Hello. I am writing to advise users of the S2 Biophysical Processor about two concerns with the algorithms used. I realize that this is not a ‘production’ issue but it a comment about data quality.
Canada Centre for Remote Sensing
There are two issues.
First, the processor does not exactly implement the Simplified Level 2 Prototype Processor algorithm described in Weiss and Baret, 2016 (https://step.esa.int/docs/extra/ATBD_S2ToolBox_L2B_V1.1.pdf) due to an error in how the calibration database was produced. This was verified by Dr. Marie Weiss. It will result in overestimates of LAI, fAPAR and fCOVER at high (>4) LAI as has been observed during validation.
We have implemented an exact version of the SL2P in the LEAF-Toolbox on Google Earth Engine (https://github.com/rfernand387/LEAF-Toolbox). It does not exhibit a bias over non-forests and, as expected to to lack of clumping, underestimates LAI over forests. Please consider verifying any L2B products from the biophysical processor using LEAF.
Second, I have noted there is an option for applying the biophysical processor to 10m resolution bands only. Validation of 10m products is sparse but indicates uncertainty does not satisfy Copernicus Global Land Service requirements. I would encourage users of the 10m processor to conduct validation over their regions of interest since our evaluation of the SL2P algorithm with only 10m bands suggests it is unlikely to meet these requirements without additional developments.
Thank you for feedback.
You refer to the old ATBD implementation v1.1.
Is it concerned the version v2.1 too (http://step.esa.int/docs/extra/ATBD_S2ToolBox_V2.1.pdf)?
These issues are a concern for V2.1. The issue was due to a bug in the original MATLAB code from Marie Weiss that incorrectly specified simulations used to calibrate the retrieval algorithm. To the knowledge of both myself and Marie this bug was never addresed in the Biophysical Processor. I have verified it persists since there are differences in a recent (V2.1) output from SNAP and my code which implemnents the correct SL2P algorithm that we (including Marie) have used for validation Validation of the Sentinel Simplified Level 2 Product Prototype Processor (SL2P) for mapping cropland biophysical variables using Sentinel-2/MSI and Landsat-8/OLI data - ScienceDirect
Validation of baseline and modified Sentinel-2 Level 2 Prototype Processor leaf area index retrievals over the United States - ScienceDirect
Note this issue was presented at the Sentinel 2 4th validation Team Meeting in 2021 and we will present if again in the 5th meeting in April.
As for the 10m resolution product. I expect the bug is also there BUT more importantly the issue is that there has been no published validation of it that indicates is is reasonable. I suggest ESA support processors that have reasonable validation or at least indicate it is not validated.
Firstly, thanks to the SNAP team for releasing SNAP 9 I noticed a couple of items related to the biophysical processor in the Sentinel-2 Toolbox change log but nothing relating directly to this issue. Was it fixed in SNAP 9 or is it still being investigated?
Your link concerns the S2 cache deletion SIITBX-288.
There is no improvement for the biophysical since 8.x version.
A new version algorithm of the INRAE should be provided soon.
Thanks for the reply. I meant to link to the full list of issues resolved in SNAP 9 (which can still be seen using the above link), and not the specific cache issue. At least three of those have “biophysical processor” in the name. In any case, I am looking forward to the new version