Distance between NURBS and ctrlpoints

I’m Japanese.
I got (ctrl)points from Kinect and generated NURBS suface object from that.

Then, I try to calculate error caused by NURBS approximation.
I want to calculate minimum distances between NURBS surface and each ctrlpoints.
My error is accumulation of the distances, but I cannot get NURBS surface equations.

How I access the NURBS parameters like equations, areas, and length?
To begin with, can I access NURBS parameters?

Thank you for reading.