2 GeoMTGM
23
As regards the root of the GeoMTGM graph, as shown in Fig. 2.2, it is represented by a geographical complex node named “Schema” whose spatial component covers the geographical area described by the graph. The root has a particular
attribute named “SRS” whose value indicates the spatial reference system used in the
graph. If the spatial component of a particular geographical object is represented with
respect to a different spatial reference system, then this information is added in the
label containing the coordinate values.
Nodes are connected through labeled edges having a direction (graphically specified by an arrow), called “relational edges.” In particular, relational edges connecting
complex nodes to their atomic nodes have labels containing the name “HasProperty,”
while those connecting complex nodes to their stream nodes have labels containing
the name “HasMMProperty”; finally, those connecting complex nodes to their geographical atomic nodes have labels containing the name “HasGeoProperty.”
The VT [18] is explicitly managed by GeoMTGM both for nodes and edges.
The VT of a (geographical) complex node is represented in its label, while the VT
of an atomic (stream) or geographical atomic node is represented in the label of the
edge between it and its parent. For example, as shown in Fig. 2.2, the VT of the
complex node Highway is [27/05/1954, now] where “now” indicates that the represented fact is currently true, while, in Fig. 2.3, the VT of the atomic node Gravity
is [03/01/2006, 03/01/2006]. Moreover, the VT of a relational edge is reported in
its label, for example, the relational edge Segment-Accident between a SubSegment
and an Accident complex node has label Segment-Accident, [27/05/1954, now]. In
general, the VT of a complex node and those of its ingoing relational edges can be
different.
As regards a relational edge through a complex node and its stream node,
besides the name “HasMMProperty,” the label of the edge contains both the VT,
the time during which the subpart of the movie has been recorded, and the specific
subpart of the stream object to which the complex node is related. For example, in
Fig. 2.3 the label of the edge between Toll-station and Traffic is HasMMProperty,
[11/02/2006,11/02/2006], [0,infinity].
Moreover, GeoMTGM allows one to represent shared spatial components, in particular it admits that the spatial attribute of a geographical entity is composed by
performing the union of the spatial attributes of other geographical entities, without duplicate geometry. This can be done by means of a new kind of edge, named
“composed-by” edge, that is defined between two geographical complex nodes and
is depicted with a big white arrow, as reported in Fig. 2.2 between Highway and
Segment or between Segment and SubSegment entities in Fig. 2.3.
Defining Geographical Properties
As previously outlined, GeoMTGM allows one to represent geographical information according to both the entity-based model and the field-based model. The first
information model is supported by the constructs explained in the previous section.
In this section, we present the construct we propose to use for GeoMTGM according
to the field-based model.
23
As regards the root of the GeoMTGM graph, as shown in Fig. 2.2, it is represented by a geographical complex node named “Schema” whose spatial component covers the geographical area described by the graph. The root has a particular
attribute named “SRS” whose value indicates the spatial reference system used in the
graph. If the spatial component of a particular geographical object is represented with
respect to a different spatial reference system, then this information is added in the
label containing the coordinate values.
Nodes are connected through labeled edges having a direction (graphically specified by an arrow), called “relational edges.” In particular, relational edges connecting
complex nodes to their atomic nodes have labels containing the name “HasProperty,”
while those connecting complex nodes to their stream nodes have labels containing
the name “HasMMProperty”; finally, those connecting complex nodes to their geographical atomic nodes have labels containing the name “HasGeoProperty.”
The VT [18] is explicitly managed by GeoMTGM both for nodes and edges.
The VT of a (geographical) complex node is represented in its label, while the VT
of an atomic (stream) or geographical atomic node is represented in the label of the
edge between it and its parent. For example, as shown in Fig. 2.2, the VT of the
complex node Highway is [27/05/1954, now] where “now” indicates that the represented fact is currently true, while, in Fig. 2.3, the VT of the atomic node Gravity
is [03/01/2006, 03/01/2006]. Moreover, the VT of a relational edge is reported in
its label, for example, the relational edge Segment-Accident between a SubSegment
and an Accident complex node has label Segment-Accident, [27/05/1954, now]. In
general, the VT of a complex node and those of its ingoing relational edges can be
different.
As regards a relational edge through a complex node and its stream node,
besides the name “HasMMProperty,” the label of the edge contains both the VT,
the time during which the subpart of the movie has been recorded, and the specific
subpart of the stream object to which the complex node is related. For example, in
Fig. 2.3 the label of the edge between Toll-station and Traffic is HasMMProperty,
[11/02/2006,11/02/2006], [0,infinity].
Moreover, GeoMTGM allows one to represent shared spatial components, in particular it admits that the spatial attribute of a geographical entity is composed by
performing the union of the spatial attributes of other geographical entities, without duplicate geometry. This can be done by means of a new kind of edge, named
“composed-by” edge, that is defined between two geographical complex nodes and
is depicted with a big white arrow, as reported in Fig. 2.2 between Highway and
Segment or between Segment and SubSegment entities in Fig. 2.3.
Defining Geographical Properties
As previously outlined, GeoMTGM allows one to represent geographical information according to both the entity-based model and the field-based model. The first
information model is supported by the constructs explained in the previous section.
In this section, we present the construct we propose to use for GeoMTGM according
to the field-based model.