About virtual variables

A virtual variable is created by applying an operator (a mathematical algorithm) to one or more input acoustic variables (operands). Operands may be raw variables or other virtual variables. Virtual variables include variables (from various variable classes), virtual lines and virtual surfaces.

Operators may be chained to create additional virtual variables; that is, by using the output of one virtual variable as input to another.

When using a long chain of virtual variables, the calculations may take a significant amount of time. A progress bar is provided to indicate how quickly the calculation is proceeding. To stop the calculation, press and hold Esc while this progress bar is displayed.

You view and manage virtual variables using the Dataflow window and the Dataflow Toolbox.

To create a virtual variable refer to Dataflow Toolbox: Create a dataflow object or Dataflow window: New.

Notes:

Virtual variables take a copy of their variable properties from their first operand, see Inheritance of Properties for more information.
Most virtual variables take unprocessed data from their operands. See About setting variable properties for more information.
If a virtual variable requires multiple operands, Echoview will only process pings that have compatible ping geometry in all operands, see Using multiple operands for more information.
You can only export data from virtual echograms if the appropriate module is licensed, see Operator licensing in Echoview
Virtual variables are unavailable if any of their operand(s) is not able to provide data (i.e. are also unavailable).
Where an operator supports operands that can be single beam or multibeam data, changing the operand data type while the echogram is open, will close the echogram.
The variety of variable operators, line operators and surface operators together with the ability to develop chains of virtual variables can potentially create circular calculation paths. Especially those involving virtual lines or surfaces specified for Exclude above or Exclude below settings, and then used as operands with Processed data, Region statistic and Cell statistic virtual variables. Circular calculations are disallowed. Console return messages can list disallowed circular calculation variables. The drag and drop of operands on the Dataflow window may also check for such circular paths.
A description of virtual echograms is provided in Higginbottom, Pauly, and Heatley (2000).

About virtual variables

See also