Sentinel-3 qulity flags usage

Optical Toolbox
#1

hi,
I’m trying to understand how to use the qualify flags in sentinel-3 efficiently but the docs I found about it are not that clear or do not specify how to use it in practice with examples.

I looked at the user guide, also here but it just has a description in words and not the meaning of the pixel values.

When I open sentinel-3 image in SNAP I also open the quality flag layer to see the values of the quality flag correspond to the pixels in one of the bands (e.g., Oa05). I understand that the pixel values in the quality flag layer are based on bits, but I did not found any explanation regarding the meaning of the values. For instance, I have clouds over a water surface and the quality flag value is 134217728, but over land the clouds gets 2283798528.

Can someone explain or direct me for a clear explanation, preferably with examples on the topic?

Thanks

Quality flag layer (OLCI)
#2

Hi Ran,

I do agree with you, that the flag documentation is - at least occassionally - a little sparse. And not always easy to find among the many documents and webpages. However, it seems to me that your main issue is first to understand how the bit-based quality flags work at all. :wink:

Each quality flag, corresponding to an integer pixel value (like your 134217728 and 2283798528 examples) is actually a package of multiple bit-value flags (in the Product Data Format Specification (PDFS) the bit-value flags are also named “words”). Your example quality flags contain at least 32 bits (or words). Each bit can take on the value of 0 or 1, and the flag value is calculated as the sum of the bit value times the bit number over all bit numbers.

to illustrate that for your example values (I leave out bit# 0-20 as they are 0 for both example pixels):

bit 21:  0          0  -  1    2097152
bit 22:  0          0  -  0          0
bit 23:  0          0  -  0          0
bit 24:  0          0  -  0          0
bit 25:  0          0  -  0          0
bit 26:  0          0  -  0          0
bit 27:  1  134217728  -  1  134217728
bit 28:  0          0  -  0          0
bit 29:  0          0  -  0          0
bit 30:  0          0  -  0          0
bit 31:  0          0  -  1 2147483648
sum   :     134217728  -    2283798528

where first column shows the bit number, second (forth) the bit value of the first (second) example pixel, third (fifth) the resulting pixel value contribution from this bit for the first (second) example. Last row shows the sum of the individial bit contributions, ie the resulting pixel value.

That is, beside the common (cloud?) flag at bit27, the second example pixel has additional flags set at bit21 and bit31.

I have no idea, what meaning the individual bits correspond to in your data, but a short description of the word meanings should be found (a) in the PDFS corresponding to your data (a number of PDFS is available from the Sentinel Documents Library - select the one you need depending e.g. on the level - L1/L2 - of your data), but also (b) in the attribute information of the quality flag entry in your data (in a field called ‘flag_meanings’).

I also have no idea, whether SNAP/s3tbx contains a tool to easily do this bit de-wrapping. I use numpy’s view and unpackbits functions for that.

I hope this helps a bit…
Best wishes,
jm