Tracking Nearest Surrounders in Moving Object Environments*

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Tracking Nearest Surrounders in Moving Object
Environments

• Ken C. K. Lee1 Josh Schiffman1 Baihua Zheng2
• Wang-Chien Lee1 Hong Va Leong3

1 Pennsylvania State University 2 Singapore
Management University 3 Hong Kong Polytechnic
University
International Conference on Pervasive Service,
Lyon, France, June 2006
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Outline

• Introduction
• Nearest Surrounder Query
• Proposed Techniques
• Performance Evaluation
• Conclusion

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Location-Based Services
Logistics
Traffic info
Geographical Information System (GIS)
Tracking
Emergency service
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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
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Nearest Surrounder Query

• What is Nearest Surrounder (NS) Query?
• A query which finds nearest objects to a query
  point at every distinct angle.
• Recently reported at ICDE06.
• Applications
• Robot football How a robot
• decide where to pass the ball?
• Digital battlefield How a soldier finds a fire
  clearing path?

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NS Query - Definition

• Given a query point, q, NS(q) searches the
  database to return a result set
• ltobject, angular rangegt
• where object is the nearest to q within the
  specified angular range.
• Example
• NS(q)
• lto1, ?g, ?a)gt, lto3, ?a, ?b)gt,
• lto6, ?b, ?c)gt, lto7, ?c, ?d)gt,
• lto8, ?d, ?e)gt, lto9, ?e, ?f)gt,
• ltNULL,?f, ?g)
• NULL means no object found
• Objects o2, o4, o5, o10, are not NS.

scenario
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Continuous NS Query

• 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
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Our Contribution

• 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.

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System Framework

• 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.

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Database Architecture

• 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 updater
• 1. Query Lookup
• 2. Partial Query Evaluation

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Safe Regions

• 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

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Safe Region Approximation

• Safe region for closed angular range
• Bounding Circle Quadrant-Based Bounding Box

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Safe Region Approximation

• Safe region for open angular range
• Bounded region of empty angular range inside the
  service area.
• Cover a lot of unnecessary space.

Approximated safe region
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Query Re-Evaluation

• 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

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Partial Evaluation - Deletion

• 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 lto1?g, ?a)gt
• from existing result.
• Add lto2?g, ?a)gt and
• ltNULL ?g, ?g)gt to the result.

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Partial Evaluation - Insertion

• 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 lto2?g , ?a)gt and ltNULL ?f, ?g)gt
• from existing result.
• Add ltNULL?f, ?h)gt,
• lto1 ?h, ?i)gt, and lto2?i, ?a)gt
• to the result.

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Performance Evaluation

• 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

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Real Datasets

• Processing time (log) elapse time for a query
  to be identified and updated.
• SRPE outperform all others, SR is the second
  best. BF, the baseline case, does the worst.

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Safe Region Approximation

• QB Box outperforms BCircle because of more
  precise safe regions.

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Conclusion Future Work

• 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