[GRASS-dev] PCA question

[Nikos Alexandris]

[Addendum]

Hamish:
> > (not talking about pan-sharpening, but in general,)
> > how about the situation where you have a map data 
> > which is loudly dominated by a signal, and you want to try and remove that
> > loud signal so that you can look at the subtle variations caused by a
> > different source that the loud signal had been masking?

Nikos:
 
> Yes, this _can_ be a perfect use-case.
 
> Especially if the presence of the feature in question, is in at least one or
> in some of the input dimensions near/close to zero. This last statement is
> based on Pielou's (flawless explanation of how PCA works) [1] and own
> experiences [2].

If the above holds true (near-zero projection, in one or some dimensions, of 
the features of interest), and in some way, these "subtle variations" do form 
distinct clusters, I'd be keen to try out non-centered PCA and (re-)confirm 
Pielou and myself (claiming, for example, higher Producer's accuracies as in 
--my-- burned area mapping).

Nikos
 
> A separation/isolation attempt of the feature in question from dominant
> variances will be "supported".  The "loud" signal would be channeled among
> the first few PCs and the "subtle variations" _could_ then be more evident
> in some of the higher order components.
 
> All in all, one has to look at the numbers -- drawing conclusions from the
> PC images is not safe!
 
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