[GRASS-user] i.atcorr range and rescale parameters for Sentinel 2 images

[Markus Metz]

On Sat, Feb 10, 2018 at 6:11 PM, roberta fagandini <robifagandini at hotmail.it>
wrote:
>
> Hi Markus and Žofie,
>
> thank you for your hints!
>
> I tested i.atcorr with range=1,10000 and rescale=1,10000. In this way,
value 10000 is assigned to all those pixels with reflectance greater than
10000 in the not-corrected image band (I'm using a blue band with min=1 and
max= 21604). If I set range=1,21604 and rescale=1,10000 in this case, the
output is an image with values ranging from 1 to 10000 (because of the
rescale parameter) but all the pixels with reflectance > 10000 in the
not-corrected image band have different values near 10000 and
proportionally rescaled but not rigidly set to 10000.

This is correct because some few pixels might have values > 10000.
Rescaling the input with 10000 means reflectance = pixel_value / 10000,
i.e. values larger than 1 can occur which is correct. Internally, i.atcorr
produces values in the range [0, 1]. These values are rescaled with the
rescale option. If you use rescale=1,10000, output values will be within
the range [1, 10000], not exceeding this range. The correct range option
for i.atcorr with sentinel2 is 1,QUANTIFICATION_VALUE. As Zofie said, you
can set the rescale option to your needs.

Markus M

>
> Just to be clearer, here some examples querying some pixels of the images
:
>
> #B02_not_corrected (original blue band)
>
> px1_value: 21604
>
> px2_value: 17869
>
> px3_value: 18500
>
> #B02_range_rescale_1,10000 (i.atcorr output with range=1,10000 and
rescale=1,10000)
>
> px1_value: 10000
>
> px2_value: 10000
>
> px3_value: 10000
>
> #B02_range1,21604_rescale1,10000 (i.atcorr output with range=1,21604 and
rescale=1,10000)
>
> px1_value: 10000
> px2_value: 9055
> px3_value: 9352
>
> Therefore the meaning of the range parameter is still not so clear to me.
Do you have other information about it and how it works?
>
> Thank you for your help!
> Roberta
> ________________________________
> Da: Žofie Cimburová <zoficimburova at gmail.com>
> Inviato: venerdì 9 febbraio 2018 21:01
> A: roberta fagandini
> Cc: grass-user at lists.osgeo.org
> Oggetto: Re: [GRASS-user] i.atcorr range and rescale parameters for
Sentinel 2 images
>
> Hi Roberta,
>
> On Thu, Feb 8, 2018 at 8:54 PM, roberta fagandini <
robifagandini at hotmail.it> wrote:
>
> Hi all,
>
> I'm testing i.atcorr (grass 7.4) with Sentinel 2A images and i have some
trouble defining the right values for range and rescale parameters. I read
in other posts and lists that the suggested value for both range and
rescale is '0,10000' while in other posts someone else suggests to use min
and max values (from r.info) for range and '1,10000' or '0, 10000' for
rescale.
>
> It's not clear to me the right meaning of these parameters and thus the
most correct values to be used.
>
> Can anyone better explain me the meaning of these two parameters?
>
>
> When I was running i.atcorr for Sentinel2 data, I used range=1,10000
(i.e. the min/max theoretical values the input data can reach).
> As for rescale, this in my understanding just sets what the output range
of the reflectances will be. So you can use for example 1-255, 0-1,
1-10000, whatever suits best to your further analysis. Relatively, the
result will be identical.
>
> Hope that helps a bit!
> Zofie
>
>
> I tested all the values suggested in other posts but I can't understand
what they exactly do.
>
>
> Thank you in advance for your help!
>
>
> Roberta
>
>
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