What technique is used to extract the control points from a NURBS surface? Is the surface interpolated from surface points or is a least squares approximation of points used? Is there a description of the method in the manuals?

I’m not sure what your question is. The location of control points determines the shape of the surface, see https://en.m.wikipedia.org/wiki/Non-uniform_rational_B-spline

To create a surface that interpolates a set of points, either direct interpolation is used if the number and ordering of three points allows it, or least squares to create a surface that approximates the points.

Hi,

if I get you right You have a Surface and want to have the controlpoints of this Surface.

To get The controlpoints use the NurbsSurface.Points Property

http://developer.rhino3d.com/api/RhinoCommonWin/html/P_Rhino_Geometry_NurbsSurface_Points.htm