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

Nikos Alexandris nikos.alexandris at felis.uni-freiburg.de
Tue Mar 31 11:07:29 EDT 2009

> >> Nikos, I'm probably missing a message (I do not see which is
> >> exactly the difference between the spot and the modis example),
> >>     

> > Hi Agus. The difference is:
> > Using SPOT (range up to max. 255):
> > i.pca == prcomp(x, center=TRUE, scale=FALSE)
> > Using MODIS (range varries between bands, up to max. ~5500):

Markus M:
> Sure? Valid range: -100 - 16000 for MODIS e.g. Surface Reflectance 8-Day 
> L3 Global 500m.

I am referring to the range of the specific MODIS bands that I've used,
that is:
# band 2
r.info -r mod_b2


# band 6
r.info -r mod_b6


# band 7
r.info -r mod_b7


> As I would use the max possible for SPOT (255) I would also use the max 
> possible for other sensors, not the observed max, otherwise statistics 
> that don't do proper normalization on the fly are not comparable between 
> different sensors.

> But any PCA should do normalization beforehand, not sure what R or i.pca 
> are doing.

That's the question _now_ (if I am right with my tests): why isn't
normalisation (=data centering in this case) performed with the MODIS
bands I use?

> Please excuse my random comments :-)

Markus, your comments are required :-)
Cheers, Nikos

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