<html><head></head><body><div class="yahoo-style-wrap" style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:13px;"><div dir="ltr" data-setdir="false"><font size="3">Dear Grass community</font></div><div dir="ltr" data-setdir="false"><font size="3"><br></font></div><div dir="ltr" data-setdir="false"><font size="3">First of all thanks for this great piece of software such as GRASS GIS!</font></div><div dir="ltr" data-setdir="false"><font size="3">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.</font></div><div dir="ltr" data-setdir="false"><font size="3"><br></font></div><div dir="ltr" data-setdir="false"><font size="3">Looking at the description of the module it says : "<span><span style="color: rgb(0, 0, 0); font-family: arial, sans-serif;">This approach provides a <b>best guess </b>about what makes sense when looking only at the DEM"</span></span></font></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><font face="arial, sans-serif" size="3"> Question 1: means that the results of the r.threshold module will depend on the resolution and the extent of the watershed?</font></div><div dir="ltr" data-setdir="false"><font face="arial, sans-serif" size="3"><br></font></div><div dir="ltr" data-setdir="false"><font face="arial, sans-serif" size="3">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:</font></div><div dir="ltr" data-setdir="false"><font face="arial, sans-serif" size="3"><br></font></div><div dir="ltr" data-setdir="false"><span style="color: rgb(106, 115, 125); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;"><i style=""><font size="3">mappatella is a matrix, in the first column the value of upslope area is stored, </font></i></span></div><div dir="ltr" data-setdir="false"><i><font size="3"><br></font></i></div><div dir="ltr" data-setdir="false"><i><font size="3"><span><span style="color: rgb(106, 115, 125); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;">in the second the number of cells, in the third the distance from origin is calculated</span></span><br></font></i></div><div dir="ltr" data-setdir="false"><i><span style="color: rgb(106, 115, 125); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;"><font size="5" style="background-color: inherit;"><br></font></span></i></div><div dir="ltr" data-setdir="false"><span style="font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;"><font size="3" style="background-color: inherit;">further in the script the distance is defined as :</font></span></div><div dir="ltr" data-setdir="false"><i><span style="font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;"><font size="3" style="background-color: inherit;"><br></font></span></i></div><div dir="ltr" data-setdir="false"><i><span style="font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;"><font size="3" style="background-color: inherit;"><span><span style="color: rgb(106, 115, 125); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;">calculating distance from origin of each point; origin of the plot is in low left point </span></span><br></font></span></i></div><div dir="ltr" data-setdir="false"><i><span style="font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;"><font size="3" style="background-color: inherit;"><br></font></span></i></div><div dir="ltr" data-setdir="false"><i><span style="font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; white-space: pre-wrap;"><font size="3" style="background-color: inherit;"><br></font></span></i></div><div dir="ltr" data-setdir="false"><font size="3">Question 2 : which is the origin? is the center of a cell of the corner of a cell?</font></div><div dir="ltr" data-setdir="false"><font size="3">Question 3: which plot? and why the low left point of the plot? </font></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><font size="3"> 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 <span style="color: rgb(34, 34, 34); font-family: Arial, sans-serif;">Distance Distribution Dynamics( </span><span style="color: rgb(34, 34, 34); font-family: Arial, sans-serif;">DDD) model for urban enviroments. <span><a href="https://onlinelibrary.wiley.com/doi/full/10.1002/hyp.10315?casa_token=ETYV_ojgMb4AAAAA%3AMWgGgX_jqbylpF6pHLoafciVZrlmCBiLCmxb7gRpsCV1F3SY7QQmOkBOIrhLzhj5BFCDSLobabm7Xw" rel="nofollow" target="_blank">https://onlinelibrary.wiley.com/doi/full/10.1002/hyp.10315?casa_token=ETYV_ojgMb4AAAAA%3AMWgGgX_jqbylpF6pHLoafciVZrlmCBiLCmxb7gRpsCV1F3SY7QQmOkBOIrhLzhj5BFCDSLobabm7Xw</a></span></span></font></div><div dir="ltr" data-setdir="false"><font size="3"><br></font></div><div dir="ltr" data-setdir="false"><font size="3">Any help would be appreciated.</font></div><div dir="ltr" data-setdir="false"><font size="3"> Thanks in advance !</font></div><div dir="ltr" data-setdir="false"><font size="3"><br></font></div><div dir="ltr" data-setdir="false"><font size="3">Best regards </font></div><div dir="ltr" data-setdir="false"><font size="3">Rengifo Ortega</font></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(106, 115, 125); font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace; font-size: 12px; white-space: pre-wrap;"><br></span></span></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><font face="arial, sans-serif" size="3"><br></font></div></div></body></html>