Hi @Anne
that's a tough one... Cost function 103 will do that for you (because it is based on sqrt(vx2 + vy2)) but not 101 (because it tries to fit vx and vy individually). In any case, you are going to need to make up vx and vy because that's what the cost functions use. You could use surface slopes (probably smoothed) in order to create a good guess for flow direction and convert velocity to vx/vy.
Let us know if you need help with that
Mathieu
