Feature characterization of rough surfaces is of growing interest in terms of a function oriented description of technical surfaces. Feature characterization requires a segmentation of significant hills and dales of the measured profile. The segmentation can be done in several ways. One method is the so called crossing-the-line segmentation which will be part of ISO 16610 part 45 and ISO 21920 part 2. The crossing-the-line segmentation described in this publication represents an extension of the algorithm proposed by Scott (Scott P, 2006, Meas. Sci. Technol. 17, 559-564) and is based on new knowledge gathered over the last ten years. As an example, the feature parameters R S m and R c according to ISO 4287:1997 are evaluated.