[GRASS-user] Automatic extraction of valley

Thu Nov 12 07:32:31 PST 2015

Thank you for this article Rengifo !

To be clearer, I've been asked by a geologist to create a polygon cover 
of valley features, the polygon shown in my example was just a quick 
doodle showing the wanted result (I would take me several weeks to 
manually digitize the whole region).

Le 2015-11-12 15:27, Rengifo Ortega a écrit :
> 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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