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 _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user