What have we learned about time?

1
What have we learned about time?
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Time Instants and Intervals

• Time is continuous but data are discrete
• At instants of time
• Over intervals of time
• Time is indexed by instantaneous time stamps
• In relational databases, intervals are time
  stamped at the beginning

Time
2010-02-23T100500Z-0800 23 Feb 2010, 1005
AM, Pacific Standard Time
2010-01-01T000000 Year 2010, Month Jan 2010,
and Day Jan 1, 2010
Time instants can be readily specified .
. Time intervals are more elusive
3
Dimensions of Time

• 2010-02-23T100000

• Begin with hourly data and then average the data
• For each day
• For each month
• For each year
• For a day of the year
• For a month of the year
• For an hour of the day

Parse a time stamp into dimensions..
. Plot the data for each
dimension
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Time Zones
Prime Meridian in Space (0º) has Universal
Coordinated Time (Z)
We need spatial-temporal reference frames.
. and functions for
projecting data in space-time
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Universal Coordinate Time (Greenwich Mean Time)
Downward Solar Radiation (W/m2) January 2003

• Universal Coordinate Time (UTC) is like
  Geographic Coordinates in space
• Local Time at any location is offset from UTC
• Offset may change seasonally (Daylight and
  Standard time)

1200 Z
GIS should transparently handle time coordinate
transformations
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Conclusions

• Time is a subtle thing
• Really continuous but data are discrete
• Time stamps play a key role
• Instantaneous at the beginning of intervals
• Time has interacting dimensions
• Hour, day, month, year
• Time has two forms
• Universal time (like geographic coordinates)
• Local time (like projected coordinates)
• We need space-time reference frames
• Tools for moving data between them

2010-02-23T100500Z-0800