Snappy Python BandMaths, Linear to DB

development python
#1

Hello everyone,

I am working with Snappy Python. I am trying to preprocess a Sentinel 1 GRD, IW product. Following are the steps I am taking for Preprocessing:
a. Subset
b. Radiometric Calibration
c. Speckle Filtering
d. Terrain Correction
e. BandMaths RVI(Radar Vegetation Index) Calculations
f. LinearToFromdB.

Now finally when I try to read the pixels, from the final processed Image for histogram and other uses I get the following error:

Code:
band = linear_to_db.getBand(“Sigma0_VH_db”)
w = band.getRasterWidth()
h = band.getRasterHeight()

print(f"Width: {w}, Height: {h}")
band_data = np.zeros(w*h, np.float32)
band.readPixels(0, 0, w, h, band_data)

image
image777×210 13.7 KB

Error message:
RuntimeError: org.esa.snap.core.gpf.OperatorException: java.lang.NullPointerException

The same problem does not occur if I do not convert the LinearToFromdB. That is if I ignore the last step. Also if I do BandMaths (for RVI) calculation after LinearToFromdB I do not get this error.

Would appreciate any help. I did not get any error in the preprocessing steps.

  1. But as per my very little understanding of Sentinel 1, conversion to dB is a good idea but what about RVI?
  2. Should the BandMaths (for RVI) calculation be done before or after LinearToFromdB in Snappy?
  3. Is there something wrong with my process?
#2

One issue of the Java-Python bridge is the incomplete error message propagation.
Depending only on
RuntimeError: org.esa.snap.core.gpf.OperatorException: java.lang.NullPointerException
it is hard to tell what the error is.

Some suggestions:

  • Save the final product to disk by using ProductIO.writeProduct(…) and work with the processed data.
  • You are loading the whole band. This might cause memory issues. Have you tried to load it in chunks? Maybe only 10 lines at once?