This paper introduces a method of edge detection. By comparison with topography

it is found that the character of edge of image shows the same valley' s character as relief map. Firstly

according to this character

primacy central point is detected by the exploring extremum method in the windows of 3×3 pixels. Then

basing on this central point

approximate normal line at±45°、0°and 90°angle are drew.if the minimums of normal line is at central point and the difference between the minimums and the maximum in the normal line is more than the threshold

this point is considered to the edge point of image. Because the brightness of this point varies with local image like valley—the valley geographic location is not always at the same level and it varies with physiognomy. So this method of edge detection is according to local minimum point in the normal line. the character of this method has self-adjusting situation of local image on detecting edge of image.