Nikos Alexandris wrote: > > The i.pca module in G7 offers a "forward/filtering/backward" PCA. I have > > an issue first with filtering and second with rescaling. > > 1) I can't see any differences in the derived Principal Components
okay, to clarify: I mean the resulting images which, initially are Principal Components (synthetic images) and, after applying filtering & backward PCA, the resulting images approach the original data -- still they are modified. > > between percent=70 and percent=99 for 4 bands which the PCs (both centered > > and scaled) are: > > > > PC1 2.78 ( 0.4947, 0.5922, 0.5743, 0.2735) [69.53%] > > PC2 1.08 ( 0.5196, 0.0517,-0.0974,-0.8473) [26.99%] > > PC3 0.11 ( 0.4210, 0.2486,-0.7926, 0.3644) [ 2.86%] > > PC4 0.03 (-0.5551, 0.7647,-0.1805,-0.2729) [ 0.63%] > > What is filtering doing actually? Shouldn't percent=70 just filter out > > the rest, somehow? Moritz Lennert wrote: > AFAIU, filtering happens after pca: pca is run on all bands, then > according to the filter percentage you chose inverse pca is run using > the principal components necessary to reach the filter percentage of > variance. In your example, 70% would use PC1 and 2 (unless variances is > rounded up) and 99% would use PC1,2,3. Any difference you see is in the > resulting images, not in the PCA. Sure -- I didn't (mean to) state otherwise. But, there are no differences in the resulting images (after PCA > Filtering > Backward PCA). Makes sense? Apologies for not being very clear. Thanks from the prompt reaction, Nikos _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
