On monitoring the top k unsafe places
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On Monitoring the top-k Unsafe Places. Donghui Zhang , Yang Du, Ling Hu Northeastern University. ICDE ’ 08, Cancun, Mexico. Problem (informal). Police cars move around and protect neighborhoods. How to monitor the places that are under-protected?. Model.

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On Monitoring the top-k Unsafe Places

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On monitoring the top k unsafe places

On Monitoring the top-k Unsafe Places

Donghui Zhang,

Yang Du, Ling Hu

Northeastern University

ICDE’08, Cancun, Mexico


Problem informal

Problem (informal)

  • Police cars move around and protect neighborhoods. How to monitor the places that are under-protected?

top-k unsafe place monitoring


Model

Model

  • A centralized server stores the map of the city, along with the required protection of every place in the city.

  • A police car reports its position updates to the server.

  • The server maintains the top-k unsafe places.

top-k unsafe place monitoring


On monitoring the top k unsafe places

  • rp: required protection

  • ap: actual protection

  • safety: ap - rp

For each place:

top-k unsafe place monitoring


Problem formal

Problem (formal)

  • Input:

    • a set of places:

      • rp

      • location (static)

    • a set of police cars:

      • protection region

      • location (keeps changing)

  • Output:

    • Continuously monitor the k places with the least safety.

top-k unsafe place monitoring


Na ve approach

Naïve Approach

  • Upon each location update (of a police car), compute the safety of every place.

  • Too slow…

top-k unsafe place monitoring


Basicctup

BasicCTUP

top-k unsafe place monitoring


Drawbacks of basicctup

Drawbacks of BasicCTUP

  • Unnecessary lower bound decreasing.

  • Too many maintained places.

  • Flashing phenomenon.

top-k unsafe place monitoring


Drawbacks of basicctup1

Drawbacks of BasicCTUP

  • Unnecessary lower bound decreasing.

  • Too many maintained places.

  • Flashing phenomenon.

P  P: decrease the lower bound;

Move again  decrease again (unnecessarily!)

top-k unsafe place monitoring


Drawbacks of basicctup2

Drawbacks of BasicCTUP

  • Unnecessary lower bound decreasing.

  • Too many maintained places.

    • Many places in an illuminated cell maybe very safe!

    • Maintaining them increases cost.

  • Flashing phenomenon.

top-k unsafe place monitoring


Drawbacks of basicctup3

Drawbacks of BasicCTUP

  • Unnecessary lower bound decreasing.

  • Too many maintained places.

  • Flashing phenomenon.

    • Assume a cell’s lower bound = SK.

    • Any police car move

       decrease lower bound

       illuminate (suppose no top-k place)

       darken

top-k unsafe place monitoring


Optctup

OptCTUP

  • Unnecessary lower bound decreasing.

    In OptCTUP, decrease only once! (use a hash to avoid future decreasing)

  • Too many maintained places.

  • Flashing phenomenon.

top-k unsafe place monitoring


Optctup1

OptCTUP

  • Unnecessary lower bound decreasing.

    In OptCTUP, in case P  F, may even increase the cell lower bound!

  • Too many maintained places.

  • Flashing phenomenon.

top-k unsafe place monitoring


On monitoring the top k unsafe places

top-k unsafe place monitoring


On monitoring the top k unsafe places

top-k unsafe place monitoring


Optctup2

OptCTUP

  • Unnecessary lower bound decreasing.

  • Too many maintained places.

    • In OptCTUP, keep all cells dark.

    • Top-k unsafe places (and some more) are maintained.

    • The lower bound of a cell is computed as if the maintained objects do not exist.

  • Flashing cells increase cost.

top-k unsafe place monitoring


Optctup3

OptCTUP

  • Unnecessary lower bound decreasing.

  • Too many maintained places.

  • Flashing phenomenon.

    • In OptCTUP, after a cell is illuminated, make sure its lower bound ≥ ∆ + SK.

    • Done by keeping some more objects in buffer.

    • This allows ∆ decreases before another illumination is necessary.

top-k unsafe place monitoring


Experimental setup

Experimental Setup

  • Network-based Generator of Moving Objects, Thomas Brinkhoff.

top-k unsafe place monitoring


Initialization

Initialization

top-k unsafe place monitoring


Update cost

Update cost

top-k unsafe place monitoring


Conclusions

Conclusions

  • Proposed the CTUP query, which is a new addition to the family of continuous location-based queries.

  • Proposed Algorithm OptCTUP.

  • May be interesting to law enforcement agencies.

Q & A...

top-k unsafe place monitoring


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