1. A computer-implemented method for modifying a spline, comprising: obtaining the spline, wherein the spline comprises a set of connected points; determining a design constraint set comprising an interpolation constraint, wherein the interpolation constraint comprises a fixed elevation for one of the connected points; projecting the spline onto the design constraint set, wherein the projecting modifies the spline by changing a first elevation of a first connected point to match the fixed elevation; and projecting the modified spline onto the design constraint set iteratively until the spline satisfies all constraints in the design constraint set.