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I'm processing Sentinel 2 L2A data in sen2r as downloaded from googlecloud and spotted an issue: if computing NDVI from bands 4 and 8 at 10 m resolution, I get an extreme offset after January 26th as exemplified by the last 7 values in the following image (this is just one pixel, but I find this consistently over many pixels):

NDVI-time series Searching the web, I learned that the issue very likely is related to the fact that since January 26th Sentinel 2 products are processed using a new baseline (v0400) which will also affect L2A products. However, I did not find a solution for the problem if working in sen2r. The relevant section of the description of the major product update states:

  1. Provision of negative radiometric values (implementing an offset): The dynamic range will be shifted by a band-dependent constant: BOA_ADD_OFFSET. This offset will allow encoding negative surface reflectances that may occur over very dark surfaces. From the user’s point of view, the L2A Bottom of Atmosphere (BOA) reflectance (L2A_BOA) shall be retrieved from the output radiometry as follows: • Digital Number DN=0 remains the “NO_DATA” value • For a given DN in [1; 1;2^15-1], the L2A BOA reflectance value will be: L2A_BOAi = (L2A_DNi + BOA_ADD_OFFSETi) / QUANTIFICATION_VALUEi The radiometric offset value will be reported in a new field in the General_Info/Product_Image_Characteristics section of the Datastrip and User Product Metadata. It will be initially set to -1000 Digital counts for all bands. It is also noted that the percentage of negative surface reflectance pixels per band will be also reported in the L2A_QUALITY report in the QI_DATA folder of the tile.

Now my question is: how to fix this issue? I want to standardize my data over time, which does not make sense with that offset. So I need some kind of harmonization between the baselines. How would that work in sen2r?

Thanks in advance for any advice.

Allan
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1 Answers1

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I suppose you may have solved this by now. But as you mentioned you will have to subtract the BOA_ADD_OFFSET from the pixel values in a band if you wish compare them with pre 22-01-25 images (Baseline v04.00).

In my processing I have preprocessed using GDAL to subtract the offset for both L1 and L2 images and clamped any resulting negative values to 0 (NODATA) before comparing them with pre 22-01-25 images.

Edward Hiscoke
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