Database Reference
In-Depth Information
A
spatial level
is a level for which the application needs to store spatial
characteristics. This is captured by its
geometry
, which is represented using
one of the spatial data types described in Sect.
11.1.1
. In Fig.
11.4
,wehave
seven spatial levels:
Supplier
,
Customer
,
City
,
State
,
Region
,
Country
,and
Continent
. On the other hand,
Product
and
Time
are nonspatial levels.
A
spatial attribute
is an attribute whose domain is a spatial data type.
For example,
CapitalGeo
is a spatial attribute of type point, while
Elevation
is a spatial attribute of type field of reals. Attributes representing continuous
fields are identified by the '
f( )
' pictogram.
A spatial level is represented using the icon of its associated spatial type to
the right of the level name. A level may be spatial independent of the fact that
it has spatial attributes (see Fig.
11.5
). For example, depending on application
requirements, a level such as
State
may be spatial (Fig.
11.5
a,b) or not
(Fig.
11.5
c) and may have spatial attributes such as
CapitalGeo
(Fig.
11.5
b,c).
a
b
c
State
State
State
StateName
StatePopulation
StateArea
CapitalName
StateName
StatePopulation
StateArea
CapitalName
CapitalGeo
StateName
StatePopulation
StateArea
CapitalName
CapitalGeo
Fig. 11.5
Examples of levels with spatial characteristics. (
a
) Spatial level. (
b
)
Spatial level with a spatial attribute. (
c
) Nonspatial level with a spatial attribute
A
spatial hierarchy
is a hierarchy that includes at least one spatial level.
For example, in Fig.
11.4
, we have two spatial hierarchies in the
Supplier
and
Customer
dimensions, which share the levels from
City
to
Continent
. Spatial
hierarchies can combine nonspatial and spatial levels. Similarly, a
spatial
dimension
is a dimension that includes at least one spatial hierarchy.
Two related spatial levels in a spatial hierarchy may involve a
topological
constraint
, expressed using the various topological relationships given in
Sect.
11.1.1
. To represent them, we use the pictograms shown in Fig.
11.2
.For
example, in Fig.
11.4
, the geometry of each state is covered by the geometry
of its corresponding region or country, depending on which level a state rolls
up to. Note that in Fig.
11.4
, there is no topological constraint between the
Supplier
and
City
levels since the location of the supplier is obtained from its
address through geocoding and the location of the city corresponds to the
center of the city.
A
spatial fact
is a fact that relates several levels, two or more of
which are spatial. A spatial fact may also have a
topological constraint
that must be satisfied by the related spatial levels: An icon in the fact
indicates the topological relationship used for specifying the constraint. In the
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