Rengifo Ortega rengifoo at yahoo.de
Thu Jun 11 04:08:52 PDT 2020

```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?
Although I understand the general aspect  of the module I still struggle to understand its details. I would appreciate some explanation to crarify  it further, since I  am using it as part of workflow within  GRASS GIS to produce inputs to a Distance Distribution Dynamics( DDD) model for urban enviroments. https://onlinelibrary.wiley.com/doi/full/10.1002/hyp.10315?casa_token=ETYV_ojgMb4AAAAA%3AMWgGgX_jqbylpF6pHLoafciVZrlmCBiLCmxb7gRpsCV1F3SY7QQmOkBOIrhLzhj5BFCDSLobabm7Xw
Any help would be appreciated. Thanks in advance !
Best regards Rengifo Ortega

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