Anderson,
I don’t think you appreciated the importance of the Murat’s comments on your
earlier post on this same topic.
In theory, there is no problem combining objects digitized at different
magnifications, or even digitized by different researchers. However, before
doing so one must carefully investigate for possible systematic biases in
digitizing, so they may be reduced to the greatest extent possible. If there is
some consistent bias in how objects are digitized in one ‘group’ relative to
the other, this will permeate into perceived differences in shape that may not
exist. A common example with older digitizing tablets would be differences in
digitizing due to the handedness of the person digitizing. Right-handed and
left-handed individuals hold the stylus differently which can result in
consistent perceived shape differences of the same objects once digitized.
Whether or not you have such an issue with your two magnifications is unclear.
However, it is impossible to evaluate this without additional replication.
Again, as Murat suggested, try digitizing each object multiple times at each
magnification. Then one could obtain estimates of the variation in digitizing
at the same magnification versus across magnifications to begin to discern
whether the between-magnification variation is greater than one might expect.
If it is, then one must dig deeper to determine why.
I would recommend sorting all of this out before embarking on your empirical
study. Otherwise, interpreting patterns in the final dataset becomes
challenging to say the least.
Best,
Dean
Dr. Dean C. Adams
Professor
Department of Ecology, Evolution, and Organismal Biology
Department of Statistics
Iowa State University
www.public.iastate.edu/~dcadams/<http://www.public.iastate.edu/~dcadams/>
phone: 515-294-3834
From: Anderson Feijo [mailto:andefe...@gmail.com]
Sent: Friday, December 29, 2017 3:19 AM
To: MORPHMET <morphmet@morphometrics.org>
Subject: [MORPHMET] Doubt about scalling photos
Hi everyone,
I am starting a new project using GM which I will work with groups with
different sizes (e.g., rodents and small carnivores). I would like to find a
way to use the whole dataset in the analyses, instead of perform set of
analyses for each sized group. So, I did a test using one skull and place the
camera in two different distances to the object (~15 cm and ~30 cm). My
expectation was after scaling (using tpsDig) I wouldn´t have any meaningful
difference. But I got two clear groups that were statistically different. So,
my question is how can I combine 2D landmarks based on photos taken from
different distances of the camera to the object. I have attached here the tps
file (10 copies of the same skull, five at ~15cm and five at ~30cm). I would be
very grateful for any suggestion.
All the best and Happy 2018!
Anderson
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