Database Reference
In-Depth Information
t
y
x
Figure 3.1 A value of type moving point (
mpoint
) and a value of type moving region
(
mregion
).
data in tables and allowing one to manipulate and combine tables, rather than
thinking of records in files containing fields in certain formats.
In a similar way, moving objects databases let the user view a vehicle moving
on a road simply as a time-dependent position (either relative to the Euclidean
plane or to the road). Mathematically, this is a function
f
:
instant
→
point
if
instant
represents a continuous domain of time and type
point
represents
(
x,y
) positions in the Euclidean plane. Such a function can be visualized in a
3D (
x,y,t
) space as shown in Figure
3.1
.
Obviously, to arrive at a powerful query language on moving objects we need
not only a simple view of data but also operations to manipulate them. What can
we do with a continuous curve as shown in Figure
3.1
?
For example, we can project it into the (
x,y
) plane. This forgets the temporal
information and returns just the path in the plane the object (e.g., vehicle) has
taken.
We can also project it on the time axis and get the time interval(s) when the
object existed (more precisely, when its movement information is available).
We can reduce it to the times when the position has certain properties, for
example, when it is inside a given region of the plane, or within some distance
to another object, perhaps even a moving object.
Amodel of data together with some operations on it is captured by the concept
of an
abstract data type
(
ADT
). Hence the idea is to model the time-dependent
position of a vehicle as an abstract data type. Because only the position as a
point is represented (ignoring the shape of the vehicle), the data type is called
moving point
(
mpoint
).
1
Similarly, for some entity for which capturing the
extent is relevant (e.g., a forest fire), the time-dependent shape and location is
represented in a data type
moving region
(
mregion
).
1
We denote data types in italics and underlined.
