# [FieldTrip] Coordinate System (Align electrode positions) Brainstrom VS fieldtrip

Hamza Fawzi Altakroury (Alumni) hamzaf at sabanciuniv.edu
Thu Jun 26 10:08:55 CEST 2014

```Thank you Dr. Horschig,

I am sorry for writing the first line of my letter wrongly.

Hamza

On Wed, Jun 25, 2014 at 5:10 PM, "Jörn M. Horschig" <
jm.horschig at donders.ru.nl> wrote:

> Dear Hamzi,
>
> this might be related to different coordinate systems, see
> http://fieldtrip.fcdonders.nl/faq/how_are_the_different_
>
> FieldTrip is using whatever coordinate system the data was spceified in,
> and it seems that brainstorm is using some fixed coordinate system and
> applied a transformation beforehands. you could e.g. notice that in
> Brainstorm the coordinates for the first and third dimension have about the
> same value, where in FieldTrip this happens for the second and third value.
> So I would assume the difference is due to a difference in coordinate
> systems.
>
> Best,
> Jörn
>
>
> On 6/25/2014 1:41 PM, Hamza Fawzi Altakroury (Alumni) wrote:
>
>> Hello,
>>
>> I was checking the one of the Fieldtrip's pages to perform an have change
>> the coordinate system of my electrodes position
>>
>> The page:
>> http://fieldtrip.fcdonders.nl/example/align_eeg_electrode_
>>
>> For doing an alignment, the fiducials positions should be given, I did
>> not have them first, so I use the brainstorm toolbox to get them from the
>> patient MRI.
>>
>> In brainstrom I found this equation for transforming from voxel to mm.
>> (given the voxel size)
>>
>> Position_mm = Position_voxel .* Voxel_size.
>>
>> While in fieldtrip I found another statement which gives (given the
>> transformation matrix) a different answer:
>>
>>
>>
>> For example using brainstorm equation I got:
>>
>> NAS = [117.1920215.8287118.9998]
>>
>>
>> And using the fieldtrip function, I got:
>>
>> NAS = [14.397349.392649.6322]
>>
>>
>> What is the difference between the two?
>>
>>
>> Thank you
>>
>>
>> --
>> Hamza Fawzi Altakroury
>> Faculty of Engineering and Natural Sciences
>> Sabancı University
>>
>>
>> _______________________________________________
>> fieldtrip mailing list
>> fieldtrip at donders.ru.nl
>> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>>
>
>
> --
> Jörn M. Horschig
> PhD Student
> Donders Institute for Brain, Cognition and Behaviour
> Centre for Cognitive Neuroimaging
> Neuronal Oscillations Group
> FieldTrip Development Team
>
> P.O. Box 9101
> NL-6500 HB Nijmegen
> The Netherlands
>
> Contact:
> E-Mail: jm.horschig at donders.ru.nl
> Tel:    +31-(0)24-36-68493
> Web: http://www.ru.nl/donders
>
> Trigon, room 2.30
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> NL-6525 EN Nijmegen
> The Netherlands
>
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--
Hamza Fawzi Altakroury