MOVING OBJECT DATABASES: BASIC CONCEPTS - by shivangi Gupta

CONTENTS Introduction Problem with Current Database Moving Object Database Approach MOST model References

INTRODUCTIONMoving object - “Objects whose position changes

continuously over time”.Eg. Moving cars, fighter aeroplane , ship etc Present database don’t know how to manage and

query such objects. Problem- Normally data stored is assumed to be

constant and doesnot change frequently.

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PROBLEMS WITH CURRENT DATABASES (CONT.)

To represent a moving object in a database, we have to update very frequently the position of the moving object

THIS IS A PROBLEM!

• Serious performance and wireless-bandwidth overhead

• If not, the answer to queries is outdated.

SOLUTION???

MOVING OBJECT DATABASE

Moving Object Databases is a database that can represent and query the moving object .

It can run powerful query languages and answer any kind of questions about movements.

eg- Which taxi is closest to a passenger request position.

Which routes are used regularly by trucks?

APPROACH Location management perspective

Spatio-temporal data perspective.

LOCATION MANAGEMENT PERSPECTIVE

It is a snapshot database that is used for 2 purposes: Maintain dynamically the locations of a set of currently

moving objects. Ask queries about the current or near future positions of the

moving objects. No history of data is kept.

Scenario- Moving object send their current positions to the database and database performs updates.

EXAMPLE- POSITIONS OF ALL TAXI-CABS IN A CITY

At a given instant of time, there is no problem. We might have a relation with a taxi-ID as a key and attributes for x and

y-coordinates to record the position. However, taxis are moving around. To keep the location information up to date, for each taxi-cab the

position has to be updated frequently. Here we encounter an unpleasant trade-off. If updates are sent and

applied to the database very often, the error in location information in the database is kept small, yet the update load becomes very high.

Conversely, if updates are sent less frequently, the errors in the recorded positions relative to the actual positions become large.

TRADE-OFF- If updates performed very often, then: - the error is small.

- the update load is very high. If the updates performed are less frequent, then - the update load is low.

- the error is large.

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TRAJECTORYTrajectory: a sequence of 2 or more locations.

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Model of a trajectory

Geometrical representation in 3D space Points (x1,y1,t1), (x2,y2,t2)

The resulting line segments comprise a polyline in 3D, the trajectory of the moving point object.

Solution- - Storing speed and direction of moving objects in

addition. - Use this information to predict near future object’s

positions. - Update overhead is reduced since Update is

performed only when velocity changes.

MOTION VECTOR For each moving object instead of storing the current

position in the database,we store a motion vector which amounts to describing the position as a function of time.

That is, if we record for an object its position at time t0 together with its speed and direction at that time, we can derive expected positions for all times after “t0”.

Motion vectors need to be updated from time to time, but much less frequently than positions.

SPATIO-TEMPORAL DATA PERSPECTIVE. It’s a database which not only stores the current state of the

spatial data, but rather the whole history of this development. This type of database is used in following situations 1.Need to go back in time to any particular instant and to

retrieve the state at that time.eg Did whales habitats move in the last 20 years?

2.For understanding how things changed, analyze when certain relationships were fulfilled. Eg Is the dead sea shrinking?

Two basic questions arises- 1. What kind of data are stored in spatio - temporal database -Point, line, region, network, partition. 2. Which kind of change may occur

-Discrete change: Construction of a building. - Continuous change: Vehicle.

Moving point- physical object moving around in the plane or a higher dimensional space, for which only the position, but not the extent, is relevant.

Ships • Are any ships heading towards shallow areas?• Find “strange” movements of ships indicating illegal dumping of waste.Air planes • Were any two planes close to a collision?Satellites• Which satellites will get close to the route of this spacecraft within the next 4 hours?

Moving region – describes an entity in the plane that changes its position as well as its extent and shape, i.e., a moving region may not only move, but also grow and shrink.

Storms • Where is the tornado heading? When will it reach Florida? Forests, lakes • How fast is the Amazon rain forest shrinking?

• What is the minimal and maximal extent of river X duringthe year?

SPATIO-TEMPORAL OBJECTS

(a) (b) a moving point a moving and shrinking region

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MOST-MOVING OBJECTS SPATIO TEMPORAL MODEL

In this a location prediction function is defined as a dynamic attribute of a moving object to facilitate its predicting future location.

The location of moving object is tracked using some efficient dead reckoning method.

Dead recknoning method - In this method an update is generated to update the location of an object whenever the deviation from its current position is greater than a pre-defined threshold.

Each server maintains a mobile object database, which adopts MOST model to record the location information of mobile object dynamically.EXAMPLE: Consider a mobile object MO ,whose last update is issued to the databae at time t0. At that time the position of MO is <x0, y0 > The scalar of speed vector is v0 and direction is a0.

If the next update from MO is at time t1, then at time t’ in the time period [t0, t1 ], the position of MO <x’, y’> can be predicted as