[GRASS-user] i.pca fixes in trunk

Markus Metz markus.metz.giswork at googlemail.com
Fri Nov 4 06:53:50 EDT 2011

Hi all,

based on the wiki for Principal Components Analysis [0], numerous
discussions in the mailing lists [1,2,3,4], particularly a comment by
Edzer Pebesma [5], and personal demand, I have fixed a few issues in
i.pca in trunk r49090.

- the faulty or missing centering of the input bands described in [0]
for should be fixed
- i.pca has a new flag -n to normalize input bands with (x - mean) / stddev
- values of the output maps are now calculated depending on the input
band transformation (centering or normalization). Is this OK?
- Eigen values, (vectors), and [percent importance] are now written to
stdout instead of stderr

The results of i.pca for the examples using SPOT imagery in the wiki
[0] are now identical to R's princomp() results. If the new -n flag is
used, the results of i.pca are identical to princomp(center = TRUE,
scale = TRUE).

Tested also with 9 input maps in a region with 400 million cells.

Markus M

[0] http://grass.osgeo.org/wiki/Principal_Components_Analysis
[1] http://lists.osgeo.org/pipermail/grass-user/2009-February/048722.html
[2] http://lists.osgeo.org/pipermail/grass-stats/2009-March/000933.html
[3] http://lists.osgeo.org/pipermail/grass-stats/2009-March/000942.html
[4] http://lists.osgeo.org/pipermail/grass-stats/2009-April/001028.html
[5] http://lists.osgeo.org/pipermail/grass-stats/2009-April/000977.html

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