[GRASS-user] i.pca vs. r.covar/m.eigensystem/r.mapcalc

[Nikos Alexandris]

My apologies for BUMPing :-)

On Sun, 2008-10-12 at 00:20 +0200, Nikos Alexandris wrote:
> I wanted to check/verify some i.pca results so I calculated 2 principal
> components using r.covar, m.eigensystem and r.mapcalc. The results are
> not the same with i.pca's output.
> 
> (Maybe this post http://www.nabble.com/Eigen-vectors-tt15686112.html was
> about the same issue)
> 
> # i.pca performed on (3) modis surface reflectance bands (2, 6 and 7  -
> 500m spatial resolution)

*** and region resolution set to 500m of course***

> r.info pca.2007.242.500.b267.1 -h
> [...]
> Eigen vectors:
>    ( -0.64 -0.65 -0.42 )
>    ( -0.71 0.29 0.64 )
>    ( 0.29 -0.70 0.65 )
> [...]

I repeated the same i.pca but setting this time a different region
resolution (250m) than before (500m) and I get slightly different
eigenvectors:

*** with region resolution=250m ***
Eigen values:
( -0.64 -0.65 -0.42 )
( -0.71 0.28 0.64 )
( -0.30 0.71 -0.64 )


I understand that the region "resolution" influences the results of
i.pca. OK, then how would someone re-produce the same PCs doing the math
in R and not in GRASS?

Could someone explain or give any hints to learn more about i.pca?

Thank you, Nikos