[GRASS-user] 回覆： Re: 回覆： Re: ridge extraction from DEM

Tue Jun 12 10:39:15 PDT 2012

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 <markus.metz.giswork at googlemail.com> 
> To: Margherita Di Leo <diregola at gmail.com> 
> CC: Michael Barton <michael.barton at asu.edu>,grass-user grass-user <grass-user at lists.osgeo.org>,Thomaswplee <thomaswplee at gmail.com> 
> 
> 
> On Tue, Jun 12, 2012 at 11:28 AM, Margherita Di Leo <diregola at gmail.com> 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 <michael.barton at asu.edu>
> > 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 <michael.barton at asu.edu>
> >> To: thomaslee at starvision.com.hk
> >> CC: grass-user grass-user <grass-user at 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, <thomaslee at 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
> >>
> >>
> >>
> >>
> >>
> >>
> >> _______________________________________________
> >> grass-user mailing list
> >> grass-user at lists.osgeo.org
> >> http://lists.osgeo.org/mailman/listinfo/grass-user
> >>
> >
> >
> >
> > --
> > Dr. Margherita Di Leo
> >
> > _______________________________________________
> > grass-user mailing list
> > grass-user at lists.osgeo.org
> > 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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