Thomas,
I don't remember posting anything about r.ppa. I did a quick search and found a post by Māris Nartišs (http://lists.osgeo.org/pipermail/grass-user/2005-October/030883.html).
r.param.scale will extract ridges. You will need to adjust the optional parameters (especially processing window size) to get the ridges you want.
An alternative method is to invert the DEM and run r.watershed on it, extracting the 'streams'. The 'streams' of the inverted DEM will follow ridges.
Michael
On Jun 11, 2012, at 9:26 AM, <thomaslee@starvision.com.hk>
wrote:
Dear Michael,
I am interested in ridge extraction but not able to find the r.ppa mentioned by
Chang, Y.C., Frigeri, A., 2002. Implementing the automatic
extraction of ridge and valley axes using the PPA algorithm in
Grass GIS. In: Open Source Free Software GIS GRASS
Users Conference, 2002.Implementing the automatic extraction of ridge and
valley axes using the PPA algorithm in Grass GISDo you have any solution since your message in 2007 about ridge extraction.
_____________________
C. Michael Barton
Visiting Scientist, Integrated Science Program
National Center for Atmospheric Research &
University Corporation for Atmospheric Research
303-497-2889 (voice)
Director, Center for Social Dynamics & Complexity
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University
www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu