On Wed, 29 Apr 2009, Nikos Alexandris wrote:
Nikos:
I tested... tested... & tested... (almost all tests below) and I just
can't understand WHY i.pca does NOT perform data centering when using
MODIS data (raw, rescaled at 0~255 or 0~254).
The attached html file (exported from Tomboy note-taker) is (I suppose)
easier to read :-)
Not easier to read, please simply post things on a website!
In your previous mail which you deleted here, you asked whether the code
of i.pca shows if the input data is centred. Yes, it is centred, the
function calc_mu(), as its name suggests, does this. The mean values are
then passed to the function calculating the covariance matrix:
calc_covariance(), where the means are subtracted from the input values of
bands before accumulating the covariance matrix. Is is possible that where
you do not see any effect of center=TRUE in prcomp that the means are all
zero? You will find reading imagery/i.pca/main.c interesting and not hard.
Roger
Cheers, Nikos
--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: [email protected]
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