[Eeglablist] Spherical coordinates (longitude and latitude) for standard 10-20 system in degrees

[Stefanie Nickels]

Hello Andreas,

Thank you for your answer.

I started looking closer at the conversion from theta and phi to longitude
and latitude because I found some sources saying that they don't map onto
each other in a one-to-one manner (e.g., this post
<http://stackoverflow.com/questions/5674149/3d-coordinates-on-a-sphere-to-latitude-and-longitude>,
see highest rated answer). After getting some help from a friend in physics
I learned that sph_theta = longitude, but that sph_phi = latitude is not
correct because phi is measured in relationship to the zenith, while
latitude is measured in relation to the equator. Thus,
the relationship between phi and latitude is that latitude = 90 - phi.
After plotting this, and comparing values from car2ph() conversion taking
the x, y, z values (which are also included in EEGLAB) I realized that this
90 - phi adjustment had already been made in EEGLAB. Thus, what is
called sph_phi in EEGLAB is actually the latitude. I guess I could have
made my life easier by not questioning whether theta and phi are the same
as longitude and latitude in the first place. :D

One very important thing I also learned, and which caused me a lot of
trouble researching this, is that physics and math people use theta and phi
in the exact opposite way (see Wiki
<https://en.wikipedia.org/wiki/Spherical_coordinate_system>). The EEGLAB
file follows the physics notation though, where theta denotes the polar
angle, while phi means the azimutal angle (at least that's how it should be
in principle, with the qualification that EEGLAB's sph_phi  is actually the
latitude (so 90-phi) as explained above).

Please let me know whether I got anything wrong about these relationships
in your (or other's) opinion!

All the best, and thank you for following up on this,
Stefanie



On Tue, Jul 5, 2016 at 8:45 AM, Andreas Widmann <widmann at uni-leipzig.de>
wrote:

> Dear Stefanie,
>
> to my understanding sph_theta and sph_phi directly reflect longitude and
> latitude, respectively. Could you please explain your problem in more
> detail?
>
> Best,
> Andreas
>
> > Am 29.06.2016 um 22:20 schrieb Stefanie Nickels <
> stefanie.nickels at gmail.com>:
> >
> > Hello all,
> >
> > I am looking for a file that contains the spherical coordinates for
> electrodes placed in the standard 10-20 system, but I specifically need the
> values in degrees of longitude and latitude.
> >
> > I am aware that eeglab contains a number of channel location lookup
> files (such as standard-10-5-cap385.elp), but they only contain the
> spherical theta and phi (and radius, which is constant). I understand that
> these three values uniquely specify the electrode's position in the sphere,
> but I don't know how to get to longitude and latitude from there. So a)
> does anyone have a file that already contains the electrode positions in
> longitude and latitude, or b) can someone point me towards the formula that
> lets me solve for longitude and latitude from theta and phi?
> >
> > Thank you all very much,
> > Stefanie
> >
> >
> > _______________________________________________
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>

On Tue, Jul 5, 2016 at 8:45 AM, Andreas Widmann <widmann at uni-leipzig.de>
wrote:

> Dear Stefanie,
>
> to my understanding sph_theta and sph_phi directly reflect longitude and
> latitude, respectively. Could you please explain your problem in more
> detail?
>
> Best,
> Andreas
>
> > Am 29.06.2016 um 22:20 schrieb Stefanie Nickels <
> stefanie.nickels at gmail.com>:
> >
> > Hello all,
> >
> > I am looking for a file that contains the spherical coordinates for
> electrodes placed in the standard 10-20 system, but I specifically need the
> values in degrees of longitude and latitude.
> >
> > I am aware that eeglab contains a number of channel location lookup
> files (such as standard-10-5-cap385.elp), but they only contain the
> spherical theta and phi (and radius, which is constant). I understand that
> these three values uniquely specify the electrode's position in the sphere,
> but I don't know how to get to longitude and latitude from there. So a)
> does anyone have a file that already contains the electrode positions in
> longitude and latitude, or b) can someone point me towards the formula that
> lets me solve for longitude and latitude from theta and phi?
> >
> > Thank you all very much,
> > Stefanie
> >
> >
> > _______________________________________________
> > Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> > To unsubscribe, send an empty email to
> eeglablist-unsubscribe at sccn.ucsd.edu
> > For digest mode, send an email with the subject "set digest mime" to
> eeglablist-request at sccn.ucsd.edu
>
>
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