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)
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.
Thanks a lot.
Regards
--
Ing. Margherita Di Leo
PhD Student Methods and Technologies for Environmental Monitoring
Department of Environmental Engineering and Physics (DIFA)
University of Basilicata Campus Macchia Romana
85100 - Potenza Italy
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)
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.
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).
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.
Hi Margherita…
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 r.broscoe.sh that: executes r.watershed for some values of threshold, applies some statistical tests and defines the best threshold through the examinated threshold values…
this is not THE way to approach the problem… but ONE way to do it… if you want you can try…
you can read more about it downloading the GRASS meeting slides here: http://www.grassmeeting2008.unipg.it/?q=node/9 (file: Cencetti et alii).
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)
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.
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).
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.
My first reply was maybe a bit short and rude, let me try again.
Both r.watershed and r.terraflow produce various output maps and it is not exactly clear to me what output you are interested in.
If you want flow accumulation, the threshold doesn't have an effect on it. Flow accumulation is calculated independent of such a threshold.
If you want rivers, you can either set the threshold in r.watershed and get stream segments or use the flow accumulation output of either module and do something like r.mapcalc "rivers = if(flow_acc > threshold, 1, null())", threshold would here be the desired number of cells draining through a given cell when using r.watershed flow accumulation or size of the area draining through a given cell for r.terraflow flow accumulation.
I'm not aware of a threshold option in r.terraflow. For r.watershed, there is no default value for threshold, you have to specify this threshold if you want stream segments, basins, or half basins, otherwise r.watershed terminates with an error.
Hope that helps more
Markus M
margherita wrote:
Hi all
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)
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.
