如何纠正Sentinel 2基线V0400偏移？
我正在从GoogleCloud下载的SEN2R中处理Sentinel 2 L2A数据，并发现了一个问题：如果以10 m的分辨率从乐队4和8计算NDVI，我在1月26日之后得到了极端偏移，如以下7个值的例证，在以下7个值中得到了例证。图像（这只是一个像素，但我在许多像素上始终发现这一点）：
- 提供负辐射值（实现偏移）： 动态范围将通过带依赖的常数转移： BOA_ADD_OFFSET。此偏移将允许编码负面表面 可能在非常黑暗的表面上发生的反射。从用户的观点 可以看，大气底部（BOA）反射率（L2A_BOA）应 从输出辐射仪中检索如下： •数字号码dn = 0仍然是“ no_data”值 •对于[1; 1; 2^15-1]，L2A BOA反射值将 是： l2a_boai =（l2a_dni + boa_add_offseti） / Quantification_Valuei 辐射偏移值将在新的一个新字段中报告 general_info/product_image_characteristics and用户 产品元数据。最初，所有乐队的数字计数最初将设置为-1000个数字计数。 还注意到，负面反射像素的百分比 频段还将在Qi_data文件夹的L2A_Quality报告中报告 瓷砖。
现在我的问题是：如何解决此问题？我想随着时间的推移标准化数据，这对该偏移没有意义。因此，我需要在基线之间进行某种协调。这在SEN2R中如何工作？
事先感谢您的任何建议。
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:
- 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.
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我想您现在可能已经解决了这个问题。但是，如您提到的那样，如果您希望将它们与22-01-25的图像（基线V04.00）进行比较，则必须从频段中的像素值减去BOA_ADD_OFFSET。
在我的处理过程中，我已经使用GDAL进行了预处理，以减去L1和L2图像的偏移，并将结果负值夹为0（Nodata），然后将其与22-01-25前图像进行比较。
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.