Section: 3.2.1 Using the Field Selector

3.2.1 Using the Field Selector

The Field Selector is used to select data sources, parameters, data times, and types of displays to use for creating visualizations.

Field Selector

The Field Selector consists of 4 panels:

Data Sources - used for selecting data sources that are currently loaded into the IDV. Formulas is a particular data source for applying formulas to other data sources.
Fields - displays the list of data choices (parameters, formulas) for the selected data source.
Displays - holds the list of display types applicable to the selected field.
Data Subset - depending on the type of data, this panel displays the list of times, levels and spatial subsetting associated with the selected data choice.

Data Sources

When the IDV reads in a data source, most of the time it only reads enough metadata to be able to create a list of data choices. Some of the datasets may be quite large and this allows you to only read those portions that you need instead of the entire dataset.

Right click the NCEP Model Data ETA data source in the Data Sources panel of the Field Selector window and select the Properties menu.
Each Data Source Properties Dialog will have one or more tabs for setting and viewing the properties of that data source. Tab through the properties of this data source.
Click the Cancel button to close the window

Data Choices (Fields)

When a data source is loaded, a set of data choices are created from the metadata. This allows you to select a subset of the dataset.

Select the NCEP Model Data ETA data source in the Data Sources panel of the Field Selector window.
In the Fields panel, you will see two tabs - 3D grid and 2D grid indicating that this dataset has both 2D and 3D fields in it.
Click on the little tab to the left of the 3D grid field to expand that tab. The list contains those 3D fields from the dataset that the IDV can display.
Select the temperature field. When you select a field, the Displays and Data Subset panels are populated with entries applicable to that field.

Derived Choices

When a data source creates the list of data choices from the parameters in a dataset, it also uses a library of pre-defined derived quantities to create a list of choices for fields that can be calculated from the fields in the dataset.

Expand the Derived tab.
In the expanded list, select Dewpoint (from T & RH). This dataset does not have a dewpoint field in it, but it does have temperature (T) and relative humidity (RH) which can be used to calculate dewpoint.

Displays

When each data choice is created, the metadata for that choice is used to determine which displays are applicable to that field.

Look at the list of available display types for your Dewpoint selection in the Displays panel.
Select the Flow Vectors (from GridRelative_u & GridRelative_v) field and notice that the list of displays changes.
Select the Sounding Data (T & dewpoint only) field and notice that you get another list of displays.
Select Dewpoint (from T & RH) again.
In the Displays panel, select Contour Plan View so we can create a horizontal slice of the data and display it as contours.

Data Subsetting

The Data Subset panel allows you to temporally and spatially subset the data you've chosen. For this exercise, we will look at subsetting by time and level.

Times

Each data choice may or may not have times associated with it which are displayed in the Times panel. If there is a set of times, you can select one of the four menu items in the drop down menu: Use Default, Use Selected, Match Time Driver, and Set As Time Driver.

You can set the time subset for all parameters in the dataset.
1. Double click (or right click and select the Properties menu) on the dataset name in the Data Sources panel to bring up the Data Source Properties editor.
2. This dataset has ten time steps. In the Times tab, select the Use Selected menu item in the drop down menu. You can now select the subset of times. Select the first five (5) times in the list (1998-06-29 12:00:00Z to 1998-06-30 12:00:00Z).
3. Click the OK button to apply this change and close the window.
4. Notice that the change is reflected in the Times tab of the Data Subset panel when you select Dewpoint this time.
You can set the time subset for a single parameter in the Times tab.
1. In the Times tab of the Data Subset panel, select Use Selected menu item.
2. In the list of times in that panel, select the last four (4) times in the list (1998-06-30 18:00:00Z to 1998-07-01 12:00:00Z).
You can set the time subset to match display driver times
1. If a user selects the Match Time Driver menu item, the IDV will check the current active view window to make sure there is a time driver available. Otherwise, an error message will appear to remind the user a driver is needed first. When the IDV confirms that a time driver is available, the time list of this dataset will be removed in the subset panel, and the times used by the time driver in the current active display will be used.
You can set the time subset to be the display time driver
1. If a user selects the Set As Time Driver menu item, the selected times of this dataset will be used as the driver times for this view window. Only one time driver is allowed in each view window, and the most recent display set as the time driver will be used.

Levels

Each data choice may or may not have levels associated with it which are displayed in the Level tab of the Data Subset panel. If you do not select a level, the first level is read in unless you are creating a display that needs a volume of data.

You can set the level subset for a single parameter in the Level panel.
1. In the Level tab, select the 850 hectopascal level.

When you are finished making all your selections, click the Create Display button to apply them and create the display. The subset of data you selected will be read from the data source and displayed in the currently active View Window.

Unidata IDV Workshop for version 6.2u2 > Accessing and Displaying Data > Working with Gridded Data