> > Date: Wed, 9 Nov 2022 11:04:02 +1300 > From: "Tony Shepherd \(FarmMaps NZ\)" <t...@farmmaps.nz> > To: "'Qgis Users List'" <qgis-user@lists.osgeo.org> > Subject: [Qgis-user] Watershed - Catchment area upstream of a point > Message-ID: <003101d8f3be$04ed8380$0ec88a80$@farmmaps.nz> > Content-Type: text/plain; charset="us-ascii" > > Hi All > > > > I'm wanting to use a DEM/DTM to automatically map the watershed/catchment > area upstream of a known point. > > > > After a bit of reading, this appears to be a not-so-simple task but > suggestions are to use the "Upslope Area" algorithm. > > > > In my case, the program fails to finish running, and the message window > shows the following error: > > SAGA Version: 7.8.2 (64 bit) > Error: tool needs graphical user interface [Upslope Area] > > C:\Users\Tony\Documents>exit > > > > Questions: > > * Does anybody know what that error means? > * Is there another tool/method that is preferable to use? >
Yes, Grass is preferable because the integration with QGIS works reliably. It doesn't necessarily need more than two steps: 1. run r.watershed and output drainage direction and stream segments. The key input parameter is "Minimum size of exterior watershed basin". You probably also want to output unique label (a catchment map) and flow accumulation (number of cells that drain through each cell"). 2. unless your point of interest happens to be at the bottom of one of the catchments already mapped, run r.water.outlet, with the drainage direction raster as input. When doing this, view the stream segments layer - if your point of interest is not exactly on a stream segment you need to specify instead a nearby point that is on a stream segment. Regards, Alister
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