Automatic Extraction of Contour Lines from Topographic Maps

The automatic extraction of contour lines and generation of digital elevation models (DEMs) from topographic maps is one of the challenging subjects mostly because of aliasing, false colors, closely spaced lines and other features causing intersection or overlapping. In this paper we present an algorithm to extract contour lines from colored images of scanned topographic maps. In our approach, we first segment the color image using adaptive thresholding to extract basic contour structure. Noise in the images is removed utilizing morphological operation. Next, the contour lines are reduced up to unitary thickness using Zhang's thinning algorithm. The bifurcation and holes that result after thinning are removed using different masks. After thinning, end points of the broken contours are identified and best candidate for connection is determined, this is performed by analyzing the Euclidean distance and direction of end points near the gap. Then broken contour lines are joined employing curve fitting technique. The performance of the algorithm is tested on several samples of topographic maps and results show good segmentation of the contour lines. This automatic extraction algorithm for contour lines from topographic maps can save significant amount of time and labor as well as improving the accuracy of the contour line extraction.