[GRASS-user] Automatic extraction of valley features

[Blumentrath, Stefan]

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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