Iteration

Dialog box

Iterations are a special type of map calculations. They are a successive repetition of a mathematical operation, using the result of one calculation as input for the next.

The calculation stops when the difference of the output compared to the input is negligible or if the number of steps is reached which was defined before. Iterations are often used in combination with neighbourhood operations.

Dialog box options:

Start map:	Select an input raster map which contains one or more starting pixels for the iteration. Open the list box and select the desired input map, or drag a raster map directly from the Catalog into this box.
Expression:	Type the expression to be used in the calculation; for more information refer to Map calculation : neighbourhood operations.
Stop criterium	Choose one of the two options:
Until no changes:	To stop the operation when pixel values do not change anymore;
Number of iterations:	To stop the operation after a certain number of iterations. Subsequently type a value for the maximum number of iterations that you want to perform.
Propagation:	Propagation is selected by default. Propagation means that the newly calculated value for a pixel is used in calculating the next pixel instead of in the next iteration.
Output raster map:	Type a name for the output raster map that will contain the result of the iteration.
Domain:	When the output map will contain values, accept the default domain, or select another existing value domain, or create your own output value domain.
Defaults:	Click this button to obtain a default domain. When the default domain is a value domain, you also obtain defaults for the value range and precision of the output map. You can also click this button to obtain defaults again after you changed the default domain, value range or precision.
Value range:	If the output map uses a value domain, accept the default value range, or specify your own range of possible values in the output map.
Precision:	If the output map uses a value domain, accept the default precision of output values, or specify your own precision.
Description:	Optionally, type a description for the output map. The description will appear in the status bar of the Main window when moving the mouse pointer over the map in a Catalog, and in the title bar of a map window when the output map is displayed. If no description is supplied, the output map will use its own definition as description.

When you click the Show button, the dependent output map will be defined, calculated and shown. When you click the Define button, the dependent output map will only be defined; if necessary, the map will be calculated later, for instance when the map is opened to be displayed.

Tips:

The start map should contain one or more starting pixels.
When working with value maps, you can generally accept the default domain Value. You only need to select or create a user-defined value domain when the values have a specific meaning for which you have or want to create a meaningful representation.