A Study On Contour Lines Extraction From Color Topographic Maps

All sorts of symbols are used to portray terrain and geographical information in maps, such as points, lines and plane. The information in traditional paper maps is not readable in computer. As the application of computer technology in town planning, water resource management, remote sensing imaging, GIS and so on, digital maps are in need. Therefore, the conversion of maps from paper into digital, namely map-vectorization is very important. In topographic maps, contour lines are the most important features to characterize three-dimensional terrain on two-dimensional map sheets, so its vectorization is in urgent. In this paper an effective method for extracting contour lines from color scanned topographical maps is proposed. It focuses on reconnecting gaps of contour lines.Firstly, an eigenvector-fitting algorithm is used in contour lines color segmentation for extracting the color keys of map features (include contour lines) in order to reduce the phenomena of color-alias and color-misalignment.Secondly, for reconnecting gaps in contour lines, linear features extracted from the gray version of the input color map are utilized to repair the contour lines segments extracted from the input color map. Unlike the existing methods, it analyses various reasons inducing gaps in contour lines and introduces corresponding solutions for different cases of gaps.Thirdly, it utilizes line tracking and Douglas-Peucker line-simplification algorithm to realize the raster-to-vector conversion. A triangulated irregular network(TIN) is constructed based on vector data, and DEM point-array data is computed by interpolating grid on TIN.Finally, the approach is applied to some representative samples and national standard topographic maps with a scale of 1:50,000, and an evaluation scheme for the results is provided. The experiments show this method applicability.