Hi Margherita..<br>time ago I worked on the "threshold problem".. just because there's not a default value for this parameter, there's a study inside the value this parameter has to assume.. and various theories.. I worked on the Broscoe Theory (1959) and produced a module (you can find it into the addons) called <a href="http://r.broscoe.sh">r.broscoe.sh</a> that: executes r.watershed for some values of threshold, applies some statistical tests and defines the best threshold through the examinated threshold values..<br>
this is not THE way to approach the problem.. but ONE way to do it.. if you want you can try..:-)<br>you can read more about it downloading the GRASS meeting slides here: <a href="http://www.grassmeeting2008.unipg.it/?q=node/9">http://www.grassmeeting2008.unipg.it/?q=node/9</a> (file: Cencetti et alii).<br>
<br>Hope this helps!<br><br>Annalisa<br><br><br><div class="gmail_quote">2009/2/5 Markus Metz <span dir="ltr"><<a href="mailto:markus.metz.giswork@googlemail.com">markus.metz.giswork@googlemail.com</a>></span><br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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margherita wrote:<br>
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Hi all<br>
<br>
i have some question about the commands r.watershed and r.terraflow. By using both commands i obtained from a DEM a flow accumulation with the algorithm of the single flow direction and a flow accumulation with the algorithm of the multi flow direction. In both cases i did not enter the treshold of cell (i.e. in the flow accumulation if a grid cell value exceeds a certain threshold, it's part of the river, if not, it isn't)<br>
<br>
Please can you let me know if there are default values of this threshold and, if yes, what they are, and if that value can be changed by the user and how.<br>
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This threshold can be set with the option threshold=some_value in r.watershed (details are in the manual, please read it). r.terraflow doesn't have such an option (details are in the manual, please read it).<br>
<br>
If the option threshold=some_value is given for r.watershed, you can also get basins, half basins, and stream segments. Maybe these stream segments are what you are looking for.<br>
<br>
Good luck,<br>
<br>
Markus M<div><div></div><div class="Wj3C7c"><br>
<br>
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