" The option can be added to MapIcosahedron but that will have to wait a bit longer. But I am not sure that would be of any help to you. Your results would depend on the number of nodes you use for the icosahedron which does not tell you much about the warping."

It is right that the results would depend on the number of nodes used in icosahedron, but I was thinking that if any resonable/ballpark number of
nodes is used, there should be a node on the stdmesh surface that is
close (say within ~1mm) to where the warped node would be on the individual
surface. Is this true at all?

"To my mind, you should compare segment length change between spherical and warped spherical. That comparison does not have to be made on the standard-mesh version of the surfaces. If you want to get an estimate relative to the segment lengths in the anatomically correct surfaces perhaps you can normalize (scale) the segment lengths on the sphere by their lengths in the anatomically correct surfaces. The programs SurfMeasures and SurfaceMetrics might be of interest to you."

Thanks, that makes sense and that is what I will prob. do. Does this approach
look ok: (1) find the coordinates of the given node on the individual original
sphere. (2) find the coords of the same node on the warped sphere. (3) calculate
the distance between the two. (4) scale the distance by a constant scale factor.
The scale facor would be determined by drawing a segment on the fiducial surface
and finding its length on the sphere. Can I use a constant scale factor regardless of the node's location? If different areas of the correct surface
get scaled by different amounts in spherical conversion, then I am not sure
if a constant scale factor would be right. (I should be asking this on the
FreeSurfer list, but just in case anyone has ideas about it here.)