# read initial head position from fif file

**URL:** <https://mne.discourse.group/t/read-initial-head-position-from-fif-file/8515>\
**Category:** Support & Discussions\
**Created:** [March 19, 2024, 6:22pm UTC](https://mne.discourse.group/t/read-initial-head-position-from-fif-file/8515 "2024-03-19T18:22:15Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Sushi](https://avatars.discourse-cdn.com/v4/letter/s/50afbb/32.png) [@Sushi](https://mne.discourse.group/u/Sushi)\
**Post date:** [March 19, 2024, 6:22pm UTC](https://mne.discourse.group/t/read-initial-head-position-from-fif-file/8515/1 "2024-03-19T18:22:16Z")

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Hello, I would like to read the initial head position (as I understand - it is calculated from the initial HPI measurement).  
I know that the info[‘dev\_head\_t’] is the transformation from device to head coordinates. Is there a way to calculate the initial head coordinates from this transformation?

Thank you!

- MNE version: 1.6.0
- operating system: Windows 10

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**Author:** ![mscheltienne](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/mscheltienne/32/827_2.png) [@mscheltienne](https://mne.discourse.group/u/mscheltienne)\
**Post date:** [March 20, 2024, 9:35am UTC](https://mne.discourse.group/t/read-initial-head-position-from-fif-file/8515/2 "2024-03-20T09:35:48Z")

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In which coordinate frame? In the device coordinate frame?  
I guess you could invert the `dev_head_t` transform and use it to map the fiducials.

Code snippet to invert an `mne.Transform` object `trans`:

```python
Transform(trans["to"], trans["from"], np.linalg.inv(trans["trans"]))

```

Mathieu

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<div class="post-metadata">

**Author:** ![Sushi](https://avatars.discourse-cdn.com/v4/letter/s/50afbb/32.png) [@Sushi](https://mne.discourse.group/u/Sushi)\
**Post date:** [March 20, 2024, 9:42pm UTC](https://mne.discourse.group/t/read-initial-head-position-from-fif-file/8515/3 "2024-03-20T21:42:35Z")

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Thank you.  
and once I have this inverted matrix, I need to multiply by the fiducials location (as defined according to the head coordinate system), is this correct?  
where can I find this fiducials location data?  
thank you!

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<div class="post-metadata">

**Author:** ![mscheltienne](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/mscheltienne/32/827_2.png) [@mscheltienne](https://mne.discourse.group/u/mscheltienne)\
**Post date:** [March 21, 2024, 7:36am UTC](https://mne.discourse.group/t/read-initial-head-position-from-fif-file/8515/4 "2024-03-21T07:36:31Z")

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IIRC, the fiducials are in `raw.info["dig"]`.

Mathieu

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**Author:** ![Sushi](https://avatars.discourse-cdn.com/v4/letter/s/50afbb/32.png) [@Sushi](https://mne.discourse.group/u/Sushi)\
**Post date:** [March 22, 2024, 7:50pm UTC](https://mne.discourse.group/t/read-initial-head-position-from-fif-file/8515/5 "2024-03-22T19:50:51Z")

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thank you!

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**Author:** ![system](https://global.discourse-cdn.com/free1/uploads/mne/original/1X/85cc6bd2b69cb698a166dc6d880fb550510d0144.jpeg) [@system](https://mne.discourse.group/u/system)\
**Post date:** [March 29, 2024, 7:51pm UTC](https://mne.discourse.group/t/read-initial-head-position-from-fif-file/8515/6 "2024-03-29T19:51:47Z")

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Closed on March 29, 2024, 7:51pm UTC

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