[GRASS-user] Creating PCA Plot

[Nikos Alexandris]

Rashad M wrote:

> This is helpful for me. So to get a pca plot i need to use just
> d.correlate with the output raster from i.pca right?

Well,

PCA (in grass i.pca) will output as many variables you will feed it to --
they will be the transformed variables.

Out of those, ususally 2-dimensional plots, or even 3-dimensional plots
can be created -- various combinations.  To exemplify, if you transform 6
Landsat bands and get 6 new Principal Components, then you would pick up
combinations of 2 PCs to created 2D scatter-plots, or at most,
combinations of 3 PCs to create 3D plots.

( We are, after all, bound to at most 3D, right? :D )

Anna K showed me also the wxIClass stuff which might help in searching for
class separabilities! <http://grasswiki.osgeo.org/wiki/WxIClass>.

It's the user's responsibility, however, to identify which PCs are of his
interest, which combination will offer useful information.

> Also i.pca changes spatial position and to output of i.pca will have the
> changed pixel position

No. It's simply a mathematical linear transformation -- does not affect
the "spatial" domain.  Dunno, in my brain I see it simply as a
redistribution of the original data's variance(s) (?).

Nikos

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