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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">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).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">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:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">r.neighbors input=dem output=dem_avg_11 size=11<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">r.mapcalc expression=”TPI_11=dem-dem_avg_11”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">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…<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">In order to address scale effects, you can use a multiscale approach with increasing neighborhoods for all the above methods…<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Cheers,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Stefan<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> grass-user [mailto:grass-user-bounces@lists.osgeo.org]
<b>On Behalf Of </b>Vaclav Petras<br>
<b>Sent:</b> 12. november 2015 13:42<br>
<b>To:</b> Jean-Roc Morreale <jr.morreale@enoreth.net><br>
<b>Cc:</b> GRASS user list <grass-user@lists.osgeo.org><br>
<b>Subject:</b> Re: [GRASS-user] Automatic extraction of valley features<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Hi,<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Nov 12, 2015 at 5:09 AM, Jean-Roc Morreale <<a href="mailto:jr.morreale@enoreth.net" target="_blank">jr.morreale@enoreth.net</a>> wrote:<o:p></o:p></p>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<p class="MsoNormal">I would like to know how to extract a valley feature from a DEM as a polygon<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
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).<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">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.<o:p></o:p></p>
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<p class="MsoNormal">Note that r.geomorphon module is in GRASS Addons repository, so use e.g., g.extension r.geomorphon to get it.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Vaclav<o:p></o:p></p>
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<p class="MsoNormal"><br>
<a href="https://grass.osgeo.org/grass70/manuals/r.param.scale.html" target="_blank">https://grass.osgeo.org/grass70/manuals/r.param.scale.html</a><br>
<a href="https://grass.osgeo.org/grass70/manuals/addons/r.geomorphon.html" target="_blank">https://grass.osgeo.org/grass70/manuals/addons/r.geomorphon.html</a><o:p></o:p></p>
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