Database Reference
In-Depth Information
From a databasing perspective, vector models require special two-dimensional
indexes to efficiently retrieve data based on a spatial search. This need for more com-
plex indexes is one of the factors that has made GI appear special, both to “outsiders”
and those in the GI community.
3.3.2
t
opoloGy
and
m
ereoloGy
GI can also be modeled topologically and mereologically. Topology expresses the
spatial relationships that exist between different objects, and mereology describes
the whole-part relationships. Neither topology nor mereology provides precise loca-
tion information; rather, their focus is on representing and preserving relationships.
Topology is often used to represent networks such as road or rail systems, where the
important aspect is the connectivity between the network components. If we take the
example of a road network, we can topologically describe the relationship between
one road stretch and another as shown in Figure 3.3. In these topological models,
the road network is symbolized as a series of links, representing stretches of roads,
and nodes, representing junctions between stretches. We are then able to model the
fact that road stretch A links to junction B, which in turn also connects to stretch C.
Topology can also be used to define the relationships between vector objects. For
example, if the outline of a building is constructed by separate line features, then
these can be topologically related as shown in
Figure 3.4
.
Mereological relationships can be used to express the fact that a particular building
is “part of” a hospital or that a road network “comprises” roads A, B, and C. There are
often strong relationships between topological relationships and mereological rela-
tionships; if we know that a building is “within” (topology) the grounds of a hospital,
it may be reasonable to assume that the building is also “part of” (mereology) the
hospital. Figure 3.4 is also an example of not only topology but also mereology. That
topological and mereological relationships can often be co-occurring has resulted
in the term
mereotopology
. Since mereotopological relationships are commonly
C
R6
R1
A
F
R8
R7
G
R2
R5
D
R10
R4
R3
E
B
R9
FIGURE 3.3
A simple road network showing the links (R1-R10) and nodes (A-G).
