[GRASS-user] i.atcorr: which is the meaning of range parameter?

[Markus Neteler]

Hi,

On Wed, Nov 13, 2019 at 2:18 PM Markus Metz
<markus.metz.giswork at gmail.com> wrote:
> On Wed, Nov 13, 2019 at 12:59 PM Veronica Andreo <veroandreo at gmail.com> wrote:
> >
> > Hi all,
> >
> > I'm using i.atcorr for atmospheric correction of Pleiades imagery. The data comes as DN in 12 bits.
>
> That is, values can be in the range 0, 4095, but the full possible range is not necessarily used.
>
> > Reading the manual of i.atcorr, I get that I must provide radiance data. Hence I converted each band to radiance using gain and bias from the metadata. Now, for the range parameter in i.atcorr, I assume I must pass the min/max output of r.info for each band. There's no extra info in the metadata about theoretical range for each band.
>
> No, the range parameter in i.atcorr needs the theoretical range.

Since it isn't that obvious yet I'd suggest to reword the description at
https://github.com/OSGeo/grass/blob/master/imagery/i.atcorr/main.cpp#L496

from:
   opts.iscl->description = _("Input range");

to something like:
   opts.iscl->description = _("Input range (theoretical range)");

(feel free to submit a PR)

> For example for Landsat, values can be in the range 1, 255 (0 is reserved for nodata IIRC), but actual band values usually cover only a part of that range. Therefore min/max of a band is not correct.

This valuable hint might to into the manual.

best
markusN

> Regarding Pleiades or other imagery, you need to get from documentation or metadata the valid range and use this as range for i.atcorr.
>
> HTH,
>
> Markus M
>
> >
> > In the i.atcorr manual and in some older emails, I read for example, that the range for Sentinel 2 is 1 to 10000. However here [1] it says: "Per-pixel radiometric measurements are provided in Top Of Atmosphere (TOA) reflectances along with the parameters to transform them into radiances". AFAIU, 1,10000 is TOA range.
> >
> > So, I'm a bit lost... What is really this range parameter representing?
> >
> > Thanks much in advance
> >
> > Vero
> >
> > [1] https://earth.esa.int/web/sentinel/user-guides/sentinel-2-msi/product-types/level-1c
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-- 
Markus Neteler, PhD
https://www.mundialis.de - free data with free software
https://grass.osgeo.org
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