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Techniques for Efficient Road-Techniques for Efficient Road-Network-Based Tracking of Network-Based Tracking of

Moving ObjectsMoving Objects

Speaker : Jia-Hui HuangSpeaker : Jia-Hui Huang

Date : 2006/10/23Date : 2006/10/23

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ReferenceReference

[1] Alminas Civilis, Christian S. Jensen, Senior and stardas Pakaln[1] Alminas Civilis, Christian S. Jensen, Senior and stardas Pakalnis, “Techniques for Efficient Road-Network-Based Trackinis, “Techniques for Efficient Road-Network-Based Tracking of Moving Objects,” IEEE Transations on knowledge and data g of Moving Objects,” IEEE Transations on knowledge and data engineering. VOL.17,NO.5, MAY 2005engineering. VOL.17,NO.5, MAY 2005

[2] [2] A. CCivilis, C.S. Jensen, J. Nenortaite, and S. PakalA. CCivilis, C.S. Jensen, J. Nenortaite, and S. Pakalnis, “Efficient Tracking of Moving Objects with Precinis, “Efficient Tracking of Moving Objects with Precision Guarantees,” Proc. Int’l Conf. Mobile and Ubiqsion Guarantees,” Proc. Int’l Conf. Mobile and Ubiquitous Systems: Networking and Services, pp. 164-173,uitous Systems: Networking and Services, pp. 164-173, 2004, extended version available as DB-TR-5, Dept. of 2004, extended version available as DB-TR-5, Dept. of Computer Science, Aalborg Univ., Denmark, http://Computer Science, Aalborg Univ., Denmark, http://

www.cs.aau.dk/DBTR/DBPublications/DBTR-5.pdf.www.cs.aau.dk/DBTR/DBPublications/DBTR-5.pdf.

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OutlineOutline

IntroductionIntroduction Existing Tracking ApproachesExisting Tracking Approaches Road Network modificationRoad Network modification Update reduction using routesUpdate reduction using routes Update reduction using acceleration Update reduction using acceleration

profileprofile SummarySummary

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Introduction (1/4)Introduction (1/4)

Location-based services ( LBSs )Location-based services ( LBSs ) Tracking geographical positions of moving Tracking geographical positions of moving

objectobject Large volumes of location updatesLarge volumes of location updates How to reduce update within predefined How to reduce update within predefined

accuracy ?accuracy ? Use road network for predicting moving Use road network for predicting moving

object’s future positions.object’s future positions.

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Introduction (2/4)Introduction (2/4)

DefinitionDefinition Road network composed of a set of segments.Road network composed of a set of segments. A segment is defined as a sequence of A segment is defined as a sequence of

coordinates.coordinates. Road network is partitioned into streets.Road network is partitioned into streets. Each segment belongs to precisely one street.Each segment belongs to precisely one street. Each segment identifies its street by referring Each segment identifies its street by referring

to a street code.to a street code.

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Introduction (3/4)Introduction (3/4)

Tracking scenarioTracking scenario GPSGPS Tracking approachTracking approach ThresholdThreshold

ChallengeChallenge Predict future Predict future

positionposition Minimize updateMinimize update

Central Database

Mobile user With

GPS

update InformPredictionapproach

moving

Mobile user With

GPS

update

InformPredictionapproach

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Introduction (4/4)Introduction (4/4)

Scenario from UML viewScenario from UML view

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OutlineOutline

IntroductionIntroduction Existing Tracking ApproachesExisting Tracking Approaches Road Network modificationRoad Network modification Update reduction using routesUpdate reduction using routes Update reduction using acceleration Update reduction using acceleration

profileprofile SummarySummary

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Existing Tracking ApproachesExisting Tracking Approaches

Three existing tracking approachesThree existing tracking approaches Point-based trackingPoint-based tracking

Represents object’s future position as the most Represents object’s future position as the most recently reported position.recently reported position.

