Available choices: Maximum Slope (Travis et al. 1975), Maximum Triangle Slope (Tarboton 1997), Least Squares Fitted Plane (Horn 1981, Costa-Cabral & Burgess 1996), Fit 2.Degree Polynom (Bauer, Rohdenburg, Bork 1985), Fit 2.Degree Polynom (Heerdegen & Beran 1982), Fit 2.Degree Polynom (Zevenbergen & Thorne 1987), Fit 3.Degree Polynom (Haralick 1983)