[GRASS-dev] PCA (i.pca) in G7: filtering and rescaling

[Nikos Alexandris]

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