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Polar/Cartesian electrode coordinates for 128-ES cap?
Posted: Thu Dec 17, 2009 10:41 pm
by Xue
Hi all,
Our lab is trying to convert the coordinates provided by BioSemi for the 128 electrode cap into the *.ced format required by the EEGLab toolbox for Matlab. However, the azimuth and latitude measurements in the BioSemi excel file do not use a standard that we recognize (or that is recognized by the program). Has anyone created a *.ced file for the 128-electrode cap, or know where we might find the polar and/or cartesian coordinates? Failing that, what is the spherical radius that was used to calculate the azimuth and latitude? Was it just forced to 1?
Thank you!
Posted: Tue Dec 22, 2009 4:24 pm
by Coen
The BioSemi site provides standard spherical coordinates, with inclination as the angle in the vertical plane measured from Cz, and Azimuth as the angle in the horizontal plane from T7 or T8.
The spherical radius is the radius of a perfect sphere that approximates the head.
Cartesian coordinates can be derived from the spherical coordinates using basic trigonometry:
x = r * sin (In) * sin (Az)
y = r * sin (In) * cos (Az)
z = r * cos (In)
Where:
"In" is the inclination (angle from Cz)
"Az" is the azimuth (angle from T7 or T8 )
with the origin in the center of the head, and with the z-axis pointing to Cz, the y-axis to the left mastoid, and the x-axis pointing to the nose
Best regards, Coen (BioSemi)
Posted: Thu Apr 22, 2010 9:01 pm
by oneiric
I have the theta (In) and phi (Az) coordinates from Biosemi, and need to convert to Cartesian coordinates,
You have provided the formula to do that, but
what is the value of r ?
Posted: Thu Apr 22, 2010 10:11 pm
by Coen
radius of the head
128 electrode coordinates
Posted: Wed Aug 10, 2011 3:51 pm
by aless
Hallo, thanks for these explanations.
I would need to know:
1. Is the radius of the head assumed to be = 1?
2. How can the idiosincratic Biosemi 128 labels (As, Bs, Cs, Ds) be correctly transformed into some international label system, like the 10-5 or similar? Has anyone done that?
Best,
Aless
Posted: Sat Aug 13, 2011 5:39 pm
by Coen
> Is the radius of the head assumed to be = 1?
The value for the radius depends on the chosen unit of measure and the size and (assumed) shape of the head.
> How can the idiosincratic Biosemi 128 labels (As, Bs, Cs, Ds) be correctly transformed into some international label system, like the 10-5 or similar?
The international BioSemi ABC layout cannot be converted into a 10-5 layout because only 22 electrodes positions overlap, see
http://www.biosemi.com/download/Cap_coords_all.xls
Best regards, Coen (BioSemi)