Subset with Shapefile looks Odd

development
1

Hello,
I am working with Snappy Python. I am trying to preprocess a Sentinel 1 GRD, IW product. Unfortunately the final output does’t match the intended area (green square in image) fully and the borders look wonky. Has anyone dealt with a similar matter?

Screenshot 2025-05-08 at 15.08.41
Screenshot 2025-05-08 at 15.08.411084×1084 154 KB

These are the steps I am taking for Preprocessing:
a. Subset
b. Apply Orbit FIle c. Thermal Noise Removal
d. Radiometric Calibration
c. Speckle Filtering
d. Terrain Correction
e. BandMaths RVI

I think I might have an error either in subset or Terrain Correction part.

Some additional info:
Shapefile and initial S1C image are in EPSG:4326 - WGS:84
Subset is set with these parameters:

    geom=str(shapefile['geometry'][area])
    geom = WKTReader().read(geom)
    parameters = snappy.HashMap()
    parameters.put('copyMetadata', True)
    parameters.put('geoRegion', geom)
    subset = snappy.GPF.createProduct('Subset', parameters, s1_read)

Terrain correction is set with these parameters:

proj = '''PROJCS["WGS 84 / UTM zone 29N",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",-9],
    PARAMETER["scale_factor",0.9996],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AXIS["Easting",EAST],
    AXIS["Northing",NORTH],
    AUTHORITY["EPSG","32629"]]''' 
    
    # Terrain-Correction Operator - snappy
    parameters = snappy.HashMap()
    parameters.put('demName', 'SRTM 3Sec')
    parameters.put('imgResamplingMethod', 'BILINEAR_INTERPOLATION')
    parameters.put('pixelSpacingInMeter', 10.0)
    parameters.put('mapProjection', proj)  # e.g., 'EPSG:4326'
    parameters.put('nodataValueAtSea', False)
    parameters.put('saveSelectedSourceBand', True)
    parameters.put('alignToStandardGrid', False)
    parameters.put('outputExtent', 'Subset')  # Use bounds of subset
    parameters.put('alignToStandardGrid', True)
    parameters.put('outputExtent', 'maxBounds')
    
    terrain_correction = snappy.GPF.createProduct('Terrain-Correction', parameters, speckle)
2

This is expected when there is significant topography in the scene. The cropping is done with average scene height but after processing, the data will follow the actual topography on ground. To guarantee complete coverage - pad your AOI of interest by max(abs(topography)) in your AOI, you can do smarter padding but this rule of thumb should work.