Implementation of road horizontal alignment as a whole for CAD

Road horizontal alignment contains three elements: straight line, circular arc, and clothoid. In AutoCAD, clothoid can only be fitted by polyline or spline, and the graphics which are separated from the road data are independent of each other. Therefore, it is necessary to develop a new curve for road horizontal alignment. Firstly, the differential approximation and series integration methods for clothoid were discussed. Secondly, the geometric formulae for road line which contain the three elements were derived as a whole. Then, the advantages and feasibility of customizing a road line class, which was derived from the curve base class, were analyzed based on ObjectARX (AutoCAD Runtime eXtension) techniques. Finally, the data structure and operations for the road line class were stressed. The experimental results show that the road line can integrate the road elements, graphics and data to implement the graphics-oriented design, which can be widely used in road alignment design.