SAR preprocessing settings

After you select a SAR file and layer or layers under Source Image Layers, the SAR-Preprocessing Settings button becomes available. When you click SAR-Preprocessing Settings, the SAR-Preprocessing Settings window appears, in which you can do any of the following:

Select the type and size of filter you want to use
Select whether to apply tail trimming and, if so, the percentage thereof

You can select from either of the following types of filter:

Enhanced Lee
Boxcar

Enhanced Lee applies a Lee adaptive-speckle filter. While reducing speckle in the image, the filter preserves the polarimetric and spatial information in the image. Spatial information is preserved by using an edge detector to identify a homogeneous local neighborhood in which to estimate the filter parameters.

Boxcar applies a boxcar filter to detected or complex-valued SAR data. It is used commonly to increase the effective number of looks (ENL) in single-look or multi-look SAR data. Speckle reduction is applied by using local averaging.

Note: It is recommended that you filter your SAR data before running segmentation on it.

To minimize the effect of backscattering, you can apply tail trimming. With SAR data, frequency histograms are often characterized by strong outliers in the rightmost tail of the data distribution; that is, point targets with a high-backscattering coefficient several magnitudes higher than the bulk of the data. Segmentation is influenced strongly by these "scatterers" and can produce substandard results over distributed targets, such as forest, water, agricultural field, bare soil, and others.

The following image, for a given SAR channel, shows that 15 percent of the brightest pixels correspond to 0.08 in intensity. With at tail trim of 15 percent, all pixels with a value higher than 0.08 will be reassigned temporarily to 0.08 for the segmentation; however, the original pixel values are used in the attribute calculation.

Figure 1. Tail trim

Tail trim

To select SAR preprocessing settings

If you already have the SAR-Preprocessing Settings window open on your desktop, proceed to step 2; otherwise, in the Object Analyst window, click SAR-Preprocessing Settings.
The SAR-Preprocessing Settings window appears.
Make sure the Apply filter check box is selected, and then in the Filter type list, select the filter you want.
Important: If you have already filtered your data, or if you do not want to apply a filter during segmentation, be sure to clear the check box.
In the Filter size box, type or select the size of the filter-window size you want to use, in pixels.
The value you enter must be an odd integer between one and 33. For example, to use a 15-by-15 filter window, enter 15. The default value is nine (9).
To apply a tail trim, select the Tail trim check box, and then in the Tail trim box below, type or select the amount of trim you want, as a percentage.
For example, to apply a tail trim of 20 percent, enter 20.
Click OK, and then return to setting up and running Segmentation.