Abstract: Ridge and valley are of essential important feature of terrain surface.They play an important role in the terrain representation because the basic structure of terrain is determined by these terrain feature.Digitized contour data contains the information of these feature lines implicitly.For many years,researchers have been working on the extraction of terrain feature lines from digitized contour lines.Up to now,existing method for feature line extraction is 2D geometrical analysis method,which includes extraction of feature points and feature line tracing.The former can be well solved by 2D geometrical analysis,but the latter is very difficult and remains unsolved because of the limitation of 2D geometrical analysis. In this paper,the author investigates the problem of the derivation of terrain feature lines,and discusses the existing method for the derivation of terrain feature line.After analyzing the existing method theoretically,the author puts forward an idea for terrain feature line tracing in which 3D terrain analysis technique is used.The new idea is based on the fact that vellay line is the line into which falling water is merged and ridge line is the line which separates falling water. The method for terrain feature line tracing developed by the new idea includes 3D terrain analysis and 2D geometrical analysis.Because 3D terrain analysis means falling water analysis in this paper,it is also called physical analysis of terrain model.In the falling water analysis of terrain,the lines into which falling water is merged and the line which separates falling water can be roughly located by falling water simulation.Because the conception of valley line and ridge line differs from the conception of the lines into slightly,some adjustment must be done before the valley line and ridge line are determined.The adjustment includes that some problems are led by differences between conceptions,and the location is not accurate by falling water simulation.The former means that there exist some mistakes at the two ends of the line into which falling water is merged because the accumulation of falling water has close relation to elevation.It leads to some part of valley line at higher position loss because the accumulation of falling water is small,and some part of the line into which falling water is merged at lower position is taken as parts of valley line because the accumulation of falling water is larger.The latter is led by the lower accuration of the data used in 3D terrain analysis.In the new method,2D geometrical analysis technique is combined with 3D terrain analysis,and the rough results of falling water simulation are used to guide the selection and ordering of feature points from 2D geometrical analysis results.In this way,the new mathod can take the advantages of 2D geometrical analysis and 3D terrain analysis,and overcome the shortcomings of them.At the end of this paper,the author gives an experimental result which shows the valley and ridge extracted by the new method is concord to the experimental terrain.