Database Reference
In-Depth Information
Querying the Model
According to the model, we can classify operators for querying moving objects
with uncertainty in two classes: (a) operators for
point queries
; (b) operators for
querying the relative position of a moving object with respect to a region, within
a given time interval. Each one of these operators corresponds a
spatio-temporal
range query
.
Operators for Point Queries
Two operators for point queries are defined in the literature:
Where At
(
T,t
): returns the
expected
location on the route of trajectory
T
at time
t
.
When At
(
T,l
): returns the times at which the moving object whose trajec-
tory is
T
is
expected
to be at location
l
. (Note that in this case the answer may
be a set of time instants, if the moving object passes through a point more
than once).
If the location
l
=
(
x
l
,y
l
) is not on the route of
T
,the
WhenAt
(
T,l
) operator
finds the set of all the points
C
on this route that are closest to
l
, and returns the
set of time instants at which the object is expected to reach each point in
C.
Operators for Spatio-Temporal Range Queries
These operators comprise a set of Boolean predicates such that each predicate
is satisfied if the moving object is inside a given region
R
during a given time
interval [
t
s
,t
e
]. Queries may ask if the condition is satisfied sometime or always
within [
t
s
,t
e
] (due to the motion of the object), and/or if, due to the uncertainty,
the object
possibly
or
definitely
satisfies the condition at some time within the
interval. The main operators corresponding to spatio-temporal range queries are:
Possibly Sometime Inside
(
T,R,ts,te
). The predicate is true iff
there exists a
PMC
(
T
) for the trajectory
T
and a time
t
∈
[
ts,te
] such that
PMC
(
T
) at time
t
is inside the region
R
.
Possibly Always Inside
(
T,R,ts,te
). The predicate is true iff there
exists a
PMC
(
T
) that is inside the region
R
for every
t
∈
[
ts,te
]
.
Always Possibly Inside
(
T,R,ts,te
). True iff for every time value
t
∈
[
ts,te
] there exists some (not necessarily unique)
PMC
(
T
) that is inside
(or on the boundary of)
R
at
t
.
Always Definitely Inside
(
T,R,ts,te
). This is true iff at every
time
t
∈
[
ts,te
], every possible motion curve
PMC
(
T
)isinregion
R.
Definitely Sometime Inside
(
T,R,tb,te
). This is true iff for every
possible motion curve
PMC
(
T
) of the trajectory
T
, there exists some time
t
∈
[
tb,te
] in which the particular motion curve is inside
R
.
