Nikos,
I'm probably missing a message (I do not see which is
exactly the difference between the spot and the modis
example), but
Perhaps you must use
r.mapcalc to rescale and not r.rescale, or if using r.rescale you must
use r.mapcalc afterwards to rewrite the output raster, as r.rescale
is modifying the categories only and not the actual raster values which
is what i.pca is probably using. Maybe, as i.pca is quite old, it
considers the categories only up to 255. In any case, using r.mapcalc
you know better what you are doing.
Also, note that you must rescale
using the mean, not max min values. In other words
r.mapcalc "band1c = band1 - mean(band1)"
Agus
Nikos Alexandris wrote:
Roger:
Good, thanks. There you say that you are using "some MODIS surface
reflectance products". I guess it will be easier to check things if the
data (GRASS location) are available, so that others can try the same
calculations. Would it be possible to make one or more test sets
available, and link them from thw Wiki?
Nikos:
Finally,
I am, sort of, free again :-).
Sorry for being late. Couldn't make it earlier. I've updated the
wiki-page [1].
I have prepaired a grass-location
containing the 3 modis bands I've used for all PCA related examples. I
will eventually upload them in gregis [check previous post] but if there is
another place
(GRASS-wiki?) to upload them would be also ok.
I have currently some access problem in gregis this why I can't upload
the MODIS bands there. Is there any other place to put them for now?
Now, I am more confused than before. Or maybe not...
There seems to be no problem with the SPOT images, that is, the results
of i.pca are (almost) identical with the results of R's prcomp(x,
center=TRUE) *with* centering. This is completely different than with
the example using MODIS data.
Perhaps it depends on the range of the input data?! The SPOT bands are
up to max. 255. The MODIS bands are:
# band 2
r.info -r mod_b2
min=0
max=5504
# band 6
r.info -r mod_b6
min=93
max=5488
# band 7
r.info -r mod_b7
min=0
max=4133
I ran the same PCA tests (in R) with the _rescaled_ as like with the
"original" MODIS bands (using r.rescale) but no luck, I get the same
contradictory results: _i.pca_ matches with _prcomp(x, center=FALSE)_.
I suspect that the range of the input data is the reason... but I am not
sure. This is why I tried to repeat the test with _rescaled_ MODIS
bands. Given that the "r.rescale" products are kind of a "rules" file
under the "cats" directory, is "r.rescale" safe? Should I instead
rescale the data with r.mapcalc or is there no difference between
r.rescale and r.mapcalc?
All explained in the *under construction* wiki. Kind regards, Nikos
---
[1] http://grass.osgeo.org/wiki/Principal_Component_Analysis
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Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
LLuis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
email: [email protected]
http://www.ija.csic.es/gt/obster
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