This paper discusses the spatial knowledge related to a line, and the characteristic points of lines is detected. According to the requirements of line generalization, new algorithms for identifying characteristic line points are presented. These characteristic points are used to improve the algorithms of line generalization. An algorithm for identifying bends is shown. In this paper, improved algorithms based on those by Douglas-Peucker, Visvalingam and Whyatt are shown. In this test, the progressive process of line generalization is emphasized.