Update when position change exceed threshold.Update when position change exceed threshold. Vector-based trackingVector-based tracking

Future position compute by speed and heading of Future position compute by speed and heading of moving object. (velocity vector)moving object. (velocity vector)

Zero-vector case of vector-based. (point)Zero-vector case of vector-based. (point) Segment-based trackingSegment-based tracking

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Existing Tracking ApproachesExisting Tracking Approaches

(a) update (a) update when exceed when exceed thresholdthreshold

(b) update only (b) update only when direction when direction changechange

(a) Point-based

(b) Vector-based

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Existing Tracking ApproachesExisting Tracking Approaches

Segment-based trackingSegment-based tracking Utilize knowledge of the road network.Utilize knowledge of the road network. Digital representation of the road network.Digital representation of the road network. Road network composed of a set of segments.Road network composed of a set of segments. Segment is in-between a pair of consecutive Segment is in-between a pair of consecutive

intersections or an intersection and a dead intersections or an intersection and a dead end.end.

Map matching positions an object on a road Map matching positions an object on a road network segment.network segment.

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Existing Tracking ApproachesExisting Tracking Approaches

Future positions of a client are given by Future positions of a client are given by a movement at constant speed along a movement at constant speed along the identified segment.the identified segment.

Update when deviates exceed thresholdUpdate when deviates exceed threshold Number of update correlated with the Number of update correlated with the

number of changes of segments.number of changes of segments.

(c) Segment-based

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Existing Tracking ApproachesExisting Tracking Approaches

Update time increases as the accuracy Update time increases as the accuracy increasesincreases

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Existing Tracking ApproachesExisting Tracking Approaches

Why improve segment-based approach ?Why improve segment-based approach ? Average length of the road segment is short.Average length of the road segment is short. It may be a relative straight road is It may be a relative straight road is

represented by several segments.represented by several segments. Road-related information can use.Road-related information can use.

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OutlineOutline

IntroductionIntroduction Existing Tracking ApproachesExisting Tracking Approaches Road Network modificationRoad Network modification Update reduction using routesUpdate reduction using routes Update reduction using acceleration Update reduction using acceleration

profileprofile SummarySummary

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Road Network modificationRoad Network modification

Number of update correlated with the Number of update correlated with the number of changes of segments.number of changes of segments.

Connect the road segment that moving Connect the road segment that moving objects would change segment as few as objects would change segment as few as possible.possible.

Two types of modificationTwo types of modification Street Code-based approachStreet Code-based approach Direction-based approachDirection-based approach

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Road Network modificationRoad Network modification

Two types of approach based onTwo types of approach based on General General segment connection algorithm ( GSC )segment connection algorithm ( GSC )

General segment connection algorithm General segment connection algorithm ( GSC )( GSC ) Iterate through all segments according some Iterate through all segments according some

specified ordering.specified ordering. At each iteration, algorithm tries to extend the At each iteration, algorithm tries to extend the

topmost or current segment.topmost or current segment.

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Road Network modificationRoad Network modification

Some parameterSome parameter rn, (road network), a set of road networkrn, (road network), a set of road network stc, a Boolean valued variable that controls stc, a Boolean valued variable that controls

connection with different street codes.connection with different street codes. rnPrioriization, canPrioritization sort order rnPrioriization, canPrioritization sort order

specificationsspecifications Sort order according segment’s propertySort order according segment’s property

Length, speed limit, ….Length, speed limit, ….

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Road Network modificationRoad Network modification Street code-based approachStreet code-based approach

Give priority to connecting with the same street code.Give priority to connecting with the same street code. In cases where several candidates with the same In cases where several candidates with the same

street code, priority given to the shortest one.street code, priority given to the shortest one. Reduces the probability that unconnected segment will Reduces the probability that unconnected segment will

be short.be short. Among candidates segment, the longest are preferred.Among candidates segment, the longest are preferred.

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Road Network modificationRoad Network modification Directional-based approachDirectional-based approach

Moving objects are expected to be moving as Moving objects are expected to be moving as directly as possible toward their destinations.directly as possible toward their destinations.

Extending segment with the same direction as Extending segment with the same direction as much as possible.much as possible.

Preference is given to segment with direction Preference is given to segment with direction at the connection point that has a small angle at the connection point that has a small angle with direction of the segment to be extended.with direction of the segment to be extended.

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Road Network modificationRoad Network modification

Property angleAvg denotes the average of the sProperty angleAvg denotes the average of the smallest angle values possible for both ends of mallest angle values possible for both ends of the segmentthe segment Eg. A segment can be extended with three segment to one sidEg. A segment can be extended with three segment to one sid

e with angles of 34, 22 and 90 degrees, then e with angles of 34, 22 and 90 degrees, then angleAvg = ( 22 +180) /2angleAvg = ( 22 +180) /2

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Road Network modificationRoad Network modification ComparisonComparison

Increase the performance of the segment-based Increase the performance of the segment-based policy and outperform the vector-based policypolicy and outperform the vector-based policy

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Road Network modificationRoad Network modification Comparison of Suburban versus cityComparison of Suburban versus city

Unmodified road network curve suggests that Unmodified road network curve suggests that the majority of updates happens due to the majority of updates happens due to segment changes, not due to speed variation.segment changes, not due to speed variation.

