2 GeoMTGM
17
exchange model (DOEM) [6], a temporal data model representing the transaction
time [18] (i.e. the time at which an item of information is stored in the database), and
the temporal graphical model (TGM) [22], a graph-based data model that is able to
represent static and dynamic aspects of semistructured data, considering the VT [18]
(i.e. the time at which a fact is true in the considered domain).
It is usual for semistructured data models to represent data as labeled graphs,
where nodes and edges suitably represent different (related) data items. In particular,
a TGM graph is a labeled graph with two types of nodes: complex nodes that describe
the objects of the represented reality and atomic nodes that represent properties of the
objects. The relationships between objects in the reality are represented by labeled
relational edges in the graph.
2.2.2 Multimedia Temporal Information
In the field of multimedia database systems, topics related to the integration of
several media objects (with their temporal aspects) have been considered in several
papers. Among them, we distinguish two different research directions: the first one
focuses on data modeling and querying issues [5, 8, 12], while the second one is more
specifically devoted to proposals of models for multimedia presentations [4, 19]. An
example of the first research direction is the work presented in [5], where a unified
data model is proposed for multimedia types, such as images, sounds, videos, and
long text data. For the second research direction, a multimedia presentation can be
defined as the composition of a set of multimedia objects that have to satisfy some
given spatiotemporal constraints during their playback; several proposals deal with
the specification of synchronization constraints between different multimedia data
items [4]. For example, in [19] the authors present a methodological approach for
checking the temporal integrity of interactive multimedia documents (IMDs), focusing on temporal properties of presentations.
2.2.3 Geographical Information and GML
In the literature, there are two approaches to the representation of geographical information [23] : the Entity-based Model and the Field-based Model. In the entity-based
model, geographical information is represented by several geographical objects that
are distinguished by an identifier and described by their attributes. The entitybased model properly represents the typical content of a traditional map, where
objects are embedded in a reference space, for example, a map can be obtained
by composing the road network, the buildings, the administrative units, and so on.
Each geographical object can be viewed as an entity, like in traditional information
systems, and each of their attributes as a property of this entity.
In the field-based model, each point of the reference space is associated to one
or several attribute values, defined as continuous functions in x and y. The altitude
above sea level is an example of a function defined over x and y, whose result is
the value of a variable h for any point in the 2D space. Measures for several phenomena can be collected as attribute values varying with the location in space, for
17
exchange model (DOEM) [6], a temporal data model representing the transaction
time [18] (i.e. the time at which an item of information is stored in the database), and
the temporal graphical model (TGM) [22], a graph-based data model that is able to
represent static and dynamic aspects of semistructured data, considering the VT [18]
(i.e. the time at which a fact is true in the considered domain).
It is usual for semistructured data models to represent data as labeled graphs,
where nodes and edges suitably represent different (related) data items. In particular,
a TGM graph is a labeled graph with two types of nodes: complex nodes that describe
the objects of the represented reality and atomic nodes that represent properties of the
objects. The relationships between objects in the reality are represented by labeled
relational edges in the graph.
2.2.2 Multimedia Temporal Information
In the field of multimedia database systems, topics related to the integration of
several media objects (with their temporal aspects) have been considered in several
papers. Among them, we distinguish two different research directions: the first one
focuses on data modeling and querying issues [5, 8, 12], while the second one is more
specifically devoted to proposals of models for multimedia presentations [4, 19]. An
example of the first research direction is the work presented in [5], where a unified
data model is proposed for multimedia types, such as images, sounds, videos, and
long text data. For the second research direction, a multimedia presentation can be
defined as the composition of a set of multimedia objects that have to satisfy some
given spatiotemporal constraints during their playback; several proposals deal with
the specification of synchronization constraints between different multimedia data
items [4]. For example, in [19] the authors present a methodological approach for
checking the temporal integrity of interactive multimedia documents (IMDs), focusing on temporal properties of presentations.
2.2.3 Geographical Information and GML
In the literature, there are two approaches to the representation of geographical information [23] : the Entity-based Model and the Field-based Model. In the entity-based
model, geographical information is represented by several geographical objects that
are distinguished by an identifier and described by their attributes. The entitybased model properly represents the typical content of a traditional map, where
objects are embedded in a reference space, for example, a map can be obtained
by composing the road network, the buildings, the administrative units, and so on.
Each geographical object can be viewed as an entity, like in traditional information
systems, and each of their attributes as a property of this entity.
In the field-based model, each point of the reference space is associated to one
or several attribute values, defined as continuous functions in x and y. The altitude
above sea level is an example of a function defined over x and y, whose result is
the value of a variable h for any point in the 2D space. Measures for several phenomena can be collected as attribute values varying with the location in space, for