Dear colleagues, I would like to thank Dr. Bameul and Dr. Reyment for their help regarding my discriminant function question. However I will appreciate any advice regarding the following problem:
It is customary to do systematic analyses on landmarks from half of the skull, considering at least two landmarks as midline points (at the axis of symmetry). Most of the recent studies work on the average of bilaterally homologous landmarks, providing a single landmark representing any homologous character in half of the skull. The main reason for this is to avoid excess of degrees of freedom in statistical contrasts. Many of you should know this better than me. The only program I know that can provide the average for a set of symmetrical landmarks is BigFix in the IMP package. The problem is that BigFix only works with landmarks oriented in an X axis of symmetry. Unaware of this requirement I made a TPS landmark matrix from bat skulls having a Y axis of symmetry, which wont work on BigFix. It will take me many days to remake this matrix if I do not find an automated method to rotate the coordinates 90 degrees to either side for having them aligned to an X axis of symmetry. 1) Is there any application or method in which I can make this rotation? 2)Is there any alternative tool to BigFix for obtaining the average from symmetrical landmarks? Another unclear method is how if statistical analyses are based on half-skull landmarks, it is possible to make the visual reconstructions of skull variability on the full set of landmarks (after reflecting the average set on the other side of the skull). This is usually done to avoid vectors of variability along the axis of symmetry which can obscure the interpretation of results. The dimensionality of the data changes if using only half or the full skull, and thus, any thin-plate spline application will be influenciated by this. It is unclear for me how then, one can obtain statistical estimates and then make the visualizations on the full skull, which will not accurately represent those estimates. Available literature is unclear about this topic, and I will be grateful if you can point towards a reference explaining this method more clearly. Thanks in advance for any advice you can provide me. Pablo Pablo Jarrin Grad. Student Department of Biology Boston University -- Replies will be sent to the list. For more information visit http://www.morphometrics.org
