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Spatio-Temporal GIS Philip Sargent May 25th 1998
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Spatio-Temporal GIS Philip Sargent May 25th 1998
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3 Goals Represent time-varying spatial data –store lots of data –manage lots of data Task-oriented operations More capable concepts
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4 Fields or Objects ? Temporal fields (rasters, TINs) or Temporal entities (objects, vectors) ? –We have to do both.
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5 What happened, where & when ? –land ownership –fires, floods Future effects of policies –models, futures, versions Generalisation –minutes to months, cycles Who needs it ?
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6 Example Change of administrative areas: R R’ R R” R’ R 19711981 1991
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7 Example Road planning: road town bypass 1996 a b c road town bypass 1996 a d c e f 1995 19961997 road town bypass 1996 a d c e f
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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
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9 t-GIS system types
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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.
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11 Relational Databases ? Conceptual mis-match. Commercial importance. Future (O)RDBMs are not purely table-based anyway. Clear OO advantage.
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12 Snapshots Complete GIS copy at each timestamp
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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
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14 Vector Snapshots Space-Time Composite: Changing attribute is the classification.
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15 Spatio-Temporal Objects
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16 Rasters and Events
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17 Change types in a t-GIS
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18 Granularity
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19 Simple Bitemporal Jan. 1998 Jan. 1997 Jan. 1996 Jan. 1995 Jan. 1998Jan. 1997Jan. 1996Jan. 1995 Transaction time Valid time
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20 Road planning road town bypass 1996 a d c e f a d d e c e f b c d
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21 Spatio-temporal operations lifetime(road) a d c e f b max-S-project(road) a d c min-S-project(road)
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22 Temporal Indexes Standard B -tree intervals + 1997 1998 Nov-Dec. 1997 Christmas Eve 1997
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23 Interpretations as Objects Forest fire objects Geometry: {…}
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24 Multiple geometries Forest fire object Geometry: {…} time-sequence of geometries
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25 Interpretations as Object Forest fire object Geometry: {(…),(…),(…)}
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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
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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.
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28 Commercial t-GIS ? Why no commercial temporal databases at all ? Why are GIS vendors nor producing t-GIS ?.
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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.
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