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Simple Filter
Author(s) Module: Olaf Conrad, (c) 2003
Author(s) Wiki Documentation: Vern Cimmery, (c) 2011 (kapcimmery at hotmail dot com)
License Wiki Documentation: Creative Commons Attribution-ShareAlike 3.0 Unported License
Description:
The Simple Filter module creates a new grid data layer by re-calculating grid cell data values for an existing data layer using a mathematical operation on the target cell value and its neighbors. The nature of this operation is determined by the values stored in a matrix or variable sized template or kernel that is centered over each grid cell as it is processed. This matrix or template is like a moving window, progressing from one cell to the next.
Figure SF.1. Settings page for the module Simple Filter.
Input
Grid system
The entry for the ‘Grid system’ parameter must be the grid system the input grid data layer (‘>>Grid’) is a part.
>>Grid
The single input parameter is the grid data layer containing grid cells you wish to be filtered using the approachs supported in this module. This is a mandatory input. The default entry is "[not set]". As noted above, this grid data layer must be a part of the grid system chosen for the 'Grid system' parameter, otherwise, it will not ba available to choose.
Output
<Filtered Grid
The output for the module is identified in the value field to the right of the ‘<Filtered Grid’ parameter. The default is “[not set]”. If you use the default the module will produce no output; the "[create]" option should be used. The default name for the output grid data layer is the name of the input grid data layer concatenated with “[Filter]”.
Options
The ‘Options’ section contains three parameters: ‘Search Mode’, ‘Filter’ and ‘Radius’. The ‘Search Mode’ and ‘Radius’ parameters control the number of neighboring grid data values that will be considered in calculation of the new value of a central cell.
Search Mode
The ‘Search Mode’ option supports two choices: “Circle” and “Square”. This option identifies whether a circular or square shaped matrix of cells will be considered around the central cell whose data value is to be re-calculated. The default entry for this parameter is “Circle”.
Filter
The ‘Filter’ parameter has three choices: “Smooth”, “Sharpen”, and “Edge”.
The "Smooth" option will average the difference between a grid cell and its surrounding cells. This is the default. The new value is calculated using the following formula:
where 
is the average value in the analysis window.
The "Sharpen" option has the opposite result from the ‘Smooth’ option. The "Sharpen" option will accentuate the differences between cells. Its formula is:
.
The "Edge" option is used to detect edges and areas of high variation in a grid. The formula for this option is:
.
Radius
The radius defines the size of the group of cells or matrix of cells that will be used. The higher the value entered for the radius the more accentuated the effect of the filter will be. The default entry is "1".
Execution Example:
This example uses the digital terrain model (DEM) for Mason County, Washington named 'MCdem' for the '>>Grid' parameter. The default value "Circle" will be used for the 'Search Mode' parameter and a 'Radius' of "10".
1. Execute the Simple Filter module.
2. Select the grid system the input grid data layer is a part for the 'Grid system' paramter.
3. Choose the DEM grid data layer 'MCdem' from the drop-down list of data layers for the grid system chosen for the '>>Grid' parameter.
4. Choose the "[create]" option for the output parameter '<Filtered Grid'.
5. In the 'Options' section, use the default values for the 'Search mode' and 'Filter' parameters.
6. In the 'Options' section, enter "10" for the 'Radius' parameter. This value defines the size of the matrix of cells contributing to the calculation of a new value for the central cell.
7. When changes have been made to the module parameters and options, click on the 'Okay' button to execute the module.
8. When module execution is complete, the output grid data layer name will appear in the 'Tab' area of the Workspace. The layer name will be the name of the grid data layer for the '>>Grid' parameter concatenated with "[Gaussian Filter]".
Notes:
1. The input grid data layer (for the '>>Grid' parameter) must be a member of the grid system chosen for the 'Grid system' parameter.
2. The single output parameter ('>Filtered Grid') must be set to the "[create]" option; "[not set]" is the default entry. If you use the default, there will be no output.
References:
Cimmery, Vern (2011): 'Gaussian Filter', SAGA References, unpublished .pdf document. [PENDING]
