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. .   ,
. 17, . 5,  2003,405–409
Guest Editorial
Semantic reference systems
WERNER KUHN
Institute for Geoinformatics, University of Mu
¨nster, Robert-Koch-Str 26–28,
48149 Mu
¨nster, Germany
e-mail: kuhn@ifgi.uni-muenster.de
(Received 12 November 2002; accepted 5 February 2003)
Four centuries after Rene
´Descartes watched a fly walk across his ceiling and
wondered how to capture its position (Gribbin 2002), we use Cartesian coordinates
routinely to describe locations. We identify the positions of entities in the real world,
transform their GIS representations from one coordinate system to another, and
integrate spatially referenced data across multiple coordinate systems. A theory of
spatial reference systems standardises the notions of geodetic datum, map projections,
and coordinate transformations (ISO 2002). Similarly, our temporal data refer unam-
biguously to temporal reference systems, such as calendars, and can be transformed
from one to another.
Geographical information systems contain spatial, temporal, and thematic data.
The first two kinds are firmly tied to reference system theories and tools. We now
need to produce the third component—semantic reference systems. Descartes might
wonder today how to establish common frames of reference for, say, a geneticist and
an entomologist to talk about that fly. They would need methods to explain the
meaning of their specialized vocabularies to each other, to detect synonyms and
homonyms, and to translate expressions. Accordingly, a theory of semantic reference
systems will enable producers and users of geographical information to explain the
meaning of thematic data, to translate this meaning from one information community
to another (OGC 1998), and to integrate data across diering semantics.
Reaching this goal may not take centuries again, but a decade or two could pass
until we see the kind of on-the-fly (so to speak) integration that is now possible
across dierent spatial reference systems in GIS (Lutz 2003). There is no need for
universal geographical information semantics, as long as we can provide the means
for any pair of information communities to define their concepts and translate
between them. All the same, work on semantic primitives and universals (Wierzbicka
1996) as well as on top-level ontologies (Sowa 2000 ) strongly suggests that a common
core of concepts exists and can be defined.
Formalizing the semantics of geographical information communities is, in any
case, much simpler than defining natural language terms. The reason is that every
information community agrees, per definition, on a shared set of concepts, expressed
in conceptual models, feature-attribute catalogues, legal documents, work practices,
International Journal of Geographical Information Science
ISSN 1365-8816 print/ISSN 1362-3087 online © 2003 Taylor & Francis Ltd
http://www.tandf.co.uk/journals
DOI: 10.1080/1365881031000114116
W. Kuhn406
regulations, and the like. These agreements are subject to inspection, analysis, and
revision (Kuhn 2001). Given this pragmatic and encouraging foundation of informa-
tion system semantics, where are we today on the path to semantic reference systems
for geographical information? What is required to reach the goal?
At first sight, semantic reference systems may sound like yet another fancy name
for what the GIS community has just learned to call ontologies. Indeed, ontologies
constitute a core component of them. They describe the conceptualisations of the
world to which the data in information systems refer (Gruber 1993 ). In other words,
they provide a frame of reference for the vocabulary used in a database. For example,
an ontology might specify what the term ‘forest’ means in one or more vegetation
databases. Current ontology work focuses on making this notion practically useful
through languages for designing and exposing ontologies (Welty and Smith 2001 ).
The spatial reference system analogy suggests something more powerful than
today’s ontology languages can oer. Producers and users of geographical informa-
tion need tools for transformations among semantic spaces and projections to sub-
spaces. A transformation may occur within or between information communities
and involves a change to the reference system (for example, adding a new axiom to
an ontology). A projection occurs typically within a community and reduces the
complexity of a semantic space (for example, by generalizing two entity classes to a
super-class).
The notions of semantic transformation and projection are suggested here in
analogy to their geometric counterparts. They do not assume any metric for semantic
spaces. All that is required are axiomatised concepts allowing for mappings between
them, i.e., a formal version of the mental spaces in Fauconnier (1994). Nevertheless,
a geometric structure for semantic spaces, such as the one proposed by Ga
¨rdenfors
(2000), would have obvious advantages for computational support and for dealing
with the core semantic issue of similarity (Rodrı
´guez and Egenhofer, 2003).
However, tools for transforming and projecting between semantic spaces need
stronger formal foundations than most current ontology languages provide. They
require sound typing, parameterised polymorphism, multiple inheritance of behavi-
our, and higher order reasoning capabilities (Frank and Kuhn 1999, Kuhn 2002).
