Principal Component Analysis.

[Cristina Isabel Seabra [Ambiente]]

Hello GRASS users:

I've seen a couple of messages about the principal components analysis commands
i.pca and r.covar.

I used i.pca to create principal components of a LANDSAT image. The results I
got using i.pca and m.eigensystem are different.

The eigenvalues are the same but the eigenvectors are different. Consequently
when I use r.mapcalc to create the principal components I get different results.

The formula I used to create the PC after I ran m.eigensystem was:

'pc1 = eigenvector*layer1 + (or -)eigenvector*layer2 ....'

For the eigenvalues there seems to be no problem but the calculation of the
eigenvectors is different in i.pca and m.eigensystem.

Does someone know which of the commands is right? One of them must have a bug,
but which one?

I really need to make this principal components analysis and I don't know which
command I should use.

Thanks for any help.

Cristina Seabra

cis at fct.unl.pt