graphic Domain
There are many dynamic phenomena that may occur in geographic space, from ‘the falling of a rain drop’ to ‘the flow of cars along a road’. Some modelling approaches conceive events and processes based on series of spatial changes over time (e.g., changes of lo- cations, change of land coverage) involving geographic entities (e.g., features, objects). These changes may characterise, for example, movement of an entity or shrinkage of a geographic feature. Other approaches take into account physical processes and chemical reactions involved in a geographic phenomenon. For instance, when investigating bush- fires, one might aim to model some physical processes (and sub-processes) that cause its ignition, such as lightning strikes or spontaneous combustion. The latter, for example, also involves other sub-processes, such as oxidation or fermentation of substance with a relatively low ignition temperature.
The representation of geographic phenomena in terms of events and processes has been investigated by researchers from several areas, such as GIScience, Knowledge Rep- resentation, and Spatio-Temporal Databases. Approaches to modelling such events and processes have been presented based on different methods (e.g., logic-based, object ori- ented, cellular automata, or agent-based), with different purposes (e.g., semantic inte- gration, reasoning about process properties, simulation, or prediction), and sometimes developed to meet the requirements of a particular application area (e.g., urban spreading, wildfires, meteorology, human population studies, ecology, geomorphology).
Claramunt and Th´eriault [19, 20] have made significant contributions to the problem of representating of geographic events and processes. In [19], the authors define a typol- ogy of spatio-temporal processes comprising three main categories:
• Evolution of a single entity: basic changes (appearance, disappearance, stability);
transformations (expansion, contraction, deformation); movements (displacement, rotation);
• Functional relationships between entities: replacement (succession, permutation);
diffusion (production, reproduction, transmission);
• Evolution of spatial structures involving several entities: restructuring (split, union,
re-allocation).
Additionally, Claramunt and Th´eriault [20] propose an event-oriented model to de- scribe the evolution of spatial entities. This model is based on a relational formalism, extends the versioning concept (in which object ‘versions’ correspond to successive states of the represented entity) and distinguishes between thematic, temporal, and spatial do- mains. A semantic formalism is presented for describing and modelling spatio-temporal processes within the geographic domain. The approach is based on Event Pattern Lan- guage (EPL) [42, 43].
More recently, Devaraju and Kuhn [27] presented a process-centric ontology to rep- resent relations between geographic processes and observed properties originating from Geo-Sensor Networks (GSNs), by defining a controlled terminology which allows an ex- plicit representation of a process and its participants. ‘A Geo-Sensor Network (GSN) is a network of sensors that monitors the properties of natural environments in different lo- cations at different times’ [27, p. 01]. Devaraju and Kuhn [27] argue that ‘facts about geo-processes can be inferred from sensor data. However, this is not easy to achieve due to the nature of sensors, which return huge amounts of data in different formats and semantics’ (p. 01). Thus the main goal of Devaraju’s and Kuhn’s research is to define
controlled terminologies that can be used to present a unified view over heterogeneous sensor data.
The work described by Devaraju and Kuhn [27] is focused on the development of an ontology to represent concepts of surface hydrology, based on DOLCE Ontology Library [41, 58]. The authors align the concepts and relationships of two processes in the hydro- logical domain (precipitation and evapotranspiration) to the general categories defined in DOLCE, such as endurant, perdurant, and quality. This initiative represents an impor- tant step to overcomimg the challenge of filling the gap between low-level measurements and high-level conceptualisations in the geographic domain. However, developing an ontology comprising a fully characterisation of geographic processes, their participating endurants and related temporal and physical properties is still the subject of exhaustive research.
Galton and Worboys [40] maintain that it is generally accepted that object, event and process are key concepts for modelling dynamic geographic phenomena; besides, of course, an appropriate representation for space and time. This chapter now discusses im- portant issues and points out several desiderata for the conceptualisation of space, time, object, event, and process for the development of an ontology for dynamic geographic phenomena.
In the modular ontology of dynamic features of reality presented by Grenon and Smith [47], continuants are described as the class of entities which ‘exist in full in any instant of time at which they exist at all and they preserve their identity over time through a variety of different sort of changes’ (p. 140), such as a person, the planet Earth, a rock and Leeds. This is the concept normally attributed to spatial objects which inhabits a spatio-temporal model. On the other hand, the authors state that, while ‘continuants are themselves subject to constant change, occurrents depend on continuant objects as their bearers’ (p. 140). According to the authors, occurrents are all bound in time and this classification comprises events and processes. Continuants are commonly referred to as endurants, and occurrents are usually referred to as perdurants.
Galton [36] agrees with Grenon and Smith (2004) in the view that continuants are time-dependent entities and occurrents are time-independent. This is to say that, whilst the properties of a continuant can be different at different times, the properties of an occurrent are possessed timelessly. Galton (2007) summarises the distinction between continuants and occurrents by saying that continuants are entities which ‘(a) can undergo change, (b) have spatial parts but not temporal parts, and (c) are wholly present at each moment of its existence’ (p. 329). In contrast, occurrents are entities which ‘(a) cannot undergo change, (b) has temporal parts, and (c) are not wholly present at any time short
of its entire duration’ (p. 330).
Nonetheless, Galton [36] disagrees with Grenon and Smith [47] in the point that pro- cesses are regarded as occurrents by the latter. Galton [36] gives several compelling ar- guments and examples to support the view that a process should be regarded as an entity which undergoes change. Thus, the author suggests to set aside the distinction between continuants and occurrents, and proposes a new dichotomy encompassing two ontologi- cal views, called EXP (experience) and HIST (history), one populated by time-dependent entities (objects and processes) and the other by time-independent entities (events), re- spectively.
Following Galton [36], in the work of this thesis, a process is regarded as an entity which is subject to change over time and to which we can ascribe certain properties (for example, a process may be described as being slowing down, or accelerating). Therefore, a comprehensive ontology of geographic processes should provide the appropriate con- ceptualisation for these distinct ontological views (i.e., one comprising time-dependent entities and the other comprising time-independent entities), and the appropriate mecha- nism to do justice with the complex interrelations involving these views.