Hi Debora,
The heads are interpolated at the observation points.
You can test this in a few minutes by making a test model with a single large triangular element, putting an observation point in the middle, and assigning fixed heads to the three vertices.
I don't have a reference for the interpolation method used. However, it appears that the three heads at the triangle vertices are used to define a plane, and that the head observations are then drawn off that plane (i.e. 2D linear interploation).
(see attachment)