Meaning of NULL when using condition in gdal_calc

Question

I am confused with the results of using a condition in gdal_calc. Initially I thought that gdal_calc was setting pixels to nodata where a condition is not met. In the documentation (http://www.gdal.org/gdal_calc.html) it is said that the following command sets values of zero and below to null.

gdal_calc.py -A input.tif --outfile=result.tif --calc="A*(A>0)" --NoDataValue=0

If I try this command:

gdal_calc.py -A input.tif --outfile=result.tif --calc="A*(A>0)" --NoDataValue=-9999

The pixels where the condition is not met are set to zero.

Should I understand that the term null used in the documentation actually refers to pixels with a value of zero?

Answer 1

You should understand the example so that

--calc="A*(A>0)" 

makes all pixels with value zero or below into zero, and after that

--NoDataValue=0

sets the metadata to tell that zero means nodata. With you modified command the zeroes are set as before but you are telling that pixels with value -9999 are nodata, thus zeroes are not nodata. I am not sure but I suppose that if input data had originally nodata set to something else than -9999 then both the metadata and those nodata pixels would be converted into -9999.