[GRASS-dev] PCA (i.pca) in G7: filtering and rescaling

Markus Metz markus.metz.giswork at gmail.com
Thu Dec 5 00:21:53 PST 2013

On Wed, Dec 4, 2013 at 11:46 PM, Nikos Alexandris
<nik at nikosalexandris.net> wrote:
> Nikos Alexandris:
>> > Here, immediate effect when percent=99.68.  I guess it gets rounded to
>> > 99.7 ?
> Moritz Lennert:
>> Actually, the man page says "percent=integer". The input is treated with
>> atoi(). I don't know if that rounds or floors..

I think it truncates = floor for positive numbers.

> Heck, overlooked that :-(.  In my very humble and ever-wannabe-scientist
> opinnion, though, we need those extra digits to make it easy rejecting last
> Principal Component(s) prior to the backward PCA. Might be one, two or
> numerous (?) depending on the dimensions.

I think it rather depends in the amount of information encoded in each PC.

Alternatively, PC selection could also be based on the Eigenvalue,
typically all PCs with an Eigenvalue >= 1 (centered and scaled input)
would be used.

Markus M

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