Dear Grass community

First of all thanks for this great piece of software such as GRASS GIS!

Recently I have been using the r.threshold module to extract a river networks at different DTM resolutions and different areal extent. I noticed that r.threshold values varies widely as a function DEM resolution and extent of watershed (areal extent). So , I have some questions, and hope someone in the community can shed some light on them.

Looking at the description of the module it says : "This approach provides a **best guess **about what makes sense when looking only at the DEM"

Question 1: means that the results of the r.threshold module will depend on the resolution and the extent of the watershed?

Looking at the source code, I realised that r.threshold used r.stats to generate a flowacc text.file ordered in ascendent order and put into a matrix called mappatella, with 3 columns defined as:

*mappatella is a matrix, in the first column the value of upslope area is stored, *

*in the second the number of cells, in the third the distance from origin is calculated*

further in the script the distance is defined as :

*calculating distance from origin of each point; origin of the plot is in low left point *

Question 2 : which is the origin? is the center of a cell of the corner of a cell?

Question 3: which plot? and why the low left point of the plot?

Any help would be appreciated.

Thanks in advance !

Best regards

Rengifo Ortega

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