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OutlineOutline

IntroductionIntroduction Existing Tracking ApproachesExisting Tracking Approaches Road Network modificationRoad Network modification Update reduction using routesUpdate reduction using routes Update reduction using acceleration Update reduction using acceleration

profileprofile SummarySummary

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Update reduction using routesUpdate reduction using routes

Constant-speed optimal policyConstant-speed optimal policy Assume moving object travels on only one Assume moving object travels on only one

segment, and no updates occur due to segment, and no updates occur due to segment change.segment change.

Assume constant moving speed.Assume constant moving speed. Gives the lower bound of the number of Gives the lower bound of the number of

update needed by segment-based policy.update needed by segment-based policy.

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Update reduction using routesUpdate reduction using routes

Use of Routes (traveler’s behavior)Use of Routes (traveler’s behavior) Assume the travelers frequently use the same Assume the travelers frequently use the same

routes to their destinations.routes to their destinations. Take advantage of knowledge of the routes Take advantage of knowledge of the routes

used by a moving object.used by a moving object. When using the segment-based policy with When using the segment-based policy with

routes, we know the future positions of an routes, we know the future positions of an object.object.

If an object deviates from its route, this is If an object deviates from its route, this is treated simply as a segment change.treated simply as a segment change.

Like constant-speed optimal policy.Like constant-speed optimal policy.

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Update reduction using routesUpdate reduction using routes

ExperimentExperiment

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OutlineOutline

IntroductionIntroduction Existing Tracking ApproachesExisting Tracking Approaches Road Network modificationRoad Network modification Update reduction using routesUpdate reduction using routes Update reduction using acceleration Update reduction using acceleration

profileprofile SummarySummary

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Update reduction using acceleration profilUpdate reduction using acceleration profilee

Segment-based policy assumes that Segment-based policy assumes that objects move at constant speed.objects move at constant speed.

Model speed as a stair function.Model speed as a stair function. More accurate modeling speed can help More accurate modeling speed can help

better predict the future position of better predict the future position of moving object.moving object. e.g. using averages of the speeds during past e.g. using averages of the speeds during past

traversals of the route.traversals of the route.

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Update reduction using acceleration Update reduction using acceleration profileprofile Acceleration profile capture the average speed variaAcceleration profile capture the average speed varia

tion.tion. Dividing the average speed variation along the route into inDividing the average speed variation along the route into in

tervals.tervals. Average acceleration is calculated for each interval.Average acceleration is calculated for each interval. define profile apf as a sequence of n+1 measuresdefine profile apf as a sequence of n+1 measures and n accelerations and n accelerations ( , ……, , , ) acceleration is valid in interval [ , )( , ……, , , ) acceleration is valid in interval [ , )

e.g. assume an object moves with speed and current location along the re.g. assume an object moves with speed and current location along the route is , belongs to the interval [ , ) oute is , belongs to the interval [ , )

then predicted position and speed of the object at time t isthen predicted position and speed of the object at time t is

im

ia

0m 0a 1nm 1na nm iaim 1im

0v0m 0m beginm

endm

predm predv

20 0 0( / 2) ,pred predm m v t a t v v at

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Update reduction using acceleration Update reduction using acceleration profileprofile ExperimentExperiment

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OutlineOutline

IntroductionIntroduction Existing Tracking ApproachesExisting Tracking Approaches Road Network modificationRoad Network modification Update reduction using routesUpdate reduction using routes Update reduction using acceleration Update reduction using acceleration

profileprofile SummarySummary

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summarysummary

Performance of basic segment-based Performance of basic segment-based tracking is sensitivetracking is sensitive Segmentation representationSegmentation representation Speed variationSpeed variation

Reduce number of update techniqueReduce number of update technique Road network modificationRoad network modification Use of routesUse of routes Use of acceleration profilesUse of acceleration profiles

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General segment connection algorithmGeneral segment connection algorithm

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Speed modelSpeed model

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Speed patternSpeed pattern