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

[Daniel Victoria]

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
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