Spatio-Temporal GIS Philip Sargent May 25th 1998

Spatio-Temporal GIS Philip Sargent May 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

7 Example Road planning: road town bypass 1996 a b c road town bypass 1996 a d c e f road town bypass 1996 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

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 Map January 1997 GIS Map January 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

18 Granularity

19 Simple Bitemporal Jan Jan Jan Jan Jan. 1998Jan. 1997Jan. 1996Jan Transaction time Valid time

20 Road planning road town bypass 1996 a d c e f a d d e c e f b c 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 Nov-Dec Christmas Eve 1997

23 Interpretations as Objects Forest fire objects Geometry: {…}

24 Multiple geometries Forest fire object 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.