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

Markus Metz markus.metz.giswork at googlemail.com
Tue Mar 31 11:00:50 EDT 2009

Nikos Alexandris wrote:
> Agustin:
>> 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):
Sure? Valid range: -100 - 16000 for MODIS e.g. Surface Reflectance 8-Day 
L3 Global 500m.
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.
Please excuse my random comments :-)

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