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Parametrization of the cord surface. The center-line c is parameterised in z and implemented as cubic spline interpolators of the x- and y- coordinates of the center-line curve. The radius r is parameterised in z and θ, the angle between the radius vector and the x-axis. The surface normal vector N is approximated as the vector that is both normal to the center-line tangent vector, and lies in the plane containing both the center-line tangent and the radius vectors.
