What does the “dl” unit mean in Sentinel images?
it means dimensionless.
Sentinel-2 Level1C products are not calibrated to bottom-of-atmosphere (BOA) reflectance yet.
Actually, L1C of Sentinel-2 is already reflectance data. But TOA and not BOA.
However, reflectance values have no unit.
See also:
https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-2-msi/level-1c/algorithm (section TOA Reflectance Computation) and
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Hi Marco,
Thanks for this nice explanation, but a question comes to my mind, might be it is very silly question,
Since the equation of TOA is the following one as it is mentioned,
and the equation of sun distance is this one,
Now my question is,
What is the difference between sen2cor in SNAP and DOS1 in QGIS?
In general there are always multiple AC algorithms available which all do it slightly different.
For S2 data there is, ICOR, sen2cor, MAJA and others, even C2RCC has its own AC over water.
They take into account for example, Water vapour, shadows (mountain cloud), sun/view angle and more.
The DOS algorithm is so to say a poor-mans AC.
So, you actually can’t compare the DOC algo with the real AC algorithms.
Different ACs have been compared and will be compared in a second round.
https://earth.esa.int/web/sppa/meetings-workshops/acix (related paper)
https://earth.esa.int/web/sppa/meetings-workshops/acix-2nd
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you’re right, sorry. In one of the old sen2cor manuals it was stated that L1C was TOA radiance but I checked it and it was changed in the meanwhile.