If it is correct depends on your research question. What I meant that your TF is defined in the volume grid in Brainstorm, and in FieldTrip. Althougth the volume grids may be different.
From the FieldTrip code and documentation, plotting volume sources on the cortical surface is done with a 3D interpolation. You may want to contact the FieldTrip team for more details on the interpolation.
- fieldtrip/ft_sourceplot.m at release · fieldtrip/fieldtrip · GitHub
- Plotting data at the channel and source level - FieldTrip toolbox
- Plotting data at the channel and source level - FieldTrip toolbox
With this 3D interpolation, only vertices in the volume grid that are closer to the cortical surface will be taken into account. This 3D interpolation process can be done in Brainstorm, it is not found in the GUI as it is not recommended. This would be possible in with the function bst_shepards() using as destination the positions of the cortical surface vertices, and as origin the positions of the volume grid vertices, it will give you a matrix of size [nSurfaceVertices, nVolumeVertices] that can be used to "project" from volume to surface. Please note that volume sources are unconstrained, thus there are 3 source time series per vertex, as such, it will be needed to flat first the volume sources to 1 time series per vertex before the 3D interpolation