Invert the DEM by multiplying all values by -1 and adding the maximum original height value. This makes the maximum height 0 and everything less than that increasingly greater than 0
Run r.watershed on the inverted DEM choosing stream segments as output. The stream segments from the inverted DEM are your ridgelines. You can keep them in raster or thin them (r.thin) and convert them to vector (r.to.vect). I've copied the GRASS user list again because this is a general question that others might be interested in. Michael On Jun 11, 2012, at 7:29 PM, Thomaswplee wrote: > How to make ridge vector line with reversed accumlated flow as in watershed? > > Still reverse DEM plus vectorization of ridge raster? > > Thomas > > > > -------- Original message -------- > Subject: Re: ridge extraction from DEM > From: Michael Barton <michael.bar...@asu.edu> > To: thomas...@starvision.com.hk > CC: grass-user grass-user <grass-user@lists.osgeo.org> > > > 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, <thomas...@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 GIS > > > > > > Do 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 > _____________________ C. Michael Barton Visiting Scientist, Integrated Science Program National Center for Atmospheric Research & University Consortium 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
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