Re: PCA versus RWA

[morphmet]

-------- Original Message --------
Subject: Re: PCA versus RWA
Date: Sat, 12 Sep 2009 18:18:32 -0700 (PDT)
From: Andy Grass <adgr...@gmail.com>
To: morphmet@morphometrics.org
References: <4a9e5d21.8000...@morphometrics.org>

How does one go about projecting the procrustes coordinates into the
tangent space?  This is actually a problem I've been wrestling with
with my own data. I compute the procrustes and pw scores in IMP and
then generally perform the PCA analysis in SAS and get two very
different results between the straight procrustes PCA and the relative
warps.



On Wed, Sep 2, 2009 at 6:55 AM, morphmet
<morphmet_modera...@morphometrics.org> wrote:

-------- Original Message --------
Subject: RE: PCA versus RWA
Date: Tue, 1 Sep 2009 11:59:38 -0700 (PDT)
From: F. James Rohlf <ro...@life.bio.sunysb.edu>
Reply-To: <ro...@life.bio.sunysb.edu>
Organization: Stony Brook University
To: <morphmet@morphometrics.org>
References: <4a9d33ab.1090...@morphometrics.org>

The partial warp scores represent a rigid rotation of the Procrustes
shape coordinates within the tangent space. Assuming one has taken the
step to first project the Procrustes shape coordinates into the
tangent space then the projections of the specimens onto the PCA axes
will be identical.

That means there is no real advantage of computing partial warp scores
unless it is on interest to see how much of the shape variation is at
large versus small spatial scales. Well, perhaps one advantage - they
give you the proper number of shape variables so that one does not
have get the eigenvectors of an unnecessarily large matrix and then
discard the last 4 (or 7 for 3D data) eigenvectors that have
eigenvalues exactly equal to zero.

=========================
F. James Rohlf
Distinguished Professor, Stony Brook University
http://life.bio.sunysb.edu/ee/rohlf

-----Original Message-----
From: morphmet [mailto:morphmet_modera...@morphometrics.org]
Sent: Tuesday, September 01, 2009 10:46 AM
To: morphmet
Subject: PCA versus RWA



-------- Original Message --------
Subject: PCA versus RWA
Date: Tue, 1 Sep 2009 06:20:00 -0700 (PDT)
From: Alexandra Wegmann <inca...@yahoo.com>
To: morphmet@morphometrics.org

Hello

Could anybody tell me when one should simply do a PCA (or CVA) on
the
procrusted shape coordinates and when one should do the PCA (or
CVA) on
the partial warp scores (relative warp analysis)? Your answer
would be
highly appreciated.



Thanks in advance


Alexandra
Master student
University of Zurich

E-Mail: inca...@yahoo.com







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