1. A computer-implemented method for modifying a spline, comprising: obtaining, in a computer, the spline, wherein the spline comprises a set of connected points, and wherein the spline is representative of a feature to be constructed outside of the computer and wherein the feature represented by the spline comprises a vertical road profile of a road; determining, in the computer, a design constraint set comprising an interpolation constraint, wherein the interpolation constraint comprises a fixed elevation for one of the connected points; projecting, in the computer, the spline onto the design constraint set, wherein the projecting comprises modifying the spline, by a minimum distance in a metric space, by mapping a first elevation of a first connected point of the set of connected points to match the fixed elevation of the design constraint set, and then moving the first elevation to the fixed elevation; projecting, in the computer, the modified spline onto the design constraint set iteratively until the spline satisfies all constraints in the design constraint set; and generating, in the computer, a computer aided design (CAD) design of the feature represented by the spline, wherein the generating is based on the projecting steps; and providing the spline for constructing the road based on the spline.