Hi Tom

Distances in the flat map should be in mm and within 10-15% of geodesics in the cortex. You could also use great circles on the sphere, which should give you about the same error percent, but you would need to scale the sphere so it has the same surface area as the cortex I believe Cheers Bruce

From: Possidente, Thomas tposside@bu.edu Sent: Monday, December 1, 2025 9:53 AM To: freesurfer@nmr.mgh.harvard.edu Subject: [Freesurfer] Flatmap Coordinates Distance

External Email - Use Caution Hi there,

I have flattened a patch from fsaverage surface using mris_flatten and I would like to find the distance between two points on the flatmap. I believe that simply calculating the distance between coordinate points on the flatmap would not be interpretable because the units are no longer in millimeters (due to distortions during flattening). Please let me know if this is correct.

I have found that I can map the vertex indices back into RAS 3D space and get the distance between 2 vertices that way, but because of cortical folding, it is different from the distance along the cortex between the two points. Is there any way to get the distance along the cortex between 2 vertices in interpretable units (ex. millimeters)?

I am using Freesurfer 7.4.1 and Matlab 2023a to read in the any surface/patch/label files.

Thank you for all the great work that you do!

Best, Tom

Thomas (Tom) Possidente | He/Him Doctoral Candidate | Boston University Brain, Behavior, & Cognition | Somers Lab