[GRASS-user] Appropriate input/output range for i.atcorr WorldView2?

[Paige Byassee]

Thank you Nikos & Marcus,

Converting to TOA radiance (or reflectance) is not really the issue... I
have already been following "Absolute Radiometric Calibration, Prepared By:
Michele A. Kuester" using the formula for L (done in R, screenshot
attached). I do think it is necessary that this be done in FLOAT32 to
retain enough detail and per DigitalGlobe recommendations.

So, if I covert to TOA radiance as you've described, I am left with 32-bit
pixels. I then import the images using r.in.gdal and launch i.atcorr. *Would
the input range then be 0, 4294967295 (i.e. (2^32) - 1)? *

If so, I wonder what a good output range might be, as I presume using
r.out.gdal on a 32-bit image could take a long time.

Thank you for your help,
Paige

On Wed, Apr 8, 2020 at 6:31 AM <nik at nikosalexandris.net> wrote:

> On 2020-04-08 15:17, Markus Neteler wrote:
> ..
> > I believe that you need to apply the formula along with the respective
> > band calibration values ABSCALFACTOR and  EFFECTIVEBANDWIDTH (the same
> > is also stored in the IMD files) from the XML metadata files:
> >
> > grep 'ABSCALFACTOR\|EFFECTIVEBANDWIDTH'
> > 058891334020_01_P001_MUL/18DEC03184338-M2AS-058891334020_01_P001.XML
> >             <ABSCALFACTOR>9.295654000000000e-03</ABSCALFACTOR>
> >
> > <EFFECTIVEBANDWIDTH>4.730000000000000e-02</EFFECTIVEBANDWIDTH>
> >             <ABSCALFACTOR>9.748051000000001e-03</ABSCALFACTOR>
> >
> > <EFFECTIVEBANDWIDTH>5.430000000000000e-02</EFFECTIVEBANDWIDTH>
> >             <ABSCALFACTOR>7.541495000000000e-03</ABSCALFACTOR>
> >
> > <EFFECTIVEBANDWIDTH>6.300000000000000e-02</EFFECTIVEBANDWIDTH>
> >             <ABSCALFACTOR>5.101088000000000e-03</ABSCALFACTOR>
> >
> > <EFFECTIVEBANDWIDTH>3.740000000000000e-02</EFFECTIVEBANDWIDTH>
> >             <ABSCALFACTOR>1.103623000000000e-02</ABSCALFACTOR>
> >
> > <EFFECTIVEBANDWIDTH>5.740000000000000e-02</EFFECTIVEBANDWIDTH>
> >             <ABSCALFACTOR>4.539619000000000e-03</ABSCALFACTOR>
> >
> > <EFFECTIVEBANDWIDTH>3.930000000000000e-02</EFFECTIVEBANDWIDTH>
> >             <ABSCALFACTOR>1.224380000000000e-02</ABSCALFACTOR>
> >
> > <EFFECTIVEBANDWIDTH>9.890000000000000e-02</EFFECTIVEBANDWIDTH>
> >             <ABSCALFACTOR>9.042234000000000e-03</ABSCALFACTOR>
> >
> > <EFFECTIVEBANDWIDTH>9.959999999999999e-02</EFFECTIVEBANDWIDTH>
> >
> > Citing from "Absolute Radiometric Calibration, Prepared By: Michele A.
> > Kuester"
> >
> https://dg-cms-uploads-production.s3.amazonaws.com/uploads/document/file/209/ABSRADCAL_FLEET_2016v0_Rel20170606.pdf
> >
> >   "The top-of-atmosphere radiance, L, in units of Wμm-1m-2sr-1, is
> > then found from the DigitalGlobe
> >     image product for each band by converting from digital numbers
> > (DN) using the equation,
> >
> >     L = Gain * DN * (abscalfactor/effective bandwidth) + Offset
> >
> >     The TDI specific "abscalfactor" and "effectiveBandwidth" are
> > delivered with the imagery in the metadata file.
> >     The digital number, DN, is the pixel value found in the imagery.
> > The Gain and Offset are the absolute
> >     radiometric calibration band dependent adjustment factors that are
> > given in Table 1. Note that these
> >     are not necessarily stagnant values and they are revisited
> > annually.
> >  "
> >
> > (the "Table 1" is found in the same PDF file above. You may want to
> > check if a newer table version exists).
> >
> > You can apply this formula with r.mapcalc, for each band to obtain TOA
> > data from DN.
>
> See also https://gitlab.com/NikosAlexandris/i.worldview.toar or
> https://github.com/NikosAlexandris/i.worldview.toar.
>
> Maybe useful (though ratehr badly programmed at the time).
>
> Nikos
>


-- 
*Paige Byassee*
Appalachian State University '19
B.S., Ecology|Certificate in GIS
(704)488-0872
byasseepaige at gmail.com
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