Furthermore, in order to relate the meaning of terms to GIS applications, they need
to place entities and relationships in the context of human activities (Kuhn 2001).
An example of a formalized semantic reference system, applying this approach to
the domain of vehicle navigation, can be found at http://musil.uni-muenster.de. It
contains a complete axiomatisation and computational solution for a simple trans-
formation (among two interpretations of a navigation data model) and projection
(simplifying a data model), and is intended as a proof of concept for the ideas
presented here.
The mapping of terms from one context to another and the construction of
common ontologies require powerful mathematical instruments of the kind found in
category theory (Bench-Capon and Malcolm, 1999). A semantic reference system
consists of ontologies that specify concepts as well as mappings between them,
embedded in a formalism that supports the computation of these mappings. These
requirements go beyond the formal apparatus underlying coordinate projections and
transformations. Most of today’s ontology formalisms, however, are even weaker
than that. How would you describe in your favourite ontology language, which
semantic aspects of a house and of a boat are projected into boathouses and
houseboats, respectively, and how they get combined (Kuhn 2002)?
Guest Editorial 407
The formal requirements for semantic reference systems are more than a theoret-
ical goal. They represent a practical necessity to achieve semantic interoperability, i.e.
the capacity of information systems or services to work together without the need
for human intervention (Harvey et al. 1999, OGC 1998). Focusing on the case of
(web) services, I see three core problems that need to be solved on the way to
semantic interoperability. Each of them identifies a special kind of matchmaking
(Sycara et al. 1999 ) between information providers and requesters. And each of them
calls for semantic reference systems to support that matchmaking:
$Service providers need to be able to determine whether a data source oers
useful semantics for a planned service (e.g. can a runomodel use the road
widths provided by a cadastral database? can the number of building levels in
that database be used to calculate building heights for noise propagation?);
$Client services need to be able to determine whether a given service oers
useful semantics as input to their processing (e.g. is an elevation model with a
certain resolution and linear interpolation sucient for a particular visibility
calculation?);
$Human users need to be able to determine whether a service provides useful
semantics to answer a question (e.g. should the question ‘which parcels touch
this road?’ be posed to the database or to the GIS?).
In all three problem statements, the term ‘useful’ indicates that the answers will
typically not be a simple yes or no, and that a process of enabling actual information
use will follow. The interesting and challenging cases are those where an information
source contains useful content that needs to be supplemented with additional
information or transformed to other contexts. These cases need the kind of projection
and transformation support that the idea of semantic reference systems suggests.
The three semantic interoperability problems raise two practical questions, which
also show the way forward to address them:
1. What needs to be stated about an information source, so that a requester
(human or machine) can assess and exploit its semantic value?
2. How does this assessment and exploitation work?
The answer to the first question clearly depends on the answer to the second: how
can we reasonably decide on the contents, let alone the representations, before
specifying their use? While most current work on information system ontologies
starts with the first, focusing on metadata and mark-up languages, the second
question raises the issues posed by the reference system analogy: What conceptualisa-
tions occur in an application? Where and how are they specified? How can they be
projected to other conceptualisations? How can they be merged? How can a common
ontology be constructed for them? How can expressions be translated from one
application context to another?
Finally, in case you wondered, spatial reference systems can be seen as a special
kind of semantic reference systems: they explain the meaning of coordinates and
how it shifts between contexts. This restriction of the dicult general semantics
problem has allowed for highly successful solutions to the spatial case. The best
strategy for progress towards general semantic reference systems is, thus, to solve
the next easiest special cases. This could be gazetteers, which translate geographical
names to coordinates. They map the semantic problem of geographic identifiers to
the geometric problem already solved by spatial reference systems. Or it could be
W. Kuhn408
cases where semantics rests primarily on geometric and topological properties, such
as those of navigation services. Ongoing work on these and other applications
confirms the utility and fruitfulness of the idea of semantic reference systems. The
semantic interoperability discussion in science (Meersman and Tari 2002) and indus-
try (OGC 1998) documents the need for them.
Acknowledgments
Our work on geographic information semantics is supported by the European
Commission, in the ACE-GIS (IST-2002-37724 ) and BRIDGE-IT ( IST-2001-34386)
projects, by the German ministry of education and science (BMBF) in the
Geotechnologies Research Program. The ideas presented have been shaped by discus-
sions with members of the semantic interoperability research group at Mu
¨nster
(http://musil.uni-muenster.de) and of the Meaning and Computation Laboratory at
the University of California at San Diego (http://www.ese.uesd.edu/users/goguen/).
Comments from anonymous reviewers helped to improve the presentation.
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