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Grade crossing is an important constituent part in road system, it is the“throat”of road traffic. The elevation planning of grade crossing is a significant design . Whether the design is reasonable affects the smoothness of the surface, driving comfort, drainage performance, needs of aesthetic. With the development of road traffic, the situation of road crossing is increasing, grade crossing’s form is more complex, engineering requirements is higher. Therefore, designers must be able to complete the design task fast and well.This paper on the basis of learning the status of development and applications of CAD technology for the elevation planning of grade crossing, combine the process and characteristics of development of Civil CAD, profound insight the situation of development of CAD software for the elevation planning of grade crossing at home and abroad, analysis the less CAD software for the elevation planning of grade crossing. Finally, according to the need of upgrade and innovation of municipal engineering MECAD software, around the actual intersection of the elevation planning, conduct relevant research and development independent, perfect, intelligent CAD system for the elevation planning of grade crossing.Through summarize various types of intersection, we can get their common characteristic: they are all polygon net graph structures; Then this paper use graph theory, topology constrained to develop software. The topology model, data storage model and constraints model will be use to format graph model. Then the smallest polygon areas can be searched automatically, district the intersection to obtain the calculating points, collect all the calculating points to generat Delaunay triangulation model. Finally, contour map can be generated automatically.The main innovation of the system is to use graph theory, topology constrained to establish the topology model, data storage model and constraints model , integrate the three to format graph model. Then the smallest polygon areas and calculating points can be generated automatically.
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