in old times, 8-bit was the best for storage/resolution (still there somehow...) but also for processing. rescaling IMHO is a legacy mostly of that time. Someone should look into i.atcorr with a specific target at modernizing this bit.
On Fri, Jun 19, 2015 at 3:20 PM Tomáš Brunclík <[email protected]> wrote: > Hello, > just a note to this discussion. I never understood the input/output > stretching parameters of i.atcor, so I have no advice here, just couple > of thoughts. > I especially don't see why these scale parameters seem to be always used > in any tutorial, either always set to 0,1 both, or worse, to actual > range of input for the input scaling parameter. I would understand their > use, if the input TOA reflectance was in percent or in reflectance > multiplied by 10000, or quantized as DN values are - then such input > stretching min,max could be used to correct for this (not based on > actual values in the image, but on its metadata, though). Similarly, if > we needed output in percent reflectance or reflectance muptiplied by > 10000, then the output streching could be used. > But if the input is in values of TOA reflectance already and we need > output in values of ground reflectance, there should be no stretching > involved at all, not even stretching to 0,1. And not even if there are > pixels in the scene having TOA reflectace over 1, and not again if there > are no pixels having reflectance over 0.5. Because both is possible - > although reflectance over 1 is of course not possible by definition, it > can be measured for surfaces not meeting measurement assumptions. > A scene band can contain pixels with TOA reflectance seemingly over 1 > very easily, nothing fishy about that. Because keep in mind, we are > speaking about *spectral directional reflectance* and it is measured > with assumption of lambertian surfaces. Reflectance value of 1 should be > value of pixel of absolutely white object with *lambertian surface*. But > there can be an object with a mirror-like surface, like water or metal > roof and if such object is reflecting light toward the sensor (glint), > it can easily reach radiance values equivalent to lambertian surface > reflectance well above 1. And of course there can be invalid pixels, > like dead pixels in CCD cameras. So the algorithm (or the user) should > not try to scale the whole input or output reflectance, regardless of if > these pixels were removed from the scene or not. If unscaled TOA > reflectance, even containing reflectances over 1 at places should be fed > to the ATCOR algorithm unscaled, the result should be ground reflectance > unscaled. > And of course, it is normal, that the result may again contain > reflectances above 1 - it could be the pixels mentioned before, but also > for example clouds - because the reflectance is true only for surfaces > on the ground. Highly reflective surfaces high above ground can again > reach radiance values higher than what absolutely white object on ground > could have, because to the cloud in 10 km altitude there comes much more > light than to the surface below filtered by the thick atmosphere down > there, and we are not correcting for this, as we do for high mountains > with DMT, do we? > > Regards, > Tomas Brunclik > > > > Yann Chemin napsal(a): > > Something is fishy here, reflectance values should always be within > > 0-1 range. > > > > Please manually remove all values outside of that range (use > > r.mapcalc) and make a histogram of your TOAR band. if the histogram is > > of expected shape, and is not truncated at 1.0 threshold then you are > > OK. if there is a problem here, that means the TOAR processing had an > > issue. Stretching output from an unknown histogram skewness could be > > giving you funny results. > > > > > > On Mon, Jun 15, 2015 at 6:47 PM Micha Silver <[email protected] > > <mailto:[email protected]>> wrote: > > > > On 6/15/2015 3:25 PM, John, Lisa wrote: > >> Hi Micha, > >> > >> did you refer to the input range? > > Yes, the input range. > > > >> I did so and checked bands 1-9 with r.info <http://r.info> -r. > >> Top of atmosphere reflectance values are all in the range of > >> -0.01 - 2.12. The negative value seems to be a outlier and I > >> check in addition with the range 0-2.12. > >> I used these values for the input range in i.atcorr -r and used > >> 0,1 as output range. > > I meant in the first i.landsat.toar step. The input (DN) values > > should be up to 700 or so. Landsat 8, with its 12 bit data can > > have max DN values of 4096. But I didn't use that full range, > > rather the actual DN range of the original images. > > > >> However I get even smaller reflectance values (0.15 respectively > >> 0.29 for vegetation in band5) > >> > >> Regards, Lisa. > >> > >> > >> This mail was received via Mail-SeCure System. > >> > >> > >> _______________________________________________ > >> grass-user mailing list > >> [email protected] <mailto:[email protected]> > >> http://lists.osgeo.org/mailman/listinfo/grass-user > >> This mail was received via Mail-SeCure System. > >> > >> > > > > _______________________________________________ > > grass-user mailing list > > [email protected] <mailto:[email protected]> > > http://lists.osgeo.org/mailman/listinfo/grass-user > > > > > > > > _______________________________________________ > > grass-user mailing list > > [email protected] > > http://lists.osgeo.org/mailman/listinfo/grass-user > > _______________________________________________ > grass-user mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/grass-user >
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