Spatio-Temporal GIS

Philip SargentMay 25th 1998

3

Goals

• Represent time-varying spatial data– store lots of data– manage lots of data

• Task-oriented operations• More capable concepts

4

Fields or Objects ?

• Temporal fields (rasters, TINs)or

• Temporal entities (objects, vectors) ?

– We have to do both.

5

• What happened, where & when ?– land ownership– fires, floods

• Future effects of policies– models, futures, versions

• Generalisation– minutes to months, cycles

Who needs it ?

6

Example

Change of administrative areas:

R R’ R’R R”R’

R

1971 1981 1991

7

Example

Road planning:

road

town

bypass1996

a

bc

road

town

bypass1996

a

d

c

e

f

1995 1996 1997

road

town

bypass1996

a

d

c

e

f

8

Two major kinds of Time

• Valid Time,synonyms:– real-time

– world-time

– event time

• User-defined time– uninterpreted value

• Transaction Time, synonyms:– database time

– registration time

– system time

– commit time

• Version information– not just time

9

t-GIS system types

Valid time Transaction time

static x x

historical x

rollback x

bitemporal

10

Types of time value

• Instants at different granularities

• Spans, Intervals (Periods)

• Relative times• Open intervals

• Often use different types for valid and transaction times.

11

Relational Databases ?

• Conceptual mis-match.

• Commercial importance.

• Future (O)RDBMs are not purely table-based anyway.

• Clear OO advantage.

12

Snapshots

Complete GIS copy at each timestamp

13

Events by Snapshots

Temporal Map Set– raster only– defines “Events” for

cells– geometry static

GIS MapJanuary 1997

GIS MapJanuary 1998

April 1997

March 1997

February 1997

14

Vector Snapshots

Space-Time Composite: Changing attribute is the classification.

15

Spatio-Temporal Objects

16

Rasters and Events

17

Change types in a t-GIS"Change"

TypeLocation

(geometry)Attribute

valueTimestampor period

I fixed select measure

II category measure select fixed

III static select measure fixed

IV trans. select fixed measure

V mutation fixed measure select

VI movement measure fixed select

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Granularity

Attributevalue

Object Segment Dataset

Valid time effectivities OO GIS - -

Transactiontime

valuehistories

objectversions

versions snapshots

19

Simple Bitemporal

Jan. 1998

Jan. 1997

Jan. 1996

Jan. 1995

Jan.

199

8

Jan.

199

7

Jan.

199

6

Jan.

199

5

Transaction time

Val

id t

ime

20

Road planning

road

town

bypass1996

a

d

c

e

f

a

d

de

c

e

f

bc

d

21

Spatio-temporal operations

lifetime(road)

a

d

c

e

f

b

max-S-project(road)

a

d

c

min-S-project(road)

22

Temporal Indexes

Standard B -tree intervals+

1997 1998

Nov-Dec. 1997

Christmas Eve 1997

23

Interpretations as Objects

Forest fire objects Geometry: {…}

Geometry: {…}Geometry: {…}Geometry: {…}

24

Multiple geometries

Forest fire object

Geometry: {…}

Geometry: {…}

Geometry: {…}

Geometry: {…}

time-sequence of

geometries

25

Interpretations as Object

Forest fire object Geometry: {(…),(…),(…)}

26

• Significant t-db community (www).• ISO SQL3 temporal plans.• Small t-GIS community (www).• No commercial t-GIS.• Existing OO GIS can be

used to provide some temporal capabilities.

Further Information

27

Who doesn’t need t-GIS ?

• Lots of people with mature, sophisticated time-sensitive tools, e.g. Statecharts etc.

• Not everyone needs a t-GIS to do temporal work.

28

Commercial t-GIS ?

• Why no commercial temporal databases at all ?

• Why are GIS vendors nor producing t-GIS ?.

29

Dynamic Schema

• What happens when the database schema itself changes with time ?– Some comparisons become inexpressible– No problem with non-temporal OO GIS– Schema stored on classes, not object instances– No problem in principle: classes are objects– Schema evolution is a fact of life.