Thank you very much Vero for your kind answer and for sharing the presentation,
which for sure will result very useful!
Da: Veronica Andreo
Inviato: venerdì 28 gennaio 2022 14:50
A: Moritz Lennert
Cc: Pilafin ; Maris Nartiss ;
GRASS user list
Oggetto: Re: [GRASS-user] R: R: R: NULL values as output of i.atcorr
Dear Ilaria,
If you use Level 2A data, you do not need to do atmospheric correction. The
products are already corrected and the only thing you need to do, if you want
floating point data, is to rescale reflectances with the equation indicated by
Moritz. Optionally removing the overshoots by setting a ceiling for values >
1, as theoretically reflectance range is [0,1].
If, instead, you are using a Level 1C product, then yes, atmospheric correction
is needed, as those products are top of atmosphere (TOA) reflectance. For
Sentinel data you have different options. Either you use i.atcorr or
i.sentinel.preproc. If you do not have a visibility layer, you need an AOD
value (or file) from a station within the scene ideally and of the ~date when
the image was taken. If you can read some Spanish, I have a presentation with
all the details:
https://gitlab.com/veroandreo/maie-procesamiento/-/blob/taller-grass-online/pdf/03_exercise_processing_s2.pdf.
In any case, i.atcorr manual page shows the process of correcting a S2 L1C
scene step by step.
hth,
Vero
El vie, 28 ene 2022 a las 12:07, Moritz Lennert (mailto:mlenn...@club.worldonline.be> >) escribió:
On 28/01/22 10:55, ilaria.ferrand...@gmail.com
<mailto:ilaria.ferrand...@gmail.com> wrote:
> Thank you Moritz, I will do some research on visibility (aerosol model
> concentration) to put a reasonable value there (any suggestion is
> appreciated, also regarding references to look at).
The man page suggests https://aeronet.gsfc.nasa.gov/.
> How can I transform digital number into radiance/reflectance?
This information should be in the metadata that accompanies the Sentinel
data. I haven't worked with this data in a couple of years, so don't
remember.
However, in the doc on
https://docs.sentinel-hub.com/api/latest/data/sentinel-2-l2a/, you can read:
For Sentinel-2 optical data, the relation between DN and REFLECTANCE
(default unit) is: DN = 1 * REFLECTANCE.
Moritz
> Thank you very much for your precious help!
>
> -Messaggio originale-
> Da: Moritz Lennert <mailto:mlenn...@club.worldonline.be> >
> Inviato: giovedì 27 gennaio 2022 17:50
> A: ilaria.ferrand...@gmail.com <mailto:ilaria.ferrand...@gmail.com> ; 'Maris
> Nartiss' mailto:maris@gmail.com> >
> Cc: 'GRASS user list' <mailto:grass-user@lists.osgeo.org> >
> Oggetto: Re: R: R: [GRASS-user] NULL values as output of i.atcorr
>
> On 27/01/22 14:54, ilaria.ferrand...@gmail.com
> <mailto:ilaria.ferrand...@gmail.com> wrote:
>> The output of r.info <http://r.info> is the following
>>
>> r.info <http://r.info> map=immagine@mapset
>>
>> ++
>>| Map: immagine@mapsetDate: Tue Nov 30 12:51:12 2021|
>>| Mapset: mapset Login of Creator: [...] |
>>| Location: WGS84_UTM32
>>|
>>| DataBase: C:\Users\[...]\grassdata |
>>| Title:immagine
>>|
>>| Timestamp: none
>>|
>>
>> ||
>>|
>>|
>>| Type of Map: raster Number of Categories: 0
>>|
>>| Data Type:CELL
>>|
>>| Rows: 10980
>>|
>>| Columns: 10980
>>|
>>| Total Cells: 120560400
>>|
>>|Projection: UTM (zone 32)
>>|
>>|N:4900020S:4790220 Res:10
>>|
>>|E: 609780W: 499980 Res:10
>>|
>>| Range of data:min = 0 max = 18876
>>|
>>|
>>|
>>| Data Description:
