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 differentiatedI 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@googlemail.com>
To: Margherita Di Leo <diregola@gmail.com>
CC: Michael Barton <michael.barton@asu.edu>,grass-user grass-user <grass-user@lists.osgeo.org>,Thomaswplee <thomaswplee@gmail.com>On Tue, Jun 12, 2012 at 11:28 AM, Margherita Di Leo <diregola@gmail.com> wrote:
Hi,
there is also r.convergence add-on:
http://grass.osgeo.org/wiki/GRASS_AddOns#r.convergenceBTW, the topographic convergence index is also available in
r.watershed in GRASS 7.Markus M
Regards,
madiOn Tue, Jun 12, 2012 at 4:24 AM, Michael Barton <michael.barton@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 0Run 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@asu.edu>
To: thomaslee@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, <thomaslee@starvision.com.hk>
wrote:Dear Michael,
I am interested in ridge extraction but not able to find the r.ppa
mentioned byChang, 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
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@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user–
Dr. Margherita Di Leo
grass-user mailing list
grass-user@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