What is (certain) SpatioTemporal Data?


 Eleanore Murphy
 4 months ago
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2 What is (certain) SpatioTemporal Data? A spatiotemporal database stores triples (oid, time, loc) In the best case, this allows to look up the location of an object at any time 2
3 What is (certain) SpatioTemporal Data? A spatiotemporal database stores triples (oid, time, loc) In the best case, this allows to look up the location of an object at any time Allows to answer queries such as Return objects that intersects some spatial window within some time interval. Q 3
4 What is uncertain SpatioTemporal Data? In most applications, this data is not complete Delays between GPS signals RFID sensors located only in certain locations Wireless sensors nodes sending infrequently to preserve power Geoapplication checkins 4
5 What is uncertain SpatioTemporal Data? Existing works Bound the set of possible (location,time) pairs of an object between observations e.g. by modeling knowledge about maximum speed 5
6 What is uncertain SpatioTemporal Data? Existing works Bound the set of possible (location,time) pairs of an object between observations e.g. by modeling knowledge about maximum speed Allows to make statements like its possible that o intersects some query window Q But how likely is this event? 6
7 Modeling SpatioTemporal Uncertainty The position of an object o a some time t is a random variable The trajectory of o follows a stochastic process, i.e. a family of random variables o(t) 7
8 Modeling SpatioTemporal Uncertainty The position of an object o a some time t is a random variable The trajectory of o follows a stochastic process, i.e. a family of random variables o(t) Given a predicate ᵠ, the event that o satisfies ᵠ is a random event. 8
9 MarkovChain Model Assumes discrete state space S and discrete time space T Given the position of an object o at time t=i, the position at t=i+ is conditionally independent of t=i Transition probabilities stored in a (sparse) State x State matrix M(o,t), called transition matrix M(o,t)[i,j] is the probability that object o will transition to state j at time t+, given o is located at state i at time t Use sparse matrix operations for efficient implementation 9
10 State of the art MonteCarlo Given a single observation, use the model to sample possible worlds. The fraction of such worlds satisfying the query predicate is an unbiased estimator of the true probability.
11 State of the art MonteCarlo Problem: Cannot handle multiple observations Most samples will miss further observations Expected number of samples to aquire a good sample grows exponentially
12 Paradigm for Query Processing in probabilistic databases Given a probabilistic database representing a exponential number of possible worlds. Given some query predicate φ, identify a polynomial set of disjunctive classes of possible worlds that are equivalent with respect to φ. Perform query processing using these classes. In the following, this paradigm will be used to answer probabilistic window queries such as Return for each object o DB the probability P o, that o intersecets a given spatial query window at any time within a given query time interval. 2
13 Apply this Paradigm to MarkovChains For window queries, we only need to consider the follow classes: The class of worlds that intersect the window regardless of their state. For the remaining worlds, one class s i for each spatial state 3
14 4 Example M o, t = M =
15 5 Example M o, t = M =
16 6 Example M o, t = M =
17 7 Example M o, t = M =
18 8 Example M o, t = M =
19 9 Example M = M = M + =
20 2 Example M = M = M + =
21 2 Example M = M = M + =
22 Example M = M = M + = P =
23 MultiObservationCase: Example 23
24 MultiObservationCase: Example We need to track where true hit worlds are located 2* S classes of equivalent worlds One class S i  corresponding to worlds where o is located in state S i, and o has not intersected the window One class S i + corresponding to worlds where o is located in state S i, and o has not intersected the window 24
25 MultiObservationCase: Example S S 2 S 3 S + S 2 + S
26 MultiObservationCase: Example S S 2 S 3 S + S 2 + S
27 MultiObservationCase: Example S S 2 S 3 S + S 2 + S
28 MultiObservationCase: Example S S 2 S 3 S + S 2 + S
29 Add knowledge to the model: Bayesian Approach S S 2 S 3 S + S 2 + S ? P ) = 29
30 Add knowledge to the model: Bayesian Approach S S 2 S 3 S + S 2 + S P ) = P( ) P( ) P( ) 3
31 Add knowledge to the model: Bayesian Approach S S 2 S 3 S + S 2 + S P ) = P( = P( ) P( ) ) P( ) P( ) 3
32 Add knowledge to the model: Bayesian Approach S S 2 S 3 S + S 2 + S P ) = P( ) P( ) P( ) = P( ) P( ) P( ) = P + P( ) 32
33 Add knowledge to the model: Bayesian Approach S S 2 S 3 S + S 2 + S P ) = P( ) P( ) P( ) = P( ) P( ) P( ) = P + P( ) = +.6 = 33
34 MultiObservationCase: Example S S 2 S 3 S + S 2 + S
35 MultiObservationCase: Example S S 2 S 3 S + S 2 + S
36 Experimental Evaluation 36
37 Experimental Evaluation 37
38 Experimental Evaluation 38
39 In many applications, uncertainty of data is inherent Ignoring uncertainty may yield wrong results Use stochastic processes to model the movement of objects between observations Augment the processes with efficient probabilistic query processing techniques Use Bayesian inference to incorporate new observations 39
40 Indexing of uncertain spatiotemporal data Different query predicates (e.g. Epsrange, knn,...) Different stochastic processes (e.g. Markovprocesses for continuous time) Perform realdata experiments, using GPS data to build the MarkovChain model. 4
41 4