Tracking Nearest Surrounders in Moving Object Environments *

6/26/2006 ICPS'06

Tracking Nearest Surrounders in Moving Object Environments*

Ken C. K. Lee1 Josh Schiffman1 Baihua Zheng2

Wang-Chien Lee1 Hong Va Leong3

1 Pennsylvania State University2 Singapore Management University 3 Hong Kong Polytechnic University

* International Conference on Pervasive Service, Lyon, France, June 2006

6/26/20062ICPS'06

Outline

Introduction Nearest Surrounder Query Proposed Techniques Performance Evaluation Conclusion

6/26/20063ICPS'06

Traffic infoNavigation

Local weather

Emergency service

Logistics

Location-Based Services

Geographical InformationSystem (GIS)Tracking

Mobile Commerce

6/26/20064ICPS'06

Moving Object Tracking One of the essential LBSs

Moving object tracking Perform a spatial query on moving objects continuously

Existing studies Continuous Window Query Continuous Nearest Neighbor Query

Above queries are based on (Euclidean) distance. Angle is also an important and interesting factor in

spatial queries. Our focus:

Nearest Surrounder Query

6/26/20065ICPS'06

What is Nearest Surrounder (NS) Query? A query which finds nearest objects to a query point

at every distinct angle. Recently reported at ICDE’06.

Applications Robot football: How a robot

decide where to pass the ball? Digital battlefield: How a soldier finds a fire clearing

path?

Nearest Surrounder Query

6/26/20066ICPS'06

Given a query point, q, NS(q) searches the database to return a result set:

{<object, angular range>}

where object is the nearest to q within the specified angular range.

Example: NS(q) =

{ <o1, [g, a)>, <o3, [a, b)>,

<o6, [b, c)>, <o7, [c, d)>,

<o8, [d, e)>, <o9, [e, f)>,

<NULL,[f, g) }

NULL means no object found Objects o2, o4, o5, o10, are not NS.

αd αe

αcq

o7

o6

O3

o1

o9o8

o5

o4 o2

o10

αb

αf

αg

αa

scenario

NS Query - Definition

6/26/20067ICPS'06

Previous work on NS Query is based on static objects.

What about the moving object environments? Scenario

o1 moved down a bit

o2 (previously blocked by o1) is

now included in the result set. Part of the empty angular range

is now covered by o1.

scenario

αd αe

αcq

o7

o6

O3

o9o8

o5

o4

o10

αaαb

αf

αh

o1

o2

αi

Continuous NS Query

6/26/20068ICPS'06

This work presents a framework for continuously tracking NS query results.

Two major costs in keeping NS query results up-to-date in a moving object environment. Query lookup Query result update

Safe region is used to address the issue of query lookup

Partial query evaluation rather than query reevaluation is used to reduce the cost of query result update.

Through experimentation, we show the efficiency of these two techniques adopted in our framework.

Our Contribution

6/26/20069ICPS'06

Users (query clients) register their queries at the database server.

Moving objects update the server periodically. Base station relays messages between wireless

and wired network Database server matches updates to NS queries.

movingobject

movingobject

change ofquery result(centralized)

database serverobject

locationupdate

basestation

querysubscription

query clientService area

System Framework

6/26/200610ICPS'06

Request queue Pending requests

Data storage Object index (R-tree) Query index (R-tree)

Processes Query processor

1. Query evaluartion

2. Determining Safe Regions for Queries

Location updater1. Query Lookup

2. Partial Query Evaluation

queryregistry

queryindex

objectindex

locationupdater

queryprocessor

requ

est q

ueue

objectlocationupdate

querysubscription/

withdraw

Database Architecture

6/26/200611ICPS'06

Used to alleviate query lookups Two types of safe regions

Safe regions for closed angular ranges Safe regions for open angular ranges

Safe regions are indexed in query index. Two indices: Closed Angular Range/Open Angular

Range

If a query whose safe region is not touched by moved objects (old/new positions), query evaluation is not needed.

Issues The more precise a safe region, the less false hits

Safe Regions

6/26/200612ICPS'06

Safe region for closed angular range

Bounding Circle Quadrant-Based Bounding Box

αd αe

αcq

o7

o6

O3

o1

o9o8

o5

o4 o2

o10

αaαb

hollowpolygon

αf

αg

αd αe

αc

o7

o6

O3

o1

o9o8

o5

o4 o2

o10

αaαb

αf

αg

q

Safe Region Approximation

6/26/200613ICPS'06

Safe region for open angular range Bounded region of empty angular range inside the

service area. Cover a lot of unnecessary space.

αd αe

αcq

o7

o6

O3

o1

o9o8

o5

o4 o2

o10

αaαb

αg

Service area

empty space

αf

empty space

safe region

Subregionnot

covered

Approximated safe region

Safe Region Approximation

6/26/200614ICPS'06

Full query revaluation Evaluate a query whenever a moved object touches its

safe region.

Partial query re-evaluation Evaluate only the portion of a NS query result affected by

moved object. Object movement considered as

deletion of the old position followed by insertion of the new position

Query Re-Evaluation

6/26/200615ICPS'06

Deletion of an object position may introduce other objects (previously hidden by the

removed object) to the result set; may introduce open angular range.

Example: Deleting o1

Remove <o1:[g, a)>

from existing result. Add <o2:[g’, a)> and

<NULL: [g, g’)> to the result.

αd αe

αcq

o7

o6

O3

o1

o9o8

o5

o4 o2

o10

αaαb

αf

αg

αg'

(deleted)

Partial Evaluation - Deletion

6/26/200616ICPS'06

Insertion of a new object position may remove some existing objects from the resultt set; may remove an open angular range.

Example: Adding o1

Remove <o2:[g ’, a)> and <NULL: [f, g’)>

from existing result. Add <NULL:[f, h)>,

<o1: [h, i)>, and <o2:[i, a)>

to the result.

αd αe

αcq

o7

o6

O3

o9o8

o5

o4

o10

αaαb

αf

αh

o1

o2

αi

αg

αg'

Partial Evaluation - Insertion

6/26/200617ICPS'06

Experiment Settings Candidates:

Brute Force (BF) (search queries and reevaluate them when objects move)

Safe Region (SR) lookup queries with safe regions as filters Partial Evaluation (PE) determine the partial change to the NS

result. Parameter settings:

Runtime environment: RedHat Linux Enterprise 3.0 on Xeon 2.6 computers

Performance Evaluation

6/26/200618ICPS'06

Processing time (log) – elapse time for a query to be identified and updated.

SR+PE outperform all others, SR is the second best. BF, the baseline case, does the worst.

Real Datasets

0.01

0.1

1

10

100

1000

RI PA

BF

SR

SR+PE

Pro

cess

ing

time

(se

c)

6/26/200619ICPS'06

QB Box outperforms BCircle because of more precise safe regions.

0.01

0.1

1

10

Uniform RI PA

BCircle

QB BBoxP

roce

ssin

g tim

e (s

ec)

1

10

100

Uniform RI PA

1

10

100

1000

10000

CP

U t

ime

(sec

)

Safe Region Approximation

6/26/200620ICPS'06

This paper studies the issues of tracking nearest surrounder query in moving object environment

Based on a centralized approach Safe region and partial evaluation are effective for

query lookup and query result updates

Future work Moving query point. Distributed approach

Conclusion & Future Work