2 GeoMTGM
29
on which the geographical property has been measured. The label of these edges
is geographical, GeoFieldName, GeoFieldValue, II, where GeoFieldName is
the name associated to the GeoField node, GeoFieldValue is the value measured
on the particular geographical complex node, and I is the temporal element on
which the property has been measured.
2.3.3 Translating GeoMTGM Graphs into XML Documents
We used GML [20] as a starting point for the definition of the rules to translate the
geographical information, represented by means of GeoMTGM graphs, into XML
documents. In particular, some tags and conventions have been extended and adopted
to implement the translation rules defined in [10] for the MTGM data model.
GML elements are used for translating geoAtomic nodes into XML elements and
are characterized by the following notation:, as depicted in Fig. 2.5.
In particular, point, line, and polygon nodes are translated into XML elements
that extend the PointType, LineStringType, and AbstractSurfaceType defined in GML, respectively. In Fig. 2.5 the XML Schema for point, line, and polygon
are shown; only the PointWithTimeType is described in detail, the other two types
are very similar to this one.
In GML the coordinates of any geographical object are encoded either as a
sequence of coord elements that encapsulate tuple components, or as a single string
contained within a coordinates element. We use a coord element instead of a
coordinates element, because it allows us to perform basic type checking and enforce constraints on the number of tuples that appear in a particular geometric instance.
The coordinates of a geometry are defined within some Spatial Reference System (SRS). In GML for all geometry elements, the SRS is specified by the attribute
srsName; instead, in GeoMTGM this attribute must be specified only for the geometric atomic node nested in the root element, and it is optional for any other geometric
entity. Moreover, unlike GML, the id attribute is required and not optional.
In the same way, for translating the VT information we have defined an element type ValidTimeType that is composed of a sequence of specialized GML
elements TimePrimitivePropertyType, named ValidIntervalType, as shown
in Fig. 2.6.
The overall main ideas underlying the designed translation technique can be summarized as follows:
• (Geographical) Complex nodes are translated into complex elements (i.e.
elements that contain other elements); in particular, they have a (nested) element
for the corresponding VT and an element for each outgoing edge.
• Atomic nodes are translated into mixed elements (i.e. elements containing both
string values and other elements); in particular, they contain the string representing their value and an element for their VT (which is contained in the label of
the ingoing relational edge).
• Geographical atomic nodes are translated into complex elements; in particular,
they have some (nested) elements for the coordinates and an element for their
VT (as for atomic nodes it is contained in the label of the ingoing relational
edge).
29
on which the geographical property has been measured. The label of these edges
is geographical, GeoFieldName, GeoFieldValue, II, where GeoFieldName is
the name associated to the GeoField node, GeoFieldValue is the value measured
on the particular geographical complex node, and I is the temporal element on
which the property has been measured.
2.3.3 Translating GeoMTGM Graphs into XML Documents
We used GML [20] as a starting point for the definition of the rules to translate the
geographical information, represented by means of GeoMTGM graphs, into XML
documents. In particular, some tags and conventions have been extended and adopted
to implement the translation rules defined in [10] for the MTGM data model.
GML elements are used for translating geoAtomic nodes into XML elements and
are characterized by the following notation:
In particular, point, line, and polygon nodes are translated into XML elements
that extend the PointType, LineStringType, and AbstractSurfaceType defined in GML, respectively. In Fig. 2.5 the XML Schema for point, line, and polygon
are shown; only the PointWithTimeType is described in detail, the other two types
are very similar to this one.
In GML the coordinates of any geographical object are encoded either as a
sequence of coord elements that encapsulate tuple components, or as a single string
contained within a coordinates element. We use a coord element instead of a
coordinates element, because it allows us to perform basic type checking and enforce constraints on the number of tuples that appear in a particular geometric instance.
The coordinates of a geometry are defined within some Spatial Reference System (SRS). In GML for all geometry elements, the SRS is specified by the attribute
srsName; instead, in GeoMTGM this attribute must be specified only for the geometric atomic node nested in the root element, and it is optional for any other geometric
entity. Moreover, unlike GML, the id attribute is required and not optional.
In the same way, for translating the VT information we have defined an element type ValidTimeType that is composed of a sequence of specialized GML
elements TimePrimitivePropertyType, named ValidIntervalType, as shown
in Fig. 2.6.
The overall main ideas underlying the designed translation technique can be summarized as follows:
• (Geographical) Complex nodes are translated into complex elements (i.e.
elements that contain other elements); in particular, they have a (nested) element
for the corresponding VT and an element for each outgoing edge.
• Atomic nodes are translated into mixed elements (i.e. elements containing both
string values and other elements); in particular, they contain the string representing their value and an element for their VT (which is contained in the label of
the ingoing relational edge).
• Geographical atomic nodes are translated into complex elements; in particular,
they have some (nested) elements for the coordinates and an element for their
VT (as for atomic nodes it is contained in the label of the ingoing relational
edge).