[GRASS-user] Report from ongoing GRASS GIS Community Sprint in Prague - i.atcoor

[Daniel Victoria]

Crap... I looked out the windows and realized the atcoor vs. flaash
comparison I just did had a major flaw. I also have a good linear
regression between uncorrected radiance and reflectance from bothe
flaash and atcoor. So a linear regression between flaash and atcoor
does not mean that atcoor is working like it should, it means that
both are correlated to the original radiance image...

But my doubth still lives on: What is comming out of the i.atcoor
module? We know it's reflectance but does a 255 value means 100%
reflectance? Or is it the max reflectance in the image and we are not
sure what that value is.

Cheers
Daniel

On Mon, May 28, 2012 at 2:41 PM, Daniel Victoria
<daniel.victoria at gmail.com> wrote:
> Hi Markus,
>
> On the i.atcoor issue, I did a quick comparison of a TM image
> corrected using i.atcoor (Grass 6.4) and Envi FLAASH, which is based
> on MODTRAN and should be similar to 6S.
>
> A detailed explanation is attached on the PDF file but basically this
> is what I did:
> 1) Correct a TM image using the same atmospheric parameters in atcoor and flaash
> 2) Generate a bunch of random points
> 3) Results from atcoor and flaash are linearly correlated but
> regression coefficients are not the same for the different bands.
>
> This makes it difficult for comparing the reflectance from i.atcoor
> with measured reflectance spectra since we are not sure what the max
> value of from i.atcoor output (255) means. Is it the maximum
> reflectance (100%)? Or is it the max reflectance in the evaluated
> image? That is, if the image has a max reflectance of 40%, then the
> output reflectance image from i.atcoor will put a value of 255 for 40%
> reflectance.
>
> Sorry for the long and confusing email. The good news is that i.atcoor
> is giving results very well correlated with the ones obtained by a
> respected atm correction scheme from a comercial package  - hurray! We
> only need to figure out the scale of the output :)
>
> Thanks
> Daniel
>
> PS - I changed the subject line so it wouldn't mix with the sextante discussion
>
>
>>
>> On Mon, May 28, 2012 at 9:42 AM, Markus Neteler <neteler at osgeo.org> wrote:
>>> On Mon, May 28, 2012 at 2:21 PM, Daniel Victoria
>>> <daniel.victoria at gmail.com> wrote:
>>>> Hi all,
>>>>
>>>> I saw on the wiki talk page that there will be some debugging in
>>>> i.atcorr. One thing I've allways been troubled by is what is the
>>>> output from the module. I know it's reflectance but the data is scaled
>>>> to 8 bits. So does it means the 100% reflectance is 255?
>>>
>>> Due to an overwhelming agenda I didn't manage to look into this.
>>> However, a clear example would be desired in order to better
>>> analyse when bad things happen. Could you provide one?
>>>
>>> thanks
>>> Markus