spatio-temporal gis philip sargent may 25th 1998
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Spatio-Temporal GIS
Philip SargentMay 25th 1998
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Goals
• Represent time-varying spatial data– store lots of data– manage lots of data
• Task-oriented operations• More capable concepts
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Fields or Objects ?
• Temporal fields (rasters, TINs)or
• Temporal entities (objects, vectors) ?
– We have to do both.
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• 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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Example
Change of administrative areas:
R R’ R’R R”R’
R
1971 1981 1991
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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
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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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t-GIS system types
Valid time Transaction time
static x x
historical x
rollback x
bitemporal
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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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Relational Databases ?
• Conceptual mis-match.
• Commercial importance.
• Future (O)RDBMs are not purely table-based anyway.
• Clear OO advantage.
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Snapshots
Complete GIS copy at each timestamp
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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
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Vector Snapshots
Space-Time Composite: Changing attribute is the classification.
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Spatio-Temporal Objects
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Rasters and Events
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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
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Simple Bitemporal
Jan. 1998
Jan. 1997
Jan. 1996
Jan. 1995
Jan.
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Jan.
199
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Jan.
199
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Jan.
199
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Transaction time
Val
id t
ime
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Road planning
road
town
bypass1996
a
d
c
e
f
a
d
de
c
e
f
bc
d
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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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Temporal Indexes
Standard B -tree intervals+
1997 1998
Nov-Dec. 1997
Christmas Eve 1997
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Interpretations as Objects
Forest fire objects Geometry: {…}
Geometry: {…}Geometry: {…}Geometry: {…}
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Multiple geometries
Forest fire object
Geometry: {…}
Geometry: {…}
Geometry: {…}
Geometry: {…}
time-sequence of
geometries
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Interpretations as Object
Forest fire object Geometry: {(…),(…),(…)}
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• 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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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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Commercial t-GIS ?
• Why no commercial temporal databases at all ?
• Why are GIS vendors nor producing t-GIS ?.
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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.