The width and the gray level of the navigation line keep almost the same in the image.Based on those characters,the navigation line can be detected robustly.Scanning the image alone the horizontal line to find the neighbor edge points whose distance is between the navigation line width minus 2 and the navigation line width multiple 2,and the gray level variance of pixels between them is small.Then the navigation line can be detected from those points by fast Hough transform roughly and refined by least squa...