What have we learned about time?

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

2010-02-23T10:05:00Z-08:00 = 23 Feb 2010, 10:05 AM, Pacific Standard Time

2010-01-01T00:00:00 = Year 2010, Month Jan 2010, and Day Jan 1, 2010

Time instants can be readily specified …. …. Time intervals are more elusive

• Begin with hourly data and then average the data:1. For each day2. For each month3. For each year4. For a day of the year5. For a month of the year6. For an hour of the day

• 2010-02-23T10:00:00

“Dimensions” of Time

Parse a time stamp into “dimensions”….. ……. Plot the data for each dimension

Time ZonesPrime Meridian in Space (0º) has Universal Coordinated Time (Z)

We need spatial-temporal reference frames……. …. and functions for “projecting” data in space-time

Universal Coordinate Time (Greenwich Mean Time)

• 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)

GIS should transparently handle time coordinate transformations ……

Downward Solar Radiation (W/m2)January 2003

1200 Z

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

Space

Time

2010-02-23T10:05:00Z-08:00