Mask returns with noise



I have several S1-IW products from September 2020 which I need to preprocess with a specific land/ desert research area (.shp file).

Here are the steps I do for each product:

(1) Product split and choose IW3 (in: Radar -> Sentinel-1 TOPS -> s-1 TOPS Split; selecting Subswath=IW3, VH).

(2) Thermal Noise Removal (in: Radar -> Radiometric).

(3) Apply Orbit File (in: Radar).

(4) Calibration to Sigma0, VH (in: Radar -> Radiometric).

(5) Deburst (in: Radar -> Sentinel-1 TOPS -> s-1 TOPS Deburst; VH).

(6) Speckle / Salt-and-Pepper removal (in Radar -> Speckle Filtering -> Single Product Speckle Filter; “Lee Sigma” Filter with a window size= 17*17).

Then, I do some Raster actions:

(7) Subset - for getting the research area rectangle (Raster -> Subset).

(8) On (7), I import the research area polygon/ vector file (.shp) via: File -> Import -> Vector Data -> ESRI Shapefile.

(9) Finally, I want to mask out the polygon with: Raster -> Masks -> Land/ Sea Mask -> Use Vector as Mask.

It seems to me that the resulting mask comes with speckles/ salt-and-pepper noise, unlike the rectangle/ subset in step 7, as if I never did step 6. ** I’m sure I applied step 9 on the subset/ rectangle from steps 7-8.

See for example:

The subset (without polygon):

The subset (with polygon - just for showing it on subset):

The masked polygon (you can see it came with much noise):

** Did someone encountered this issue? Is it a bug? Can you please help?



the amount of speckle is the same before and after the masking, it is just the image conrasts which change.
Once you apply the same color stretch to the image before and after masking, they will look alike.

You can select identical min / max values in the Color manipulation tab.