Nikos: > > The thing is by multiplying by 0.0001 thing are worse concerning the > > *eigenvalues* (the eigenvectors are the same):
> > # use of i.pca gives > > r.info -h pca_mod_b267.1 [...] > > Eigen values, (vectors), and [percent importance]: > > PC1 6307563.04 (-0.6353,-0.6485,-0.4192)[98.71%] > > PC2 78023.63 (-0.7124, 0.2828, 0.6422)[1.22%] > > PC3 4504.60 (-0.2979, 0.7067,-0.6417)[0.07%] > > # using i.pca gives > > r.info -h pca.mod_x.1 [...] > > Eigen values, (vectors), and [percent importance]: > > PC1 0.06 (-0.6353,-0.6485,-0.4192)[98.71%] > > PC2 0.00 (-0.7124, 0.2828, 0.6422)[1.22%] > > PC3 0.00 (-0.2979, 0.7067,-0.6417)[0.07%] Markus M: > OK, I don't have the full discussion on i.pca in my head, so I don't > know how much sense my comments make. The loadings and percentages > explained variance are identical, that's good. Yep. > The Eigenvalues are not, it seems they were calculated from unstandardised > (raw) values. Note: the percent importance is nothing else than just transforemd eigenvalues (that is: sum-up all eigenvalues and say the sum is the 100%, take then the percent of each eigenvalue). The fact that the 2nd an d 3rd eigenvalues in the above example are 0.00 is a (another) print-out/report issue I think. However, the multiplication of the MODIS bands with the recommended factor (0.0001) does nothing to the way i.pca treats the data. ## I am convinced that i.pca wrongly _depends_ currently on the range of the input data whether to apply data centering or not. I just need to rescale the MODIS bands in to 0,255 and confirm my skepsis. If I am wrong then it might be even more complicated!! ## > For imagery processing, that may be desired, for other applications AFAIK it > is required that input variables variables (here different bands) are > standardised first so they can be combined and principal components > extracted. As Augustin Lobo and others suggested, data centering should be performed. And me, with my limited knowledge and experience think the same. > I'm more familiar with non-spatial PCA, so it's high time I read the > manual of i.pca, and the new wiki page on it... Markus, I might missed to update some important sentences. So "handle with care" :-) Cheers, Nikos _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
