[GRASS-stats] Re: [GRASS-user] Testing i.pca ~ prcomp(), m.eigensystem ~ princomp()

[Edzer Pebesma]

I tried to improve the wiki page, but never got access.
--
Edzer


>
> 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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