Why set a temporal reference
Similar to a spatial reference, a temporal reference provides information about how a particular time was measured. The temporal reference is important because it allows the ArcGIS software to integrate datasets that were recorded in different time zones.
How temporal reference is used to integrate data
Suppose you have two different sets of weather sensor data that were recorded using the local time zone for the sensor. One set of data is recorded in Pacific time zone ([GMT-08:00] Pacific Time [US & Canada]), and the other set of data is recorded in eastern time zone ([GMT-05:00] Eastern Time [US & Canada]). If these two sets of data are added as layers in ArcGIS and time enabled, it will be possible to visualize these datasets using the time slider. However, a problem occurs when the two datasets are played back together, because 5:00 a.m. in the Pacific time zone is not the same time as 5:00 a.m. in the eastern time zone. If this data is animated without modification, data that was recorded at 5:00 a.m. Pacific time will be shown at the same time as data that was recorded at 5:00 a.m. eastern time. In reality, these two sets of data were collected three hours apart.
This situation can be corrected by setting the temporal reference in each layer. When time is enabled on the Pacific time zone layer, you should set its time zone to Pacific time zone. When enabling time on the eastern time zone layer, you should set its time zone to eastern time zone. By setting these values, the time slider can be used to animate the data, and data collected at 5:00 a.m. in Pacific time zone will appear at the same time as data collected at 8:00 a.m. eastern time zone.
In addition to integrating the two different time zones in the data, ArcGIS will also account for the time zone you prefer to use for visualizing the data. For example, suppose you prefer to see the time slider display time to use the central time zone ([GMT-06:00] Central Time [US & Canada]). When the time slider is set to show 5:00 a.m. central, the Pacific time zone layer will show the data recorded at 3:00 a.m., and the eastern time zone layer will show the data recorded at 6:00 a.m.
Choosing a time zone for your data
Time zones are supported in ArcGIS through a layer's temporal reference. If a layer is enabled, you can specify a temporal reference, which includes the time zone and the rules for daylight saving time.
The time zone of your data is ideally the time zone in which the time values were collected. When enabling time on data, you can specify the time zone on the Time tab of the Layer Properties dialog box. By default, no time zone is set on the layer. The time zone can be set to one of the predefined values in the drop-down list. The list of predefined values includes entries for every global time zone.
- There are sometimes several entries that all have the same offset from GMT. This is because there are different regions that are in the same time zone but follow different rules for daylight saving time.
- By specifying a temporal reference, you are setting both the time zone and the rules for daylight saving time.
Changing the time zone of your data
You can convert the time values collected in one time zone to another time zone using the Convert Time Zone geoprocessing tool. Converting the time values from one time zone to another time zone helps normalize temporal data from different time zones. This helps improve display and query performance for visualizing temporal data from different time zones when visualizing temporal data using the time slider.
For example, in the following illustration, the time values in the Input_Time field, which are collected in Pacific time ([GMT-08:00] Pacific Time [US & Canada]), are converted into time values for the eastern time zone ([GMT-05:00] Eastern Time [US & Canada]) and stored in the Output_Time field. Basically, using the Convert Time Zone geoprocessing tool, the three-hour time difference between the eastern and Pacific time zones has been added to the time values in the Input_Time field and output to the Output_Time field.
Daylight saving time
Daylight saving time rules are part of the data's temporal reference. The rules tell ArcGIS how daylight saving time affects the clock that was used to record the data. Using this information, ArcGIS can properly integrate date that was recorded using daylight saving time with data that was recorded without daylight saving time.