standardization
may not solve the problem of interoperability by itself, because of the following reasons
(Stoimenov and Dordevic
-
Kajan, 2002):



The construction and maintenance of a single and integrated model is a hard task.



The requirement to communicate with geospatial sources that do not conform the
adopted standard will be always present.



Existing geospat
ial sources have their own models which may not always be mapped to
the common model without information loss.



The standards/specifications are subject to continuous change, but systems will not all
simultaneously change to conform.


6.

ESTABLISHING THE SYN
E
RGY OF GEOSPATIAL CO
NTENT SOURCES

The problem of interoperability may be overcome by the adoption of one of the well
-
established methods that allow the synergy of heterogeneous information sources (Garcia
Molina et al, 2000)

(Figure 3)
: (a) federated datab
ases; (b) data warehouses; and (c)
mediators.


Federated databases
adopt the simplest architecture. Information sources are independent
to each other. The synergy is accomplished by a separate middleware component
(connector) per source pair (Figure 3
a
).
This component is responsible for the

9

interoperability between the two sources. This architecture is ideal when a limited number
of sources are involved; otherwise numerous connectors must be built and being
maintained. Notice that for N sources, Nx(N
-
1) c
onnectors are required for a full network.


In
data warehouses
, the contents of two or more sources are collected and combined into a
common schema. Then a single repository, called
warehouse
, accommodates all
inform
ation content (Figure 3b
). The populati
on of the warehouse is accomplished by an
individual content
extractor

per source and a
combiner
, which maps the content of each
source into the common schema.


Mediators
, similarly to warehouses, provide a unified view to the contents of two or more
sour
ces. However, a mediator does not accommodate any content. Key component of
mediator architecture (Figure 3c) is the middleware, which is called
wrapper
. A wrapper
plays the role of the translator between the mediator and an individual source. Notice that
the mediator may be able to judge if a query posed by the user may find useful content to
an individual source. This is accomplished through the use of appropriate metadata and
meta
-
services at the mediator level.
Mediator technology is more flexible and w
ould be
ideal for the development of an SDI (
Stoimenov and Derdevic
-
Kajan, 2002)
. Obviously, it
implementation raises all issues related to semantic heterogeneity as presented in the
previous Sections.




(a)

(b)


10


(c)

Figure 3.
Alternative methods
to allow the synergy of heterogeneous information sources:
(a) federated databases (the arrows represent the connectors); (b) data warehouses (the
arrows represent content flow); and (c) mediators (the arrows represent content flow).


7.

DISCUSSION

The paper

provides an overview of the role of semantics in a spatial data infrastructure or
service
-
driven infrastructure (SDI), and the alternative architectures of the middleware
between the clients (users and applications) and the servers (data and services).


I
t is widely recognized, that the integration of semantics (of both data and services) at all
components of an SDI middleware (metadata, gateways and interfaces) may promote the
SDI functionality and usefulness. Currently, technology provides enhanced tools

and
methods for implementing this task (models, repositories, s/w tools); however GI science is
not mature to make full use of them, provided that the issues of semantic heterogeneity
must be overcome.


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