说明：  在autocad里对pline折线进行光滑的程序，目的是实现等高线的光滑，相比autocad里提供s样条光滑，可以实现光滑后仍然过原来的折线顶点，从而保证等高线的准确
(Pline in autocad in on line to smooth the process in order to achieve a smooth contour, which offers s compared to autocad spline smoothing can be achieved even after smooth line through the original vertex, thus ensuring the accuracy of contour lines)