[GRASS-user] Inverse PCA in GRASS

[Markus Neteler]

HamishB wrote:
> 
> Michael Perdue wrote:
>> I think what your asking is if anyone knows how to apply the  
>> transformation matrix that was used to derive you pca components in  
>> reverse to get back to the original coordinate system. Something like  
>> what is done in a decorrelation stretch?
>> Hopefully someone more knowledgeable than I can elaborate, but the  
>> general idea is as follows;
>> 
>> if the i.pca is capable of divulging the eigenvectors, you can use  
>> r.mapcalc to apply them in reverse. else;
>> 
>> r.covar -> m.eigensystem -> mapcalc -> "do something with your  
>> principal components" -> mapcalc (inverse rotation) -> "original  
>> coordinate system"
>> 
>> You can take a look at the bottom of the r.covar page for more info.  
>> The big "if" is on the status of m.eigensystem. I tried the link to  
>> the manual page and it is broken so I'm unsure of it's status.
> 
> 
> m.eigensystem has not been ported from GRASS 5 to GRASS 6.
> 
> It is available from the GRASS 5 source code:
>   grass-5.4.0/src.contrib/CERL/misc/m.eigensystem/
> 
> online:
>  
> http://freegis.org/cgi-bin/viewcvs.cgi/grass/src.contrib/CERL/misc/m.eigensystem/
> 
> 
> While it is written in Fortan77 I see no reason why it couldn't be
> compiled with g77 or gfortran and run with modern versions of GRASS.
> The GRASS raster/imagery engines and file formats have not changed since
> GRASS 5 so there shouldn't be many problems.
> 
> FWIW ./configure still checks for f2c.h
> 

I have added m.eigensystem to GRASS Addons SVN with a slightly
updated Makefile. Grab here:
http://trac.osgeo.org/grass/browser/grass-addons/m.eigensystem

or from SVN
http://trac.osgeo.org/grass/wiki/DownloadSource
-> Subversion GRASS Addons source code repository

Markus
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