Thomas, Again, you'd have to look to see if this can give you the results you need.
The word "ridge" is a qualitative term that refers to a general landform category. For your purposes, how high does a landform need to be to qualify as a ridge: 1m, 10m, 100m, 1000m? Similarly, how long does it need to be and what is the acceptable range of profile slope along a ridge-top? These feature extraction methods deal in numbers, so they will probably never exactly match the subjective perception of a landform as a ridge. This is especially true if you are interested in how people (in the past and in the present) perceive a landform and its suitability for settlement. But you should be able to get reasonably close to some kind of consistent perception of what is a ridge, and the GIS methods have the advantage of being explicit, transparent, and repeatable--essential for science of course. But there is probably no "best" method for ID of ridges. I haven't tried the convergence method, but will do so to see how it turns out. Michael On Jun 12, 2012, at 5:41 AM, Thomaswplee wrote: > What is the difference on convergence and ridge? > Both local convex quadratically double differentiated > > I am interesting in application of geomorphology vs social science such as > terrain analysis related to human settlement > > Thomas > > > > -------- Original message -------- > Subject: Re: [GRASS-user] 回覆： Re: ridge extraction from DEM > From: Markus Metz <[email protected]> > To: Margherita Di Leo <[email protected]> > CC: Michael Barton <[email protected]>,grass-user grass-user > <[email protected]>,Thomaswplee <[email protected]> > > > On Tue, Jun 12, 2012 at 11:28 AM, Margherita Di Leo <[email protected]> > wrote: > > Hi, > > > > there is also r.convergence add-on: > > http://grass.osgeo.org/wiki/GRASS_AddOns#r.convergence > > BTW, the topographic convergence index is also available in > r.watershed in GRASS 7. > > Markus M > > > > > Regards, > > madi > > > > On Tue, Jun 12, 2012 at 4:24 AM, Michael Barton <[email protected]> > > wrote: > >> > >> 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 <[email protected]> > >> To: [email protected] > >> CC: grass-user grass-user <[email protected]> > >> > >> > >> 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, <[email protected]> > >> 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 > >> > >> > >> > >> > >> > >> > >> _______________________________________________ > >> grass-user mailing list > >> [email protected] > >> http://lists.osgeo.org/mailman/listinfo/grass-user > >> > > > > > > > > -- > > Dr. Margherita Di Leo > > > > _______________________________________________ > > grass-user mailing list > > [email protected] > > http://lists.osgeo.org/mailman/listinfo/grass-user > > _____________________ 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
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