[Eeglablist] Coordinate systems question

[Balázs László]

Hi,
I am trying to write code to digest electrode positions data for a Polhemus Patriot 
digitizer. I would like to match up results with some standard coordinates, however I am 
puzzled with the various location files available. The problem is with the origins/axes.
Neuroscan sample files seem to have axes thru preauriculars and nasion. These files 
were probably made with a digitzer, not having much attention paid to precision. Other 
files, like the ones in the EEGLAB sample locations seem to have an origin somewhat 
higher, close to the level of Fp1-Fp2-O1-O2. What are the origin and axes here? If I 
understand correctly, the system should not influence the localisation, until it is 
coregistered with te fiducial points (LE,RE, Nasion), since the head model is getting 
transformed to the MRI head using the coordinates of the fiducials .
Anyway, I would like to see a bit more clearly.
Cheers,
Laszlo
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Laszlo Balazs, Ph.D.
Institute for Psychology, Budapest