A Study On Reconstruction Algorithm Of Contour Lines Extraction From Color Topographic Maps

Topographic map, which is mainly composed of colored lines, is used to express the geographic and topographic information. Contour line is an important component of the topographic map and the main source to build Digital Elevation Model, so the research of automatic extraction and vectorization of contour lines has been a research hotspot and difficulty in the field of topographic map vector quantization. Due to contours break for intensive distribution on the topographic map, it’s the key issues in the process of extracting contour lines automatically that how to extract the complete contour lines from color scanned topographic map, especially reconstruct the contour lines from broken ones.This paper studies a method that extracting contour lines automatically from a color scanning raster topographic map, designs and implements two repairing algorithms for reconstructing the broken contour lines based on two different graphics theories. The main work in this paper is as follows:1. Study the color segmentation method on color maps. To get the complete contour lines from raster topographic maps, this thesis studies the basic principle and several classic segmentation algorithms of color image segmentation, and discusses post-processing operation after segmenting in detail, including de-noising.2. Design and implement a new algorithm for semi-automatic connection of disconnected contour lines based on the Min Pair of Breakpoints Method proposed by R.Samet. The traditional Min Pair of Breakpoints Method more likely connect the breaking large extent contour lines incorrectly, aiming at which this paper proposes an improved breakpoint matching method. By calculating the main trend direction angle of the contour lines where the breakpoints are, based on the combination of the Euclidean distance weights and angle control weights between breakpoints, we can get the optimal matching point from the candidates. During connecting breakpoints iteratively, this method can ensure the correct connections between breakpoints with different breaking degrees by dynamically changing the threshold. The experiment results show that, regardless of the distance and directions between breakpoints, the improved method can be a very good solution to solve the breakpoint connection problem.3. Design and implement a new algorithm for automatic connection of broken contour lines based on crust Curve Reconstruction Method proposed by Amenta. The traditional crust Method can only reconstruct single-edge curves, aiming at which this paper proposes an improved curve reconstruction method on contour lines. This method firstly preprocesses the points to refinement point sets, and then combines the single edge of the sets and crust Method based on Voronoi diagram and Delaunay triangulation, at last skillfully use Voronoi diagram vertex to regress point axis. This method not only can effectively eliminate the influence of the noise in extracted edge data points, but also can obtain satisfactory contour lines curves for different shape of point sets. The experiment results show that the improved contour lines curve reconstruction method is applicable to maps of various sizes and complexity with high universality.