[GRASS-dev] Re: i.cca eigenvector computation strangeness

Hamish hamish_b at yahoo.com
Tue Aug 18 02:26:12 EDT 2009

Soeren wrote:
> I have verified every step between the old and the new
> version of i.cca. The difference is only in the eigen vector
> computation.
> So the big question is ... which result is correct???

is it more a matter of popular convention than correctness?
i.e. does it depend on your reference frame / POV ?

> The new one looks more reasonable to me, but well this is
> only one example ... .
> Do i have found a bug in the numerical recipes eigen vector
> code, or is the sign of eigen vectors not fixed and changes
> with the chosen algorithm?

perhaps the output of the i.pca module, the m.eigensystem module (addons),
and R-stats can give you some confidence in one flavour over another.

I'd guess it is highly likely that you are correct and this source was
adapted from Fortran beginnings.



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