Derived Data

7.3.2 Derived Data

The IDV uses the formula mechanism to automatically create "derived" quantities based on the fields in a particular data source. When the data source is read in, the list of derived quantities is evaluated against the list of variables in the data. Users can define their own derived quantities by:

Defining an IDV formula by name, description, and mathematical formula.
Using special operators (D1, D2, ... DN) for the variable names.
Specifying the binding between the special operators and the parameters names or system aliases.
Saving the formula and loading or reloading a data source.

See the Formulas section for an overview of creating IDV formulas.

Create a formula select the Edit->Formulas->Create Formula menu item. This brings up the Formula Editor dialog box used to define a formula:

The minimal information you need to provide is the name of the formula and the actual formula.

You can use the Derived tab in the Formula Editor to set the bindings between formula parameters and fields/aliases. You need to check the Create derived quantities checkbox to enable this as a system formula.

The parameters to be bound are denoted by operands D1, D2, D3, etc in the formula itself. If you need user input, you can use other field names and the users will be prompted to supply those fields.

You can define the bindings for the operands using parameter groups or by specifying the parameters to use. Right clicking in the Parameter input field will pop up a menu that allows you to select system parameter aliases or parameters from fields loaded into the IDV already.

Categories specify the types of data that are allowed in this formula. For example, if your derived formula is valid only for 3D Grids, you can select that category from the popup list. Access the list by right clicking in the Categories input fields.

You can define specific output categories if the type of the data that the formula creates is different from the categories that the input operands require. In general, the output categories will be the same as the input categories. For more information on data categories, see the Formulas section.

To save the formula, click on Add formula. A new entry appears in the Fields panel of the Field Selector. If you want to apply the formula to an existing data source, you need to reload that data source. Right click on the name of the data source in the Data Sources section of the Field Selector and select Reload Data.

Unidata's Integrated Data Viewer > Miscellaneous > Data Analysis