Re: [MORPHMET] symmetrizing data with bilateral symmetry

[andrea cardini]

Dear Eli,
I'd say that it depends on what you want to do.

If quantifying asymmetry is one of your aims, then of course you must 
have landmarks on both sides.


If not, I'd tend to say that it's better to have the bilateral landmarks 
and then decide whether or not to 'symmetrize' the data.
However, even in this second case, as I suggested with an example in a 
paper which should be soon out in Syst. Biol., there might be cases when 
you might decide to landmark just one side to save time and money and 
maximize sampling during data collection. Then, often, by simply 
reconstructing the side with no landmarks by mirror-reflection, you 
might get a better approximation of the total configuration you would 
have had by landmarking both sides.


There might be also cases when you really want to measure just one side 
because your question is specific to that side, but I've never found one 
such case in the literature.


To symmetrize the data in MorphoJ you just need to:
1) load them as data with object symmetry;
2) do the Procrustes superimposition (and check that MorphoJ is 
'guessing' the correct pairs of bilateral landmarks);

3) save size and the symmetric component (export from the file menu);
4) restore size by multiplying the symmetric shape coordinates by the 
centroid size of the corresponding individual.
Of course, if you do all your analyses in MorphoJ, you just need to 
select each time only the symmetric component.


Cheers

Andrea


On 05/07/16 16:36, Elahep wrote:


Dear all,

I have got quite confused about object symmetry! If analyzing symmetry 
is not our purpose is this necessary to do symmetrizing procedure for 
the data that have bilateral symmetry?
I have seen some people choose only one side of the bilaterally 
symmetric object to study its shape variation (for example between 
geographic areas). In this case is this better to put landmarks in 
both sides of the midline or considering just one side is enough? If I 
want to do the symmetrizing how can I do it with tps series or Morphoj?


Any help would be appreciated!

Eli
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Dr. Andrea Cardini
Researcher, Dipartimento di Scienze Chimiche e Geologiche, Università di Modena 
e Reggio Emilia, Via Campi, 103 - 41125 Modena - Italy
tel. 0039 059 2058472

Adjunct Associate Professor, School of Anatomy, Physiology and Human Biology, 
The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, 
Australia

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[MORPHMET] symmetrizing data with bilateral symmetry

[Elahep]

Dear all,

I have got quite confused about object symmetry! If analyzing symmetry is 
not our purpose is this necessary to do symmetrizing procedure for the data 
that have bilateral symmetry?
I have seen some people choose only one side of the bilaterally symmetric 
object to study its shape variation (for example between geographic areas). 
In this case is this better to put landmarks in both sides of the midline 
or considering just one side is enough? If I want to do the symmetrizing 
how can I do it with tps series or Morphoj?

Any help would be appreciated!

Eli

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MORPHMET may be accessed via its webpage at http://www.morphometrics.org
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