[GRASS-user] Automatic extraction of valley

Thu Nov 12 06:27:03 PST 2015

Dear Jean,
If I understand you correctly what you are looking for is to make a  swath profile along curved geomorphic features e.g botton valleys. I think  that if  you want to this in  grass gis, it would be enough to  create a  mask using the the polygon you show in your picture and then extract that area using r.mapcalc. You also can use the geomorphons approach,  described by Vaclav, but you have to be careful with the parameters, since they will yield different results depending on your parameters.I would rather use gemorphons instead of r.param.scale, since it is scale independent.  If you want something more specific I suggest you to take a  look at this paper it might give some  ideas about the topic.
http://www.earth-surf-dynam.net/2/97/2014/esurf-2-97-2014.pdf

Best regards..

Rengifo Ortega

> 
> Message: 4
> Date: Thu, 12 Nov 2015 11:09:48 +0100
> From: Jean-Roc Morreale <jr.morreale at enoreth.net>
> To: grass-user at lists.osgeo.org
> Subject: [GRASS-user] Automatic extraction of valley features
> Message-ID: <ea169b38242db4eab8424817c2c2149a at enoreth.net>
> Content-Type: text/plain; charset=US-ASCII; format=flowed
> 
> Hi,
> 
> I would like to know how to extract a valley feature from a DEM as a 
> polygon : the wanted result isn't equal to a watershed or a sub-basin, 
> it is the cumulated shape of the two slopes and the bottom where the 
> stream is located.
> 
> As I do not master the geomorphological vocabulary, here is a image 
> showing the desired outcome :
> https://drive.google.com/open?id=0B1sJg8zddqSPUk9BcTdncVRNNHc
> 
> I'm using GRASS7, I've tried r.watershed and several r.stream.* modules 
> but my lack of knowledge in this domain is blocking. The elevation data 
> used is the EU-DEM.
> 
> Regards,
> Jean-Roc
> 
> 
> ------------------------------
> 
> Message: 5
> Date: Thu, 12 Nov 2015 07:41:47 -0500
> From: Vaclav Petras <wenzeslaus at gmail.com>
> To: Jean-Roc Morreale <jr.morreale at enoreth.net>
> Cc: GRASS user list <grass-user at lists.osgeo.org>
> Subject: Re: [GRASS-user] Automatic extraction of valley features
> Message-ID:
> 	<CABo5uVs7Qrs5F+mRX_baN29+XUTFMuOxixAXFscBG7n8hAYvRA at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> 
> Hi,
> 
> On Thu, Nov 12, 2015 at 5:09 AM, Jean-Roc Morreale <jr.morreale at enoreth.net>
> wrote:
> 
> > I would like to know how to extract a valley feature from a DEM as a
> > polygon
> 
> 
> The classic way would be to use r.param.scale with method=feature, but I
> would recommend using r.geomorphon by Stepinski and Jasiewicz (2011, 2013).
> 
> Both modules will give you a raster map. You can use r.to.vect to get
> vector polygons. To get just the valley use either raster algebra
> (r.mapcalc) on the raster map or v.extract on the vector map.
> 
> Note that r.geomorphon module is in GRASS Addons repository, so use e.g.,
> g.extension r.geomorphon to get it.
> 
> Vaclav
> 
> https://grass.osgeo.org/grass70/manuals/r.param.scale.html
> https://grass.osgeo.org/grass70/manuals/addons/r.geomorphon.html
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> ------------------------------
> 
> Message: 6
> Date: Thu, 12 Nov 2015 12:59:47 +0000
> From: "Blumentrath, Stefan" <Stefan.Blumentrath at nina.no>
> To: Vaclav Petras <wenzeslaus at gmail.com>, Jean-Roc Morreale
> 	<jr.morreale at enoreth.net>
> Cc: GRASS user list <grass-user at lists.osgeo.org>
> Subject: Re: [GRASS-user] Automatic extraction of valley features
> Message-ID: <a483ccb78ff7490284f5dba6982ce0eb at NINSRV23.nina.no>
> Content-Type: text/plain; charset="utf-8"
> 
> Hi,
> 
> Something to have in mind is that your definition of a valley will be heavily scale dependent, e.g. what depth and width (and possibly even length) do you consider as a valley. As a consequence, also results from r.param.scale and r.geomorphon are likewise scale dependent (where scale is - amongst others - accounted for in neighborhood parameter).
> 
> Another option to identify valleys is the Topographic Position Index (TPI), which is defined as the altitude of a pixel in relation to the average altitude of the pixels in the neighborhood. In GRASS terms:
> r.neighbors input=dem output=dem_avg_11 size=11
> r.mapcalc expression=”TPI_11=dem-dem_avg_11”
> 
> Negative TPI values indicate valleys. However, a drawback of TPI is that it may also give negative values at the foot of a slope, and that the signal will “fork” in wide U-valleys…
> 
> In order to address scale effects, you can use a multiscale approach with increasing neighborhoods for all the above methods…
> 
> Cheers,
> Stefan
> 
> From: grass-user [mailto:grass-user-bounces at lists.osgeo.org] On Behalf Of Vaclav Petras
> Sent: 12. november 2015 13:42
> To: Jean-Roc Morreale <jr.morreale at enoreth.net>
> Cc: GRASS user list <grass-user at lists.osgeo.org>
> Subject: Re: [GRASS-user] Automatic extraction of valley features
> 
> Hi,
> On Thu, Nov 12, 2015 at 5:09 AM, Jean-Roc Morreale <jr.morreale at enoreth.net<mailto:jr.morreale at enoreth.net>> wrote:
> I would like to know how to extract a valley feature from a DEM as a polygon
> 
> The classic way would be to use r.param.scale with method=feature, but I would recommend using r.geomorphon by Stepinski and Jasiewicz (2011, 2013).
> Both modules will give you a raster map. You can use r.to.vect to get vector polygons. To get just the valley use either raster algebra (r.mapcalc) on the raster map or v.extract on the vector map.
> Note that r.geomorphon module is in GRASS Addons repository, so use e.g., g.extension r.geomorphon to get it.
> 
> Vaclav
> 
> https://grass.osgeo.org/grass70/manuals/r.param.scale.html
> https://grass.osgeo.org/grass70/manuals/addons/r.geomorphon.html
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Rengifo Yahoo <rengifoo at yahoo.de>